Diagnosing Lithium cells? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: November 17, 2015, 08:58:13 pm
content: I got a box full of used 18650 cells with unknown history, and i'd like to find out what the history was.To that end i built a battery analyzer that can charge and discharge the cells while logging the current and voltage curves.The question is - now what?Specifically, i remember seeing several papers that described in detail what sorts of abuse or wear leave what types of signs on the discharge curves.I.e. being charged below zero would make the cell linger around 4V, then drop fast is the only one i remember.Anyone knows where to find info like that?

author: dmwahl
author_rank: Contributor
timestamp: November 17, 2015, 10:12:17 pm
content: The main things you'll see are reduced capacity and increased internal resistance, which will show up as greater droop in the voltage curve at a given load.Nice tester though, do you have the design posted anywhere?

author: doobedoobedo
author_rank: Regular Contributor
timestamp: November 17, 2015, 11:59:22 pm
content: I agree totally with dmwahlDo an internal resistance test, it'll tell you much more than discharge curves alone.Mind you if you're only going to use them for low current applications I don't suppose it much matters.

author: crazy horse
author_rank: Contributor
timestamp: November 18, 2015, 04:34:46 am
content: http://batteryuniversity.com/learn/article/testing_lithium_based_batteriesLots of good battery information on this site.

author: amyk
author_rank: Super Contributor
timestamp: November 18, 2015, 05:30:42 am
content: Quote from: crazy horse on November 18, 2015, 04:34:46 amhttps://en.wikipedia.org/wiki/Talk%3ALithium-ion_battery#Dubious_information_originating_from_discredited_source

author: Artlav
author_rank: Frequent Contributor
timestamp: November 18, 2015, 06:47:55 am
content: Interesting.Conversely, that talk page is the only reference i could find about the "battery university" being discredited.Do anyone have any details?I thought it was a good and useful site, and there is a lot of valid info there.

author: eas
author_rank: Frequent Contributor
timestamp: November 18, 2015, 08:51:59 am
content: Quote from: amyk on November 18, 2015, 05:30:42 amWhat a clusterf*ck that whole discussion is. It reads like a cargo-cult version of intellectual rigor.

author: crazy horse
author_rank: Contributor
timestamp: November 18, 2015, 09:08:52 am
content: It does seem if it is a bit personal.

author: ez24
author_rank: Super Contributor
timestamp: November 18, 2015, 09:12:42 am
content: Interesting

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 18, 2015, 09:42:27 am
content: Hi,I can contribute this information. I recently measured an 18650 Cell made by PK Cell, nominal capacity 2200 mAh.This is my measurement setup:The equipment was controlled using HPIB.These are the results:The horizontal axis is depth of discharge, 0 is fully charged, 1 is fully discharged.The ESR was measured by interrupting the discharge current and measuring the change in voltage.I also measured a Ultracell Rose 4200mAh cell obtained from eBay. This graph show the raw data. The blips shows the voltage when the load was turned off to measure ESR.This battery had a measured capacity of 2250 mAh.Regards,Jay_Diddy_B

author: Artlav
author_rank: Frequent Contributor
timestamp: November 18, 2015, 01:07:08 pm
content: Quote from: Jay_Diddy_B on November 18, 2015, 09:42:27 amThat's interesting.What i plan on doing, is to add ESR logging during discharge.Measure voltage, turn the load off, wait 100us, measure voltage, subtract, do the math, turn the load back on.Would that be a valid method?The descriptions i google up mention the inverse being done - that is, load the cell for a millisecond, measure the voltage drop.More importantly, does the length of the delay matter?When the discharge switches off at the end, the cell quickly gains voltage, from 3 to 3.6 in a few seconds.So, how long is long enough, and how long is too long?Quote from: dmwahl on November 17, 2015, 10:12:17 pmNo, i don't. It's a pretty straightforward thing, but if there is interest i can describe it.One thing is, it's only a few days old and i've already fixed a few big bugs, so there won't be any quality in hurry.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 18, 2015, 01:40:27 pm
content: Artlav,I think the best way to measure the ESR would be to reduce (or increase) the load current and calculate R= delta V / delta I.I would use a scope to determine if the timing matters.I had to use relatively long timing, because I had the DMM free-running, I could have shortened the time if I knew when the DMMs are going to read.Being consistent is important, if you want to compare cells.Regards,Jay_Diddy_B

author: ralphd
author_rank: Frequent Contributor
timestamp: November 18, 2015, 02:19:38 pm
content: Quote from: amyk on November 18, 2015, 05:30:42 amThe wiki post is dubious. Battery University published verifiable details. The SEM photos might be hard to do on your own, but charge/discharge and IR measurements are easy to verify.http://batteryuniversity.com/learn/article/how_to_restore_nickel_based_batteries

author: dmwahl
author_rank: Contributor
timestamp: November 18, 2015, 03:20:54 pm
content: However you end up measuring internal resistance, be sure to use a 4 wire connection so that the resistance of your test leads doesn't affect the result. You may have thought of that already, but just wanted to mention it.

author: Artlav
author_rank: Frequent Contributor
timestamp: November 18, 2015, 03:52:54 pm
content: Quote from: Jay_Diddy_B on November 18, 2015, 01:40:27 pmOk, made it and checked - the voltage settles within about 1us, i suspect more due to the FET switching delay than anything else.After that it stays flat for hundreds of us. So 100us is good.Haven't tried to find the upper limit, however.How valid the measurements are is a big question.I'm getting 90mV (+-5mV, 3.9 Ohm load to open circuit+10k sense divider) of difference on both near-dead and half-decent cells, and it's not changing much at all from full (4.1V) to empty (3.2V).Starting at 90mV at 4V, it dropped to 80mV at 3.2V, or 87 mOhm to 97 mOhm increase near the end."Near-dead" as in they started at about 1000mAh, and are down to 400mAh after 3 charge-discharge cycles."Half-decent" as in they are at 1200mAh out of the rated 2200mAh, and it changed little over the same 3 cycles.2 of each, all at once.Sounds like the ESR is not really that indicative of anything.Confirmed on a scope, so it's not a rig error.Should i look at absolute values, or am i doing something wrong?Quote from: dmwahl on November 18, 2015, 03:20:54 pmNaturally.

author: doobedoobedo
author_rank: Regular Contributor
timestamp: November 18, 2015, 09:58:19 pm
content: A Li-ion that's lost half it's capacity and has 87-97mOhm internal resistance is not half-decent. It's mostly dead.It'd still work for something that doesn't draw much current (less than an amp) and only requires a single cell if you're determined to use it.

author: eas
author_rank: Frequent Contributor
timestamp: November 18, 2015, 11:54:17 pm
content: Member HJK has published curves on a huge number of LiIon cellson his site. Seeing how new cells of the type you are testing perform should be useful in evaluating old cells.From the reading I've done, the internal resistance of "modern" (ie, those made in the last decade or more) lithium ion cells is of limited use on its own for determining general cell health because most cells are now formulated to maintain a relatively low IR over their target lifespan. The preliminary testing I did with a half-dozen or so used and new-old-stock 18650 cells bore this out. There was a correlation between cell capacity/cycle count, IR and calendar age, but just by eyeballing things I could tell it didn't have great predictive power.Furthermore, I found in limited tests that cells that were at less than 2/3rds their original capacity still could have a long useful life in front of them. After 50+ cycles, the cells I tested only had a further 1-2% reduction in capacity. This was at relatively high discharge/charge rates too. I don't recall the exact values, but my discharges were at over 1.5A. Not enough for some applications, but not a trickle to an 8-bit MCU either.Oh, also, yes, the amount of rest time when doing IR tests is probably significant, but as long as you are consistent in your methodology, it should be useful for relative measurements. Keep in mind that the IR numbers from battery manufacturers are usually obtained using an AC impedance measurement technique. It's probably not the most useful measurement for a lot of purposes since it bears little resemblance to the DC workloads cells are subject to, but it's quick, and it apparently works well enough for matching cells for packs.

