LiPo charging -end of cycle current rates - Page 1
Forum section: electronics / projects

author: Skimask
author_rank: Super Contributor
timestamp: April 03, 2016, 02:50:14 am
content: Q about proper charging of 18650 type cells...For the purposes of discussion, we'll just assume that the cell being used is a standard 18650 type rated at 2000 mAH, rated at 1C charge/discharge, max of 4.2v, low cutoff at 2.75v...Normal LiPo charging starts of with CC, in this case 1C, 2000mA, until the cell reaches 4.2v, at which point it switches over to CV, until the current drops to 1/10 C, in this case 200mA.So, maybe one guy is going to push that and charge with 5C. Charging would stop when the CC level dropped to 1/10C of the charger, which would be 1000mA rather than 200mA.Likely not going to do the cell any favors by pushing it harder and probably going to shorten the lifespan of the cell significantly. 'nuff said.Now, maybe another guy is charging those same style LiPo's with a cheap USB charger which is only going to get you 500mA at most. Assume that the charger is 'properly' designed and charging stops when CC level drops to 1/10C of the charger, which in this case is 50mA.Does this present an issue in future? eg. charging longer at a lower current, making the LiPo sit at the same absolute maximum voltage for a longer period?

author: sleemanj
author_rank: Super Contributor
timestamp: April 03, 2016, 04:16:30 am
content: Quote from: evb149 on April 03, 2016, 03:41:04 amThe TP4056 (very common charge controller) terminates charge once the rate has dropped to 1/10th of the normal charge rate.

author: Skimask
author_rank: Super Contributor
timestamp: April 03, 2016, 04:47:42 am
content: Yes, I do have a bunch of those TP4056 modules...the newer kind with the microUSB as well as the DW01A protection IC. Can ya beat the ebay price? I mean compared to using a DS2764 & MAX1811 combo?Guess I should've been a bit more specific earlier. Probably should've stated 2 "1/10" C ratings. The 1/10C rating of the LiPo being charged, and the 1/10C of the max current capacity of the charger.But, post #2 clarified what I was thinking in the first place. Sitting there at low C levels trying to get down to 1/10C of the programmed charge rate of a particular charger when charging a cell of significantly higher capacity (eg. charging a 2000mAh cell with a 500mAh charger programmed to stop at 1/10C in the CC phase as the TP4056 does), is more than likely will be bad for LiPo health.So, in the short term, stick with the TP4056 as they are so damn cheap.In the long term, switch over to something that could be programmed to terminate the charge at a lower voltage, say 4.1v or so, under slower charging conditions, or maybe even a timed shut off would be good enough.Either/Or, question clarified...

author: linux-works
author_rank: Super Contributor
timestamp: April 03, 2016, 05:03:12 am
content: interesting about the tp4056 (and also the adafruit boards that use alternate chips); if you start the charge with the programming set at one point, then change it, you can run the charge for longer.say you start off at 500ma and you run until it gets near 1/10 or 50ma. if you change the resistor to something closer to 10k, you are now at 100ma charge level and the stop point is 10ma. you get a bit more duration which, I think, is a bit more charge.i found this because I'm building a diy charger and studying that TP chip along with some others.fun lcd display stuff:using an arduino to talk to a digipot that talks to the charger board. works finealso using a 4ch a/d board (16bit) to measure current and voltage and log/relay it remotely.

author: Skimask
author_rank: Super Contributor
timestamp: April 03, 2016, 05:10:40 am
content: Quote from: evb149 on April 03, 2016, 05:02:54 amExactly what I was thinking. As long as a guy knows what he's dealing with, it's all good, or at least good enough.QuoteNo programmable V(charge) cutoff with the TP4056, but it does have a programmable current resistor, as noted above, which can be switched in/out as required with a simple MOSFET. Might come in handy if the LiPo C rating was lower than the max current capability of the charger. Not in my case though.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 03, 2016, 05:46:06 am
content: A better way to use the TP4056 for larger cells is like ebay item 371430693191 . Put multiple chips in parallel and program each for a part of the total current. The derating plus the chip's built-in thermal limiting should give you decent balancing between the chips. For the example cell, use 3 chips and program each for 0.67 A. That would give you a total charging current of 2000 ma and a cutoff current of 0.2 A.I haven't tried this, and don't know any of the sellers.Ed

author: HKJ
author_rank: Super Contributor
timestamp: April 03, 2016, 06:05:24 am
content: The charge_rate/10 termination current is a short-cut used in many chargers and not what the cell specifications says. Typically 18650 cells are specified for a termination current in the 50-100mA range.Cells will wear down faster at 4.2 volt, but it is a process that take a years, i.e. a hour more or less charging time is not significant, but storing the battery fully charger or partial charged is.