author: Artlav
author_rank: Frequent Contributor
timestamp: November 19, 2015, 10:44:35 am
content: Quote from: doobedoobedo on November 18, 2015, 09:58:19 pmCuriously enough, the graphs in eas' link show that 100mOhm is about average IR for these cells.Also, i'm charging them to 4.1V, so the comparable capacity would be about 1500mAh... Which is still close to half of 2200mAh.Well, it would go into the weak bin, i guess.I've got a few genuine Sanyos int the rig at the moment, should be a nice reference point on what a good cell looks like.Quote from: eas on November 18, 2015, 11:54:17 pmIs there a dependence on the voltage they were found at (presumably after spending a while at it)?The bad cells were found at 2.85V, and over 6 cycles they went from 600mAh to 200-300mAh.The better cells were found at 3.75V, and they kept at around 1200-1300mAh over the same 6 cycles.Here are the curves. Voltage on the top, truncated to 3V, current on the bottom.There is a bit of inconsistency as i tweaked the rig along the way, but the patterns are still here.Good cell, charges:Good cell, discharges:Bad cell, charges.The spikes on the first curve is me probing the current and freezing components to check thermal stability.Interesting how it goes to 4.1V almost at once.Bad cell, discharges.It looks almost as if the cell settled at some value, after the stress burned out whatever unstable capacity was in there.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2015, 07:29:40 pm
content: No official standard to measure DC resistance. Dropping the load off for several seconds every now and then and calculating R = dU/dI gives good approximations. Keep the method fixed between the test for proper comparability.Or if you do full cycles - both charge and discharge - you can just calculate the DCR from the graph when the graphs are aligned on the common axis (mAh instead of time, unless the charge and discharge current is same). For example, if you charged at 1A and discharged at 2A, look at voltages at the same aligned point (for example, 500 mAh before the cell was full; and 500 mAh after the discharge started. If it was 4.00V while charging and 3.70V while discharging, you'll get R = dU/dI = 0.30V/3A = 100 mOhm.Often, official "end-of-life" is either 30% drop in capacity or 100% increase in DCR, whichever comes first. Some typical laptop 18650 cells show a DCR of about 50 mOhm when new, so 100 mOhm would be end-of-life. But increased DCR may not be significant in your application at all! Cells with increased resistance can still offer good efficiency and good amount energy when discharged and charged more slowly. Similarly, if you are fine with capacity slightly less than the rated 70%, why toss them.In addition to these two mechanisms, leakage current a.k.a. self-discharge can also increase significantly in some cases. Leave a rather fully charged cell alone. Measure the voltage after 24 hours, and again after a week or so. You will see if some cells leak significantly more than the others.For safety reasons, toss any cells that show below about 2V without even trying to charge them. For cells reading between about 2V and 3.0V, precharge them at very low current (C/50 to C/100) to normal empty voltage levels (to about 3.3V) before normal charging.Never charge li-ion cell, new or used, below 0 degC. Very low currents may be acceptable, but don't risk it. It will at least ruin the life and might cause safety issues in the long run, or in substandard cells. Keeping the temperature higher (30-40 deg C instead of room temp) while charging is good for the life.

author: doobedoobedo
author_rank: Regular Contributor
timestamp: November 19, 2015, 11:36:32 pm
content: Lithium cells don't like to be stored discharged, the closer they are to around 3.8V the less they dislike it, so yes that could explain the good v bad cells especially if they've been left like that for quite a while.I'll fess up and say I don't really use round cells much, I use a lot of LiPos though and the 2S2P 1500mAh 35C pack I have in front of me has DCR of 15.1mOhms (tested using voltage drop/current calculation) so 100mOhms for a single cell sounds crazy high in comparison.

author: eas
author_rank: Frequent Contributor
timestamp: November 20, 2015, 01:08:37 am
content: Initial voltage of recovered cells can be useful for screening, but I think some people put too much stock in it and pitch cells that could have a long useful second life. Low voltage could be a sign of cells with high self-discharge, or the time since last charge, or a flaw in the pack design. Cells may be compromised, or not.I like Siwastaja's approach. Pitch anything below 2v. Precharge anything between 2 and 3. Charge, check self-discharge after ~12 hours. Any that have dropped by more than about 0.1v below termination voltage could be put through a gentle charge/discharge cycle and checked again.Not surprised that cells that only showed 600mAh capacity quickly deteriorated. As for their capacity loss leveling off, my understanding is that capacity loss decline is generally due to the Li-ions in the electrolyte reacting with electrode or other electrolyte materials, typically during charging.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 20, 2015, 07:02:39 am
content: Quote from: Siwastaja on November 19, 2015, 07:29:40 pmDoesn't high temperature age the cells faster?I recommend limiting charge voltage to 4.1V/cell and charge rate to 1/2C if that is OK in your application.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 20, 2015, 07:16:05 am
content: Quote from: doobedoobedo on November 19, 2015, 11:36:32 pmSource on this? I have heard the opposite. Of course avoid storing at 0% SoC, but something like 20% could be very good for the life. My own research is still going on, but I'll get exact results later.QuoteLithium ion cells are made for many different applications. They can optimize the internal structure (basically, active surface area to active mass ratio) to produce so calledenergy cellsor so calledpower cells(and anything between these). Both offer near 100% efficiency at extremely slow charge/discharge. Power cells can provide higher currents without overheating. But this is a compromise that lowers the energy density (Wh/kg) and increases price per energy ($/kWh). So you always try to use energy cells in slow discharge, slow charge applications, like laptops, electric vehicles etc. Many 18650 energy cells work best when fully discharged during more than 1.5 hours (0.7C), even though they are often specified 3C peak, 2C continous discharge. If end-of-life is considered 100% increase in DCR, then the rating goes down even more. Power cells are used in power tools; many 18650 power cells are specified to 8C to 12C, i.e. full discharge in less than 6 minutes.100mOhm DCR is perfectly normal for an aged 18650 laptop-type "energy" cell which has doubled its internal resistance, but can still be great if the application discharges it slowly (below 0.5C). World is full of low-power applications for these recycled cells - the leakage is more important.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 20, 2015, 07:28:29 am
content: Quote from: NiHaoMike on November 20, 2015, 07:02:39 amIn storage, higher temperature causes the cells to age faster due to electrolyte reaction.But in actual cycling, lithium plating during charging is the most prominent reaction.I came across a really good research paper some time ago where they tested this on Panasonic production cells, and it was surprising how much charging at 45degC instead of 25degC helped with the life. Charging is where most of the cycling damage occurs, and as we all (hopefully) know, at 0 deg C it is completely forbidden because it can destroy the cell almost immediately, so this is a very logical finding indeed.But when in storage, keep the cells as cool as possible.Another interesting finding in their paper was thatstoringat 80-90% SoC didn't provide any benefit compared to 100%; you really need to store below 60% (preferably below 50%) to get the benefit. OTOH, my findings show that it's thechargingthat does the damage between (80%)-90%-100%, so the traditional "only charge to 4.1V" tip is a good one anyway.Summary for best life:* Charge at slightly elevated temperatures to slow down lithium plating - but don't go to extremes, over 45degC is often forbidden by the manufacturer and may cause electrolyte reactions.* Discharge temperature doesn't matter so much, but higher temperature reduces energy losses.* Store as cool as possible. Store at 40-50% SoC.* Limit charge rate. 0.5C is the real maximum with many cells! If you can, further limit it slightly after 4.0V - kind of pre-CV-phase.

author: doobedoobedo
author_rank: Regular Contributor
timestamp: November 20, 2015, 10:54:59 am
content: Quote from: Siwastaja on November 20, 2015, 07:16:05 amBatteries from the factory are charged for maximum shelf life. Measure a new one, for LiPos they're within a couple of hundredths of a volt off 3.8V. Look at the Lithium 'storage' setting on any hobby charger, it charges/discharges to 3.8V.Finally for a real-world experiment take 3 identical LiPos. Discharge one to 3V, fully charge one and leave the other at 3.8V. Leave them for 6 months on a shelf.The discharged cell will have puffed considerably and is only fit for the bin, the fully charged cell may well have puffed a bit, but it won't be like the 3V balloon and the 3.8V cell won't have changed.Quote from: Siwastaja on November 20, 2015, 07:16:05 amNo argument with this.

author: dmwahl
author_rank: Contributor
timestamp: November 20, 2015, 03:56:33 pm
content: Quote from: SiwastajaCould you post a link to this? I'd be interested in reading it.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 20, 2015, 05:25:52 pm
content: Quote from: doobedoobedo on November 20, 2015, 10:54:59 amNo source for the claim that they are especially charged for "maximum shelf life". They are meant to be integrated into products as soon as possible. There can be other reasons for the typical 50% shipping SoC. "Good enough" shelf life is experienced at 50% to 60% SoC, no doubt about that. It doesn't mean it couldn't be even better at 30% SoC. Also, I have really seen some that are shipped clearly below 50% (3.5 something volts), but cannot remember which brand, now. It was some of the leading Japanese or Korean 18650.QuoteI have more than 50 cells sitting at 3 different temperatures, charged to different levels, two different chemistries from four manufacturers. Now that I'm fired from the Uni research position due to some office politics, I'm not tied anymore into current scientific standard of selling copyrights to a corporation or restricting access to the scientific work from non-paying members of public, so I can publicly post the results. We'll see!3V(open-circuit) sample is overdischarged, don't do it. I'm not surprised if it puffs. Even if it did store well at exactly 0% SoC, even a small leakage will cause it to overdischarge. Also, 0% is not specified as exactly as 100% is (which would be a CC-CV method). For this reason only, I'd try to avoid even short-term storage below 10% SoC. I would stay away from the clear "knee" part of the curve, which would start at about 15 - 30% SoC, depending on chemistry.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 20, 2015, 05:32:22 pm
content: Quote from: dmwahl on November 20, 2015, 03:56:33 pmSorry, I read a version on physical paper and I have forgotten where I put it, can't remember the authors, and it will be behind some paywall anyway. The title was something like "effect of regenerative braking on battery life". Their main test subject was to test short charge pulses during discharge, and they showed that they actually benefited the cell life (possibly by reducing the total SoC window, because the amount of energy discharged stayed constant), busting the myth that short charge pulses are detrimental to the batteries. But they had other interesting findings, too, related to shelf life.

author: doobedoobedo
author_rank: Regular Contributor
timestamp: November 20, 2015, 06:49:51 pm
content: You're right, but if I was making batteries I'd send them out at the pretty much universally accepted storage charge. You never know how long they're going to be sitting on a distributor's shelf after all.I'd love to see evidence that it really doesn't matter so long as it's between the two steep drops in the discharge curve.

author: Artlav
author_rank: Frequent Contributor
timestamp: November 20, 2015, 10:08:07 pm
content: Anyone else like collecting and looking at the data?Well, here are some curves of good Sanyo R1112 cells, nominal 2400mAh at 4.2V, with a few years of laptop duty behind it (found at above 3V).One cell, 3 charges, 4 dischargesThe capacity stays the same within the noise.These can be though of as a reference for what a good cell should look like (and also confirms that the rig is repeatable to within 1%.).3 charge cycles of one cell.Not as consistent as discharge, apparently dependent on how long the cell had to recover after the discharge.4 discharge cycles of one cell.Start automatically exactly 5 minutes after charge termination, and so super consistent.4 of these cells charging at once.And 4 of them discharging at once.ESR is around 80-90mOhm on all of them.I'm now cycling a couple more of Sanyos, that were found at 2.94V, and they look great so far.And a couple of ICR18650-24E, that were found at 2.63V.These are the same as the BAD cells before, and they show capacity degradation after 3 cycles as well. Looks like a pattern.I guess that the low voltage being bad is cell brand dependent. Good ones can survive a dip into 2V just fine, lower quality or different chemistry ones would degrade.Hopefully will know for sure once i go through the whole box in a few weeks.Quote from: Siwastaja on November 20, 2015, 05:32:22 pmSounds like this one?http://www.a3ps.at/site/sites/default/files/downloads/evs28/papers/A3-01.pdfQuote from: Siwastaja on November 19, 2015, 07:29:40 pmI had a good cell running in a weather station through a Russian winter (as low as -30*C, while being solar charged every day at about C/50-C/100.By spring it was still alive and have the same capacity as it had in the summer.So yeah, low currents work i guess.Quote from: Siwastaja on November 20, 2015, 05:25:52 pmI'd love to to see your results.I guess i can bend the dump resistors towards the cells and compare them at different high temperatures, but that feels like a recipe for disaster.Quote from: Siwastaja on November 20, 2015, 05:25:52 pmHm...Should i program the rig to give the cells some charge after the last cycle?As of right now the cells go into the done bin essentially discharged to 0%, but they all recover to about 3.8V after a few hours.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 21, 2015, 08:35:53 am
content: Quote from: Artlav on November 20, 2015, 10:08:07 pmYes! Thanks! Apparently they also tested 5-month storage at 0% SoC (not specified more accurately), and it was the best storage condition. (I'm still not recommending storing at 0%, but by all means go for 30% instead of the "traditional" 50%).QuoteThis is not surprising. Still, it's generally explicitly forbidden by all manufacturers. In reality, lithium plating is a function of charging current (lowest best), voltage (i.e., termination SoC, lowest best) and temperature (highest best), but manufacturers go the easy way and define the temperature limit as a step function - no charging below 0 degC at all, and full charging current acceptable above 0 deg C. Unfortunately, I have a strong "educated feeling" that charging at full specified charging currents slightly over 0degC will shorten the cell life considerably, especially near 4.2V. On the other hand, there is nothing wrong to do a occasional, very slow charge slighty below 0 degC. Avoid charging fully, however.QuoteI was thinking the same with my rig (which is a non-dissipative bidirectional converter), but never did it. It would be important only if you goreallyclose to 0%, or slightly below by accident. 0% is kinda fuzzy, anyway. You can also use "safer" limits like 3.0 to 3.1 volts ending voltage, but then the resulting capacity reading is slightly pessimistic, especially for cells with higher DCR.QuoteThis sounds extremely strange. Double-check the numbers. Empty cell should show about 3.3V open-circuit voltage. 3.8V is near 50%. Extemely weak cells with huge internal resistance could do this, having enough resistive voltage drop to trigger your low-voltage cut-off limit while they in reality are at 50%, but you would notice this from very crappy capacity (like <1000 mAh for any 18650) and abnormal heating for such a short cycle.What is your low-voltage cutoff limit? It also depends on current. 2.8V at 0.5C is generally good for laptop cells, but increased DCR leaves some juice behind unless you lower the cut-off voltage (possibly not recommended) or add a CV phase to the discharge. (CV discharging at 2.8V may not be recommended either, so.....)

author: Artlav
author_rank: Frequent Contributor
timestamp: November 21, 2015, 10:51:37 am
content: Quote from: Siwastaja on November 21, 2015, 08:35:53 amJust checked, and yes, only bad cells are like that.The Sanyos got up to 3.4V overnight, the so-so are at 3.6-3.7, while the bad ones are at as much as 3.9V, even though all were discharged to 3.0V.DC ESR is around 100mOhm for all of them, so i'm confused.Hm, with low capacities the discharge current (average of 0.9A, 3.9Ohm load) is getting to 1C and above, while the good ones are getting 0.5C. Maybe that's the problem?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 21, 2015, 11:13:31 am
content: Quote from: Artlav on November 21, 2015, 10:51:37 amDCR is not a constant - it's a function of temperature and SoC. DCR can really shoot up when the cell is nearly empty. Let's say it is 100 mOhm when measured from full cell, then it might even be as much as 400 mOhm from an empty cell.If you are discharging at 0.9A, then there is a resistive voltage drop of 400mOhm*0.9A = 0.36V. So when you are measuring terminal voltage of 3.0V, the open-circuit voltage would be 3.36V, which is more than 0%.There are other effects in play too (some of which I don't fully understand, either), but this should give you an idea why the increased DCR makes it harder to fully drain the cell, and why cells with increased DCR work better in slow-discharge applications. You are measuring the voltageafterthe internal "resistor", and the bigger the "resistor", the higher the "actual" cell voltage still is. So you are just not fully discharging them."Power" type RC toy cells (which have rather low DCR) may specify 3.0V or even more as the stopping voltage, but these 18650 energy cells are specified lower, typically between 2.5V and 2.8V. To fully discharge them, you need to go to lower than 3.0V, or accept that you are not getting full capacity.Your assumption of "higher C rating" is also correct; as the cell now has less active material that does the work, it also has less surface area to do the work. The remaining bits are working harder, which you can measure as an increased DC resistance.So there are two mechanisms (approximately):- Loss of active material - reduces capacity and increases DC resistance linearly. Caused by cycling.- Electrolyte reaction - reaction products produce "gunk" that slows down ions from getting where they need to go - increases DC resistance [sometimes gunk can even completely block parts of the structure, which can be seen as capacity loss]. Caused by calendar aging.Both may be equally important in a typical used laptop battery. In a badly aged battery, "real" full capacity can only be measured really slowly. It is often better to try to simulate your intended usage.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 21, 2015, 11:23:15 am
content: Attached is my (incomplete) study of DCR vs. SoC on several (new) commercial cells. Your mileage may vary, because there is no one valid standard of measuring DCR, and also because temperature affects the results --- DCR is obtained here as a difference of three curves obtained on three different cycles with different temperature characteristics because of heating effect of DCR itself! There is something unexpected going on because Samsung ICR18650-26H is getting better results than expected, compared to newer NCA cells which perform better in other tests. Oh well. Hope this gives you something to think about!