author: Skimask
author_rank: Super Contributor
timestamp: April 03, 2016, 06:26:53 am
content: Quote from: edpalmer42 on April 03, 2016, 05:46:06 amThat was one of the next things I was going to play with...take a few of the modules I do have, parallel them as required, remove and/or bypass the DW01A from all but one of them, and see what happens.Might not work all that well though. Tolerances between each module might mean that one of them is going to cutoff early and stress out the other ones. Don't know. Isn't going to hurt to throw something together to try at any rate...

author: edpalmer42
author_rank: Super Contributor
timestamp: April 03, 2016, 07:04:31 am
content: Quote from: Skimask on April 03, 2016, 06:26:53 amI'm not sure if you can overstress a chip. If one has a slightly lower voltage setting, it will stop providing current while another one with a slightly higher voltage setting keeps providing only as much current as it's programmed for. If two chips have slightly different programmed current settings, they just provide as much current as they want. In any case, the chip has internal limiting to protect itself.Ed

author: Seekonk
author_rank: Super Contributor
timestamp: April 03, 2016, 07:23:51 am
content: I have a number of the TP4056 boards that come standard at 1A. Best of luck changing that resistor. Had some iffy cells and put a 3.6 ohm resistor in series with power. Spec says .5 ohm max. They seem to work fine with this, still cut out normally at .1A.

author: linux-works
author_rank: Super Contributor
timestamp: April 03, 2016, 01:50:47 pm
content: changing the resistor was not hard. I'm over 50 and I could do itI didn't know there was a half ohm series max limit. where did you see that? maybe I need to re-read the spec sheet.I'm using approx .1 ohms as my I-sense series R, and I'm also going thru a logic mosfet as a switch, so that adds some R as well.when I meter-read the voltage across the lipo, I'm not seeing anything close to 4.20, and I think I should (?). I'm seeing it term at about 4.17. I'm wondering if that's because I have some 'junk' in series and that's too much for the TP to handle?I have a setup where I can read current and voltage (soon I'll data log it over scpi) and I'm using some good meters to calibrate a 16bit 4ch a/d board that I'm using to give the controller feedback on the current actual values of E and I.on lousy chargers, I do see voltages that get to 4.2 and even a bit over, which is worrying. but on the TP modules I have, its not getting up to 4.2. anyone do any quant analysis on this chip and have any info to share?given that there are many fakes of this chip, it may not be useful to try to engineer a solution using it. for that reason, I'm also trying the chip that adafruit has picked for their micro-lipo usb charger modules. both are controlled by a single R and digipots work well on both circuits.

author: amyk
author_rank: Super Contributor
timestamp: April 03, 2016, 03:02:25 pm
content: You should read this thread entirely - it debunks some common misconceptions about charging/discharging/storing li-ion chemistry:https://www.eevblog.com/forum/projects/diagnosing-lithium-cells/4.17 is less than 1% out from 4.20, the TP4056 is in fact specified for1.5%which means its regulated voltage may be anywhere from 4.137 to 4.263.Storing at 100% charged is not as bad as widely believed --- the huge capacity degradation likely comes from high temperatures (an observation of someone who kept cells in a big non-climate-controlled warehouse somewhere in South Asia?) See my post in that thread:https://www.eevblog.com/forum/projects/diagnosing-lithium-cells/msg804996/#msg804996Also this one and the others from the same author:https://www.eevblog.com/forum/projects/diagnosing-lithium-cells/msg805589/#msg805589