author: amyk
author_rank: Super Contributor
timestamp: November 21, 2015, 05:41:39 pm
content: Quote from: Siwastaja on November 21, 2015, 08:35:53 amVery interesting. Figure 6 in that paper gives very different numbers fromthe BatteryUniversity, namely:At 25 degrees and 40%, BU gives 96% after 1 year whereas the paper has at 45% equivalent of 96.4% (98.5% after 5 months, assuming the trend is exponential). They agree on this.At 40 degrees and 40%, BU gives85%after 1 year whereas the paper will be equivalent to93.0%At 25 degrees and 100%, BU gives80%after3 months, or41%after a year, whereas the paper will be equivalent to94.6%after a year.At 40 degrees and 100%, BU gives65%after3 months, or17.9%after a year, whereas the paper will be equivalent to88.4%.Who has stored lion cells at 100% and 25 degrees for a year? Do they end up having less than half their original capacity afterwards?A few more interesting things to read, which seem to be closer to the paper above than BU's numbers:http://ramasamy.uga.edu/pdf/2005%20-%20JPS%20-%20Calendar%20Life.pdfhttp://www.che.sc.edu/faculty/popov/drbnp/WebSite/MSA-calendar.pdfhttps://www2.unece.org/wiki/download/attachments/8126481/EVE-06-05e.pdf?api=v2(NCA cells, after 3.45yearsat 100% or 50% and 25 degrees, with almost no degradation, but slight degradation with 40C and very rapid degradation at 60C.)https://books.google.com/books?id=LxwRBwAAQBAJ&pg=PA407&lpg=PA400&ots=iP97D3lkzqhttps://hal.archives-ouvertes.fr/hal-00876555/document(also no degradation at 30C, slight at 45C, and significant degradation at 60C.)It looks like temperature is the biggest factor. Don't store batteries in hot places, but if you must, then keep them around 40% charged. Otherwise 100% charged is fine.Investigation on internal resistance vs aging:http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/32/026/32026613.pdf...and then there's this design note from Linear, showing only 80% remaining capacity when stored at 100% for a year.http://www.linear.com/docs/28778

author: Siwastaja
author_rank: Super Contributor
timestamp: November 21, 2015, 06:17:31 pm
content: Quote from: amyk on November 21, 2015, 05:41:39 pmThe problem is that "Battery University" has been totally discredited numerous times, and is mostly matter of joke in the industry. I have stopped considering anything posted there even as a grain of salt. Not worth of your time. I have the feeling they have tried to up the quality a bit and have removed some of the most horrid urban myths (some of which may have first originated from them, such as "li-ion has 3% per month self discharge" myth). Still, try to find other sources if at all possible. Even though they may have valid data sometimes, they are not just reliable enough and it's impossible to know whether to trust them "this time" or not.In any case, thanks to the inspiration from this thread, I continued my research. I originally planned that I'd let the cells sit for a year, but they are a bit overdue now! Just had other things to do. But I started to do "measurements after" right now. I have all kind of samples from full to empty, at different temperatures. I charged them in July-August 2014.

author: Artlav
author_rank: Frequent Contributor
timestamp: November 22, 2015, 09:27:50 am
content: Quote from: Siwastaja on November 21, 2015, 11:13:31 amAnd it seem to.The way i measure DCR is to turn off the load, wait 100us, then measure the voltage and compare it with the one before the load.It works, verified both on a scope that the rig does it right, and with a resistor in series with the cell.However, measured llike that the DCR does not ever change.I.e. here it is for a bunch of so-so cells that recovered to 3.7-3.8V after discharge to 3.0V:The resolution isn't that great, since the measurement limit is about 5mOhm, but it should be good enough to tell 50mOhm apart from 100mOhm, or the whole 400mOhm.But what i get is a flat line, essentially, and looking at the raw data the voltagedropgoes from 90mV full to 65mV empty.That's why i'm confused about the cell not discharging all the way.Maybe there is more than just ohmic resistance?I.e. equivalent parallel capacitance of some sort?If so, then i have no idea how long to wait - with the load disconnected, the voltage just keep going up without stopping for minutes.Quote from: amyk on November 21, 2015, 05:41:39 pmI'm pretty sure i did. A quadcopter i made crashed a few years ago with the battery still almost full, and was sitting in a box for several years before i bothered to rebuild it.It still flies about as long as before, so the battery didn't suffer any noticeable capacity loss.Quote from: Siwastaja on November 21, 2015, 06:17:31 pmWhich raises the question - how can i know about it? As in, how are people supposed to find out that it is discredited?The info on it sounds kinda true, and the only mentions of it being discredited is the wiki talk page and this thread.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 22, 2015, 03:58:12 pm
content: Quote from: Artlav on November 22, 2015, 09:27:50 amThe problem with this is that you are not measuring DC resistance, but AC impedance at fairly high frequency - 10 kHz, which is even higher than the industry standard 1 kHz for AC impedance measurements. The problem with AC impedance again is that the number has little practical usage in battery system design. It can be useful in cell diagnostics when manufacturing cells, however. Even the pure capacitance of the cell due to its "electrolytic capacitor" type physical construction is enough to give short, 100 us current peaks with lower ESR.It is indeed true that the AC impedance does not shoot up like DC resistance does at low SoC - AC impedance is most of the time smaller than the DC resistance.DC resistance, however, is the important one as it explains (or models) the voltage drop that causes your battery management to stop discharge at low-voltage cutoff limit. It also defines resistive heating, and you can indeed feel many 18650 heat up considerably faster at the end of discharge -- due to the combination of increased DC (i.e. ionic) resistance, and enthropy heating. At high discharge currents, I have measured many 18650's to first slowly rise in temperature to about 45-50 degC, and then jump over 60 degC during the last 10% of the SoC. But it's hard to quantify the amount of resistive heating at play here, because it's partially chemical enthropy - and the cell actually cools down when you start charging the empty, heated cell right away - it cools faster than with no current!QuoteYou are exactly on the right track here. There is only "ohmic" resistance, because ohm by definition is the unit of resistance, but there are several sources for the resistance - in the DC measurement, ionic resistance is by far the most important, and it is caused by the ions slowly "swimming" and struggling to find their homes in the anode and cathode lattices. This is orders of magnitude slower than electrons shooting in the copper and aluminium current collectors.Methods to measure DCR vary, but I have been getting comparable - i.e., "consistent enough" results ranging from 3-4 second step to 20-30 second step.Mostof the AC effects go away by simply waiting for one second. I also do it by using two (sometimes three) different current levels, instead of stopping charge/discharge completely. My current methodology is: every 120 seconds of charging or discharging at current I:10 second step at I1 = 0.85*I, V1= voltage measured at the last second;10 second step at I2 = 1.3*I, V2=voltage measured at the last second;10 second step at I3 = 0.85*I, V3=voltage measured at the last second.R = ((V1+V3)/2 - V2) / ((I1+I3)/2 - I2)That graph I posted is different, though; it's R=dU/dI on whole discharge curves at once! It's the best way to avoid AC effects completely, but you get heating effects, because at higher current, the cell runs hotter, and hotter cell has lower DCR. Heating effects don't affect the pulse method in the same way, because of the thermal mass.QuoteI can't say, really! I can't "disprove" them, I don't have time, I'm not interested, etc. I have seen some valid discreditations but they have been lost somewhere in the vast land of the Internet. Also, their site seems to change quite a bit. It is well possible that most of their info is valid now. It's just my $0.02 that I don't trust them, which is the case with many people on this field, too. Scientific community also isn't interested in random websites calling themselves "university", they refer to peer-reviewed journals (which are sometimes jokes, too, but that's a different subject altogether)...But I remember for sure that the "3% per month self discharge" myth originated from there.Findingreallycredible sources is difficult! You basically need to go to the very sources which describe the exact methods in a replicable manner, and make your own conclusions.

author: amyk
author_rank: Super Contributor
timestamp: November 22, 2015, 04:21:50 pm
content: Quote from: Siwastaja on November 22, 2015, 03:58:12 pm...or test them yourself, as is happening in this thread.Another interesting study I found, involves cycling and storing cells for over 10 years:http://www.dtic.mil/get-tr-doc/pdf?AD=ADA515369They do degrade, but not as much as you'd think - there's a cell from 1994 in there that has over 100K cycles and appears to still have more than half its original capacity left.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 22, 2015, 04:30:43 pm
content: I am finishing the characterization of the first sample of my self-discharge/calendar aging test:Samsung ICR18650-26HCapacity at start = 2554 mAhDC resistance at start = 70 mOhmCharged fully to 4.20V, C/40 stop for storageStored for 1.36 years at between 21 to 25 degC, on average close to 22.5 deg CCapacity has INCREASED by 0.5% per year (so no practical effect; I expected at least some loss of capacity!)DC resistance has increased by 7.8% per year (as expected)Reversible leakage (i.e., self discharge) was 4.2% per year (slightly more than I expected, however this was a full cell).Info like this coming from other samples with NCA chemistry instead of LCO, different storage SoC and different temperature, but it takes time because I cycle them three times at 0.5C to average capacity and DCR, so over 12 hours per cell to test them.

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 22, 2015, 04:36:25 pm
content: Quote from: Artlav on November 22, 2015, 09:27:50 amThe correct way to measure the DCR of a cell/battery is to compare the difference in voltage at two different non-zero currents close in time to each other (ideally by bouncing back and forth between the two test currents, then taking the average of the voltages at each). For example, if a 4000mAh cell reads 3.250V at 1A and 3.200V at 2A then the DCR = (3.250 - 3.200) / (2A - 1A) = 0.05R

author: KL27x
author_rank: Super Contributor
timestamp: November 23, 2015, 06:01:20 am
content: ^That sounds like a way better method that could remove other potentially unstable variables!Regarding Battery University, call me a paranoid conspiracy theorist. But I think a lot of the information on that site was encouraged by some interested parties.Ten years ago, it seems like anytime a thread like this popped up on the internet, you'd get 20 morons saying the same thing. Repeating something out of a datasheet (that did nothing to support their views in most cases. Views which largely consisted of how you needed to do XYZ and alpha to use li ion batteries, and that if you don't use a dedicated charging IC and balance cells and have an EE degree, you'll burn your house down). And I'm sure that 99% of them believed everything they said, simply because they heard/read someone else say the same thing in a convincing manner. You wouldn't believe the number of safety experts who had no first hand experience or knowledge.And then... a few days later a picture of a burned down building or a smoking airplane would show on the front page of the news machine with a story about the dangers of li ion batteries. Or someone would post such pictures in the thread, with a vague backstory. I don't recall ever reading about casualties, curiously. As if in the entire existence of li ion batteries exploding and burning, they have never killed anyone? Maybe that would be too scary? For some reason we were all very well educated on thedangersof li ion batteries, while leaving all the important bits quite vague and ambiguous. The general populace was encouraged to use these batteries, but only in the devices that originally contained them. This was safe. When done, we were supposed to toss the environmentally dangerous "heavy metal" containing and "dead" batteries in a recycling bin.Meanwhile, there is probably 0.3 laptops and 0.7 smart phones with these incendiary devices built-in for every man, woman, and child on every passenger flight in America, not in the cargo hold, but on peoples' persons. At a time when you couldn't carry a pair of nailclippers in your carryon.Maybe it's because you can't sell as much milk if everyone has their own cow? And think about this: What business is Battery University involved in? How do they make their money? Aren't they in the business of selling various flavors of milk? Last I checked, they sold batteries and/or advertised links to such stores. So you might say the people behind Battery University were an interested party. You might even say they had a vested interest in the dissemination of this kind of knowledge.In the 1970/80's a sci fi (pop sci?) writer named Robert Heinlein wrote of an invention called the Capstone. It was a portable energy storage device that could store and release massive amounts of energy with near 100% efficiency. It didn't generate any energy. It didn't blow up cities. It was simply a really good battery. The inventor never patented it, because patents expire. Instead he kept it a trade secret. And the Capstone empire quietly grew through shell corporations until it has its fingers in everything. This wasn't a particular book, as I recall, but rather a consistent theme throughout one of his major fictional worlds.Heinlein didn't predict the internet. With the internet, you don't need patent or trade secrets. If you have enough resources, you could actually strategically misdirect, subtlely encourage, and anonomously support the vast majority of morons on it. Where did all these li ion experts come from? An unsubstantiated website. A mysterious "I worked for . I did research/design and have degrees in this and that. And it's more complicated than that. Because of [this vagery, that vagery, and the other vagery in super complicated technical speak, which if you dissect and understand, isn't really saying anything]... and you'll burn your house down" - poster who appeared out of nowhere and then disappeared. Then sit back and watch all these armchair experts appear and repeat what they read, because they know how to use Google. And they get an emotional reward from remembering what they learned... or rather what they were taught.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 23, 2015, 08:00:31 am
content: Yes! I remember the times, just some 5 years ago, when internet forums were flooded full of this shit:1) "lithium-ion cells are extremely complex and difficult to charge and require special algorithms provided by special ICs" -- false; li-ion is the simplest battery type, practically the only type that can be charged by CC only/voltage stop, or CC-CV, and can also be partially charged. There only is voltage limit and current limit, nothing else. Any proper lab PSU does the exact algorithm. Li-ion is the closest real battery to the "ideal" battery. This claim seems to be a mixup with NiMH batteries, or just pure stupidity.2) "lithium-ion cells blow up instantly when you do anything wrong" -- false, as we can clearly see. OTOH, it's essential to have good safety precautions and know what you are doing. Li-ion cellsarehightech, and it really is dangerous when manufactured by some random no-name sweatshop; or even the good cells if they are severely misused.3) "There is this LIPO thing which is completely different OMG!!1" -- false, there is no such a thing as "lipo" at all. This is a complete joke in the industry. This misinformation existed in Wikipedia, too. I started the great cleanup process of the Wikipedia "LIPO" page about a year ago, and the time seemed ripe for it; it didn't seem to cause much backlash.And every time something completely dumb was going on, magically, Battery University website was used as a referencea lot.That's why I still cringe every time I see anyone use Battery University as a reference; I won't even bother clicking the link. Not interested.But I can see that things aremuchbetter now, in many ways. People are using SMD components now and not recommending that you should place every DIP IC in those bloody DIP sockets anymore. And people are talking less shit about li-ion, because they are actually using and testing the cells, and hence getting real-world data.BTW, for a LCO cell charged to only 4.00V instead of 4.20V and stored at room temp, the self discharge went down from 4.2%/year to 1.4%/year and DCR rise from 7.8%/year to 2.1%/year. No noticeable loss of capacity in either sample. Yes yes, I'm writing some kind of paper after I have all the samples tested.

author: KL27x
author_rank: Super Contributor
timestamp: November 23, 2015, 05:38:43 pm
content: QuoteNow that's cool. It's good to see some actual data.This data is suggestive that SoC may, in fact, affect DCR, which is interesting. It's also interesting to put this into perspective.At 2.1% decrease in DCR per year, a battery would have a DCR half life of 33 years.And stored at full voltage, the half-life would be reduced to 8 1/2 years... But with a self-discharge rate of 4.2% in the first year, the voltage of the battery wouldn't stay full the whole time. In some number of years it would hit the same curve as the 4.00V cell. So in storage, we might think that batteries would last a pretty long time in either scenario. Did you charge them and store them floating?Because, OTOH, if a cell were to be kept topped off at 4.2V all the time, the decrease in the DCR could logically be presumed to have been way, way higher over a year. Considering the 4V scenario resulted in 2.1%, that 7.8% decrease appears to be the average of an expotential curve that started out who knows how high.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 23, 2015, 06:32:12 pm
content: Yes, my method was charge-and-forget; probably most of the self-discharge happened rather quickly, which helped the cell to store better. A floating test would be interesting, too. Anyhow, it's widely accepted that when floated, you don't want to float at 100%, but rather at 90% or 80%. This has been a standard feature in laptop battery configuration for years.

author: amyk
author_rank: Super Contributor
timestamp: November 24, 2015, 04:43:50 am
content: Quote from: Siwastaja on November 23, 2015, 08:00:31 amI think the Chinese are largely responsible for debunking these points, because of the huge amount of products they've created which use lion cells yet have ridiculously simple charging circuitry; nothing more than a 4.2V regulated supply with current-limiting resistor. Granted, some of their cells do come to an exciting end, and that's what makes the news, but that neglects thehugenumber of others which operate uneventfully. 18650s are quickly becoming the new AA as can be seen by the numbers of products using them (replaceably) now. I'm optimistic that in a few more years, they'll become even more popular once the mysticism around them disappears.

author: Artlav
author_rank: Frequent Contributor
timestamp: November 24, 2015, 10:16:51 am
content: Here is something curious.13.1.1 and 13.1.2 are Samsung ICR18650-22B, 11.3.1 and 11.3.2 are Samsung ICR18650-22F.Both have nearly identical specs, and both got stable capacity over 4 cycles.Both were found at mid-3V voltage.And yet, they are quite different - the F got half of it's rated capacity.The interesting thing is how the curves change - the slope is capacity dependent like that.It really is as if they have different DCR.Quote from: Siwastaja on November 21, 2015, 06:17:31 pmGlad to be an inspiration.Quote from: Siwastaja on November 22, 2015, 03:58:12 pmHow would that work if you know voltage and current along the graph, and they are not the same?I tried to find it by matching the graphs by capacity spent/accepted, then doing (Vc-Vd)/abs(Ic-Id), and i'm getting something that does not look right at all.It is proportional to the cell's crappyness, however.The edgy green line is the computed DCR, at 100mOhms per bright line:Quote from: Siwastaja on November 22, 2015, 03:58:12 pmI mean that in a sense that we are measuring more than just a resistor.Quote from: Siwastaja on November 22, 2015, 03:58:12 pmQuote from: MagicSmoker on November 22, 2015, 04:36:25 pmOk... I got to try that.Quote from: Siwastaja on November 22, 2015, 03:58:12 pmYeah, i think i understand how stuff like antivaxers spread now. It's just not obvious what is true and what isn't when you didn't do a lot of searching.Quote from: Siwastaja on November 23, 2015, 08:00:31 amHaving done an exercise similar to this thread's one to a bunch of NiMH AAs, i cringe on that one as well.With Nickel, you don't "charge" a battery, you feed the current into it that makes some chemical reactions go at a rate almost independent of the charge level.The only way to know you are done is to look for tiny variations in voltage or charge fast enough for the battery to heat up rapidly when the active material gets depleted.While Li-Ion is pretty much a capacitor - current in is dependent on the state of charge. Limit it and done.Just WTF.Quote from: Siwastaja on November 23, 2015, 08:00:31 amNow that is an easy one to believe in.I had at one point (2009-ish) crashed a quadcopter enough to puncture the LiPo. One tiny little hole, and yet it was smoking in 10 seconds and ablaze in 15.Teaches you to respect these...And now, here is this guy who tries to make this kind of cells go off again, and again, and again with no luck.Kinda makes you wonder if maybe the technology changed over the years?Or i was just "lucky".

author: Artlav
author_rank: Frequent Contributor
timestamp: November 24, 2015, 01:17:27 pm
content: Wait, no. I was stupid.I matched the capacity backwards - discharge from top to bottom with charge from bottom to empty.When done right, i get something much more interesting.Here is the above one, at 100mOhm per bright line:And a comparison of the two cells i talked about earlier, at 1 Ohm per bright line.The DCR goes up to several Ohms near the end (but i guess that's method artifact by then).So yeah, it kinda adds up with the incomplete depletion.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 24, 2015, 02:27:39 pm
content: Quote from: Artlav on November 24, 2015, 10:16:51 amYou plot the voltages on common X axis, which must be charge (mAh). Your current integration must be fairly accurate. You need to start at the same point, i.e., have fairly well standardized 100% charge before you do the discharges. Then, you will get two voltage curves which show different amount of voltage sag compared to (imaginary) Open Circuit Voltage, but they overlap in time just fine. This is because even remotely good cells have Coulombic efficiency over 99.9% (typically 99.97%), so if the same cell is discharged by x mAh twice, it's (exactly enough) at the same SoC, regardless of the current it was discharged at - as long as you stay within limits of no damage occuring to the cell.QuoteYes, it's kind of a variable resistor that someone turns in interesting ways depending on their mood. Of course, resistance is only a number that models the actual phenomenon, which is a voltage drop. Voltage drop is caused by ions going to their places too slowly. It's like a traffic jam; road seems full, and voltage indicates "fuller" (or emptier) battery, whereas in fact the cars are still on their way, not at home yet. Voltage "updates slowly".Quote from: Siwastaja on November 23, 2015, 08:00:31 amYes, about 99.97-99.99% of the charge fed into li-ion battery does exactly the right chemical reaction which is kind of transparent to the user, and the rest slowly degrades the battery, as long as the simple constraints are met - no huge overcurrents, no huge overvoltages, no charging at very low temperatures. Li-ion also stores quite well at any SoC, so no huge constraints here. Fully charged? No problem. Battery at 50%? No problem. Empty battery? No problem. Just don't overdischarge or overcharge.Also, nowadays even the cheapest laptop cells can provide quite nice discharge currents at good efficiency. Compare the 1C of the cheapest energy-type li-ion cell to the 0.2C of lead-acid which is sometimes considered "quick discharge" or otherwise the Peukert effect messes everything up; no Peukert at all on li-ion!QuotePuncture reactions have been getting much better (on quality cells only, of course; all RC LiPo stuff is shit anyway) lately, due to advanced separator design. PTC in 18650 cap can only protect against external shorts. But when the battery is internally shorted (because of the puncture), the electrodes heat up but before thermal runaway, so-called "shutdown separator" melts and shuts down any ion transfer, stopping current generation. If this all happens quickly and evenly enough inside the battery, it can be made so that no spot reaches the thermal runaway temperature (which for LCO or NCA is about 160 deg C). But these mechanisms are still somewhat unreliable. Much better than nothing, but never design a product so that the cell could be punctured, even a high quality one, even though it is unlikely that it goes all flamey. But it happened to Tesla when enough nearby cells were physically crushed at the same time.QuoteIndeed!But never trust random noname Hobbyking batteries. From the safety viewpoint, they range from "extremely dangerous" to "slightly horrible". (And anything else than Sony, Sanyo, Panasonic, Samsung, LG and a few others are "nonames".)

author: ralphd
author_rank: Frequent Contributor
timestamp: November 25, 2015, 12:57:36 am
content: I call BS on claims that lithium ion (Co, Mn) are the simplest to charge. LiFePO4 are much easier. You can even use 4 in series to replace 12V lead acid battieries in cars, motorcycles, etc.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 25, 2015, 06:58:54 am
content: Quote from: ralphd on November 25, 2015, 12:57:36 amI can see so many shining, elaborate arguments right here, in reply to my long expert posts!QuoteBTW. Your sentences don't even make any sense. LiFePO4 is lithium ion. My post included LiFePO4. LiFePO4 is not easier than LiFePO4.... even tough LFP is a rarely used niche product mostly manufactured by substandard companies, which was one of the reasons I decided to ditch it. The energy density is horribly poor, especially the Chinese cells are now lacking so much behind -- 95 Wh/kg versus 270 Wh/kg of the state-of-the-art NCA cells. LFP is also too expensive because of the Chinese price fixing strategy. 30-40% higher price for some 60-70% worse product just doesn't cut it.And, LiFePO4 is more difficult than LCO/NCA/NCM in one regard -- it has very flat curve with no indication of SoC. It is harder to use in some applications (like my bidirectional battery cycling workstation which needs a "storage" battery kept at 50%). OTOH, flatter curve can be nice in some cases. It's even closer to the "ideal" battery model, or would be if there was no knee sections in the curve.QuoteThe only reason you cannot use 4 LCO/NCA/NMC in series to replace a 12V battery isbecause the voltage is higherand your car bulbs will burn out with too much voltage. Nothing to do with difficulty of charging.You can replace a 24V car battery with a 7s NCA. I'd do it without cell-by-cell monitoring, but it's not recommended. It's exactly the same for LFP! The "LFP never requires a BMS" internet forum bullshit is just based on fact that no one professionally uses LFP, and no quality manufacturer makes it, so nothing matters the slightest. You can claim anything and do anything. And of course it works. NCAissometimes professionally used without BMS, for example by BOSCH.Once again: li-ion (inluding LFP, d'oh) charging iseasiest to understand. Simple chemical reactions, practically no side reactions messing things up. No Peukert effect, and charge in = charge out. Near to ideal battery model. Noextremely complexside reaction stuff from lead acid or NiCd/NiMh. No specialized charger IC needed at all for good results, becausethere is no magic. Li-ion charger ICs are just CCCV power supplies.

author: amyk
author_rank: Super Contributor
timestamp: November 25, 2015, 08:26:29 am
content: Quote from: Siwastaja on November 25, 2015, 06:58:54 amHe is talking about 4S LFP, which is around 12V nominal.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 25, 2015, 10:28:22 am
content: Quote from: amyk on November 25, 2015, 08:26:29 amI know, you probably missed his point, and hence my reply too.

author: KL27x
author_rank: Super Contributor
timestamp: November 25, 2015, 08:30:43 pm
content: QuoteThe truth is that li ions cells do not even need CC-CV. This is just one fancy way to describe it. To distill this algorithm even further, all you need is current and voltage limiting. This is why a 4.2V current source with a series resistor works perfectly fine and is not the cause of cells igniting. As long as the value of the resistor limits the maximum charge current to within acceptable limit, this method is perfectly safe and perfectly capable. Since we know the minimum float voltage of a li ion cell in the usable range, and in fact your application and/or battery circuit must define that lower limit if you want your batteries to last, ohms law is all it takes to choose the right resistor. It will take longer to charge the cell, particularly the last 20%, but there is no safety concern. I suggested this as a valid and capable method of charging cells 7-8 years ago. I never purchased or reverse engineered any li ion charging circuit, although I have ripped several out and thrown them in the trash. I read the datasheet of a li ion charging IC and understood what the graphs meant on a basic level, rather than reading more into it than what it was. Good thing there are engineers in China that don't get their education from random commercial websites with ties to commercial industry.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 27, 2015, 07:26:58 am
content: Quote from: KL27x on November 25, 2015, 08:30:43 pmExactly!Li-ion is easy, because there are only limits. It's easier to follow limits that are completely stateless than to think about steps. There are no steps. You just follow a few very easy rules all the time.Another good thing is that you don'tneedto do anything to keep the cell in shape. Don't want to fully charge? Then don't charge fully. Ditch the "CV" phase, make a CC circuit and a voltage latch that disables the charger at 4.2V. Charge as much or as little as you want to, as long as you follow the simple limiting values.Constant voltage supply, such as an LM317 regulator set at 4.2V, with a series resistor is perfectly fine, if you confirm manually that you don't start charging overdischarged cells (there is a minimum voltage limit for charging!).So we have:Voltage range for safe charging & discharging (typically, 2.5V to 4.2V)Maximum charging current (typically at least 0.5C) and maximum discharging current (typically at least 1C)Temperature range for safe charging (typically, 0 degC to 45 degC)Temperature range for safe discharging (typically, -20 degC to 60 degC)All of these make perfect sense and are easy to obey with simple circuitry.And every time someone asks: "Can I do it my way?" The only question is: "Are you following the limits?" If yes, then it simply doesn't matter. Li-ion is a simple charge sponge for the user. For the manufacturer, it's full of expensive, complex high-tech manufacturing processes.Optimizing for maximum life requires some extra knowledge, but it is not hugely important. Just following the limits gives better lifetime than any other widely used battery technology there has been. The same goes for bending the limits; some are not hugely important, and if you know what you are doing, you can do more.

author: djQUAN
author_rank: Frequent Contributor
timestamp: November 27, 2015, 08:49:24 am
content: I fully agree with the "Li-ION charging is easiest" statement.You put current in and limit voltage that's it. Although the voltage limit should be pretty accurate but that is not a problem these days with the available devices.Lead acid needs a four step charge process. which has bulk, absorb, float and optionally, terminate and needs to be done with timers and different voltages.And it doesn't really like staying in a partially discharged state which occurs in quite a bit of applications.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 27, 2015, 09:37:11 am
content: Quote from: djQUAN on November 27, 2015, 08:49:24 amRelatively accurate, yes, but not overly so. Any half-decent voltage reference will do, as long as it's not complete shit. And the voltage isn't temperature-dependent like lead acid termination voltage is!I tested Samsung ICR18650-26H cycled with different end voltages. The highest was 4.30. I terminated the test after 100 cycles - there was approx. 0% capacity fade on all samples, including the one charged to 4.30V on every cycle.Datasheets often specify maximum at 4.25V.And, if you are wise, you implement normal charging to only 4.15V, or even 4.10V. That way, you have quite a lot of margin before 4.30V, after which the risk of severe damage really starts going up.I did some 10 cycles (maybe 8 or 9) charged to 4.45V and didn't see any capacity fade, either, but I'm quite sure I would have seen problems if I had carried on cycling. It can happen rather suddenly; severe capacity drop could happen during a few cycles after 20-50 cycles. And there is significant safety issue with charging to such high voltages. Charging to 4.45V brought about 20% more usable energy storage capacity! Don't do this at home, or at least use proper precautions such as closed steel casing without flammable materials nearby, even though the risk of quality cells blowing up from such "slight" abuse is smallish because of their advanced separator design (among other things). For example, I haven't been able to blow up a Sony 18650 cell by applying 30V from a 10A power supply for an hour or so, not even venting, but that doesn't mean itcouldn'tgo up in flames. The protections are good, but not perfect and not meant to be actually tried in actual products.But if you are even remotely interested about safety (as everyone probably should), the most solid tip is: only use genuine cells from the world-class major manufacturers, which are Panasonic, Sanyo, Sony, Samsung, LG and possibly a few Chinese ones like Lishen, but none of the ones praised by hobbyists are any good. You are at a risk, albeit small, when using such products, even when obeying the limits, but especially in the case of abuse.

author: Artlav
author_rank: Frequent Contributor
timestamp: November 27, 2015, 11:16:44 am
content: Meanwhile, i tried several ways of measuring DCR.0.1 second wait every 10 seconds, and 1 second wait every minute.Of the four methods (curves, 0.0001s, 0.1s, 1s) none give the same results.-100us one does not differentiate anything, and basically always reports around 90mOhms.-0.1s method gives 170-200mOhm curve for the good cells.-1s method gives a 200mOhm to 300mOhm curve for the same good cells, and 600-700mOhm curve for bad cells. Also, it gives significant artifacts on the discharge curve, the voltage takes about 5 seconds to settle back to the pre-measure level.-The curves method give a 50mOhm to a peak of several Ohms near the end more or less straight curve for a good cell, and 100mOhm up to a similar peak for a bad cell.All in all, the 0.1s method seem to be the best, as it differentiate good from bad without interfering with the rest of the process.The curves method might be a close second, since it also seem differentiating, but that need some more work to clean it up.Quote from: Siwastaja on November 25, 2015, 06:58:54 amHm, i heard differently. Specifically, that there were many industrial applications, car manufacturers using them to replace lead-acid starter batteries in the newer cars, various hobby-grade electric vehicles, go carts and so on.The advantages are supposed to be good cycle life and abuse resistance (no thermal runaway), even if the energy density is less.I got a bank of 40152 LiFePO4 cells working as a UPS for the 24V lights&trinkets wiring i have at home (you can see the the rig plugged into it on the first post's picture).However, all i can say is that they work.Haven't done any stress tests on them.I did a table of $/KWh for various cells on on sale, and they came out the cheapest at about $600 per KWh, above only the crap chineese and noname cells.Now, used 18650 can be as low as $100 per KWh, but they come in a variety of condition and quality, and by the time you filter out the good ones you're back into $400-$500 range.

author: djQUAN
author_rank: Frequent Contributor
timestamp: November 27, 2015, 12:20:30 pm
content: I have a bunch of recycled 18650's from old laptops and tested each one (charge then discharge at 1A to see remaining capacity) and most are still pretty good. I use a hobby charger for testing and charging packs.Edit: I have experienced overheating of some cells when charging but it occurred even when the cells have been discharged only down to 3.2V. Charging current is reasonable for a single string of 18650's (1A) It got pretty hot and I was able to pull it out on time so I thought those cells were toast. Left it to cool and waited for fireworks but nothing happened so I charged them again a day later but at a lower rate (300mA) and it terminated and is now back in service as if nothing had happened. Anyone know what was at play here? Cells were laptop pulls (genuine Sanyo cells) but still have some life in them.I also have a couple packs of LiFePO4, the big ones 40152 also. I used a 4S one as a portable battery pack and another 4S3P pack as ahuge power bank.From my limited experience with those, LiFePO4 seems pretty good to me compared to a lead acid of similar size for the application.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 30, 2015, 06:42:11 am
content: Quote from: Artlav on November 27, 2015, 11:16:44 amThis is not surprising, since you are not measuring DC resistance, but AC impedance. You have already found the answer by yourself, you just need to realize what you are seeing:QuoteThis is what DC resistance is all about! It's the voltage sag between two different current levels. To actually see the DC component, you need to use a long enough pulse that all AC effects are removed, i.e., the voltage needs to settle back fully.I haven't even tried going much below 5 seconds, because I don't want to see AC effects. Cell chemistry is "sluggish". You'll need a rather long pulse. IIRC, I use 3 or 4 seconds in one system, a bulk cycling setup, and 10 seconds in another, a more precision setup.QuoteYes, you have to choose between perfectly smooth discharge curve (no data lacking), or a good DCR measurement curve. You can't get both at once. Or you can do two different discharges and get both perfect discharge curves, and a full DCR curve calculated from the two curves.I like the pulse method because it gives almost perfect discharge curve and a slightly quantized but good enough DCR curve, on one go.However, I don't see much use for "perfect" discharge curve. You can measure the DCR once every two minutes, for example, and use a 10-second pulse. Then, you'll have about 20 seconds of missing data, which you can fill in with linear interpolation. The error in discharge curve during the interpolated sections will be rather minimal; not much happens during 20 seconds when discharging at 0.5C or maybe 1C. Just don't try to get the DCR from the very last maybe 0 to 5% SoC.QuoteIt's probably good enough to differentiate between good and bad, it gives idea of the real DC resistance, and you'll get clean curves, so go for it.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 30, 2015, 07:32:55 am
content: Quote from: Artlav on November 27, 2015, 11:16:44 amPartially true, yes.The problem with forum hearsay is the it's never questioned or checked, the same "facts" circulate year to year.There wassignificantacademic (and probably industrial) interest in the LFP chemistry in early 2000's, more than 10 years ago. It was a promising technology for a few reasons:(1) It is cobalt-free - cobalt is somewhat significant cost factor in the traditional LCO chemistry, also slightly toxic.(2) Significantly higher onset temperature for thermal runaway (compared to LCO), so, safer(3) Lower voltage helps slow down electrolyte reactions (overgrowing SEI layer) - longer calendar life(4) Lower voltage helps slow down lithium plating - longer cycle life, higher charging C rate(5) The cathode lattice structure allows it to be fully charged - LCO cathode can only be charged to about 50%(6) The energy density tradeoff is only about 20-30% compared to LCO - a good tradeoff!(7) Finally, the price could be much lower than LCO! Li-ion is hugely expensive - this could be the revolution!But the problem, as always, is that the ideas and practice do not always meet. In fact, everything went the opposite way:(1) They got the manganese and nickel work better in the cathode (the problem of Mn in cathode was its dissolution in electrolyte, not sure about what was holding nickel back). The most state-of-the art NCA chemistry has gotten rid of 80% of the cobalt, and NMC has lost 67% of it. These new chemistries are both better than LCO in all respects, not compromises.(2) This holds true; thermal runaway is a lot less likely on LFP cathode. It is not impossible, however. Anyone who claims that, is lying. However, thermal runaway onset temperature on cathode is only a small part of complete li-ion safety. LFP still has thermal runaway, it has exactly the same burning electrolyte composition which can shoot out of the cell and ignite. The DIY EV community has had several LFP related fires that resulted in total loss of the property. I'm not saying that LFP is extremely dangerous, but I'm just comparing. It may not be safest at all. We don't have enough real safety data to say one way or the other, but my gut feeling says that in practice, the real LFP may be considerably worse in safety.While LCO, NCA and NMC are at the risk of thermal runaway at lower onset temperatures (at about 160 to 200 deg C, instead of about 350 deg C for LFP, IIRC, feel free to check the numbers), the safety can be brought by advanced design ideas, mostly in the separator film. Such R&D is only done by the Big Players. There was this paper (again, can't remember the title or the authors, sorry) where they tested short-circuiting and overcharging (12V supply, IIRC) commercial 18650 LCO and 25650 LFP cells, and all LFP samples reacted much more badly because of the lack of protections and safety design.The problem with LFP safety is that the manufacturers are small players with lack of advanced safety measures, and they are trusting too much in "safe" chemisty. And while the cathode chemistry isa lotsafer, the resulting complete product may be less safe.(3&4) Electrolyte and electrode design has come a long way. Nowadays, 4.20V is just fine. Modern cells still do increase their DCR in storage, but the fully charged calendar life has at least doubled from a few years to nearly 10 years. The modern cells also exhibit better structure on the electrodes, causing better ion transfer to the lattice, causing less lithium plating even at higher voltages.They could even go as far as go and release first 4.30V then 4.35V LCO cells to increase the energy density. Now the best products are back to 4.20V, but they do have leeway in the reactions, and very good life.So, the problems that LFP promised to solve, were solved in another ways, without the huge tradeoffs of LFP.(5) LFP cathode can be fully charged, but so what? It's gravimetric capacity is hugely poor, so even when it can be fully utilized, it's still horribly poor compared to LCO, NCA or NCM.(6) Before 2005-2007, when the LFP was still a "thing", a typical state-of-the-art LCO cell was about 150-160 Wh/kg. LFP was expected to be at about 120-130 Wh/kg, only a 20% tradeoff in energy density. But LFP was rather "fully developed" from the beginning, and large companies with proper R&D didn't see it could work out. LCO went to about 220 Wh/kg, and then became NCA which is now at about 270 Wh/kg on the state of the art products, whereas LFP is still at 120-130 Wh/kg. The Tesla's battery pack, which gives the car a 500km driving range, weighs something like 500 kg which is already a lot. If done in LFP, it would weigh more than 1000 kg, which would be totally impossible.(7) Price. So finally, most of the LFP, which is a tiny market share on li-ion, probably less than a percent, are produced by smallish Chinese producers with no or substandard quality control. They clearly have a price fixing agreement going on. The prices have gone down much slower than on any other li-ion chemistry. 6-7 years ago it was different, but in 2015, NCA cells from Samsung or LG or maybe even Panasonic are so much cheaper than the Chinese LFP, per energy - about $200 / kWh vs. $300/kWh.DIY EV conversions done in LFP are very limited in range because of the poor energy density, both gravimetric and volumetric, and high price of the LFP. They are mostly of sub 100-mile design.LFP still has one advantage, and it's the voltage range which is compatible with 12V lead acid system, so a 4s LFP pack can be used as a direct replacement for a 6s lead acid pack. People love to think that there is more to this, and attribute this to the "safety" of LFP or "no BMS" stuff, but really, it's basically only about the voltage (both average voltage and the curve shape). LFP has lower voltage (which partly attributes to the low energy density), and while it's generally a bad thing, it happens to help with this particular scenario, by pure luck.QuoteYou can get proper Samsung NCA cells, not the most modern stuff, but still almost 2x better energy density than any LFP, for $200/kWh once you find the right distributors and can buy a large enough batch. I have purchased from a local Finnish distributor.QuoteSounds horrible. I would buy used 18650 for $100/kWh only if they are properly characterized and still show at least 80% of original capacity and <150% of original DCR.I have considered making a good multichannel test rig for quickly characterizing a lot of 18650's, but then I would pay something like $20-30/kWh from "raw" cells that were only selected with a multimeter (voltage at least 2.0V), because I would expect that at least half of them are rejected, and testing also takes time and energy, even when fully automatized.I like to try to answer any questions regarding this or anything else li-ion related.