author: Skimask
author_rank: Super Contributor
timestamp: April 03, 2016, 06:38:32 pm
content: @edpalmer42 - I agree about the built-in protection keeping the chip from killing itself, but these are, after all, eBay partsAlong those same lines, I can confirm the thermal limiting works as advertised. Watched my Flir E4 centered on the chip while monitoring charge currents, etc. Temperature comes up too far, current goes drops off.I've got 3 of the TP4056 modules out playing now. All three of them are cutting out above 4.2, right at about 4.21. Not too bad. Might be the 20ga wires, might be the breadboard. Who knows. Who cares. The price was still right.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 04, 2016, 02:31:15 pm
content: The only relevant specification is the cell datasheet.The cell is considered full (100%) when the charging is terminated at the current listed in the cell specsheet. Typically, this is C/20, i.e., 100mA for a 2000mAh cell.Without knowing the cell, the charger can only guess. If the termination current is too large, the cell doesn't get 100% full. If the termination current is too small, the cell gets slightly overcharged; but very slightly so, because the typical termination rate (C/20) is very low to begin with.If the termination current equals to charging current, then there is no CV phase. It is not needed per se.On a typical low-cost 18650 energy cell:- Charging at 0.5C and stopping without CV phase (at 0.5C, when 4.20V is reached) gives you about 85-93% SoC- Charging fully (stopping at C/20) gives you about 4.15-4.19V OCV- Stopping at way too low current (going on much too long), i.e., like C/100, gets you to about 101-102% and does relatively little damage.You can think the cell as a constant voltage source/sink (very close to one you can draw in Spice), voltage varying depending on the state-of-charge (percentage of total charge storage), with some resistance in series (about 0.05 Ohm for a typical unused low-cost 18650). Now, some voltage differential must exist between the Open-Circuit Voltage and charger output so that current can flow through the internal resistance of the cell. At C/20 (100mA in our example), this voltage differential would be very small, and the cell will end up at about 4.18V OCV (open circuit voltage). If you never ended the charge, the cell would sit at 4.20V OCV, which would be about 101% or maybe 102% state-of-charge. Nothing special, nothing dangerous, and nothingthatspecial to the cell life.It's a myth that storing at 100% is extremely detrimental to the cell life; floating there is not a big deal either (but avoid ripple currents to be sure). Actually, storing at 80-90% gives little benefit; on some cells, storing at 100% may be surprisingly better than at 80%! Shelf life starts getting better when going below 60-70%, optimum storage being at or below 50%.Now, charging current is extremely important!At 5C, you will almost instantly totally kill practically any li-ion cell! Poor quality ones may blow up catasrophically; but don't do it even with quality cells.Some 18650's may be specified to 1C charge, but if it's an energy cell (i.e., a laptop cell, not a power tool cell), this 1C spec is actually near the real hard limit. Going any further will bring the cell life down quickly. It can happen in a sudden way: the cell may be fine when charged at 1.0C for hundreds of cycles, but die suddenly after 20 cycles when charged at just 1.2C. Don't trust the 1C spec, anything can happend, depending on the temperature. Hotter is better when charging at higher currents, because the chemical reaction rate is higher; the desired reactions (li+ intercalation in graphite lattice) happen more easily.You can destroy a cell easily if you charge in a combination of specified conditions; i.e., a cell is specified at 1C charge at +25 degC, and it's specified it's OK to charge at >0degC, but the combination of, say, +5degCand1C current is not specified.Limit the charging to 0.5C for good life on almost any li-ion cell. You can use higher currents at lower state-of-charge (inside small temperature range), but start derating the current after you hit 4.0V, before the actual CV phase. Try to be at 0.5C when you hit 4.2V. Chemically, you are doing the same thing you do with CV phase; slowing down the rate while approaching the point where the reaction doesn't happen easily (and there is a close-by competiting reaction, lithium plating, you don't want to happen).Charging is analogous to filling a glass with water to the brim, where any spilling is detrimental. You slow down while approaching the top. You should slow down even before the 4.20V mark, but after that, it's super important (hence the CV phase). The usual spec of 0.5C charge is rated at just before the 4.20V mark, so you could actually go a bit faster below that, but they never bother plotting actual curves in cell datasheets; the li-ion cell data is seriously lacking compared to semiconductor data, for example.BTW, please try to avoid the term "lipo". At worst, it's an idiotic oxymoron, and at best, it's just a very confusing term for a form factor. We should do everything in our power to get rid of the word altogether.

author: linux-works
author_rank: Super Contributor
timestamp: April 04, 2016, 06:23:27 pm
content: interesting info; I'm still reading up and experimenting on what a good charger should do and also what it should NOT do.I have been charging my quadcopter toy cells at 1c. maybe I should start using c/2 like you suggested. I've heard that other places, too, and since I'm not in a huge hurry, I'll give it a try.on my next iteration of my diy charger board, I plan to remove the tp charger board and do my own current source. I have a DAC chip on my arduino test board and that will drive a typical logic mosfet to limit current. I can measure the actual current value so the cpu will be my feedback loop and regulator. by doing my own, I can stop the charge at c/20 instead of the c/10 that the TP board (and the adafruit boards) all run at.one question: I'm assuming that I'm going to have to set the voltage level a bit higher than 4.20 since I will have a mosfet in series for current lim and also a fixed small value R for current sense (.1 or so). I can vary the voltage by that series mosfet, but I'm wondering if I should also vary the source voltage so that I get an effective 4.2v at the battery terminals, after all that series crap has down their various drops (?)

author: HKJ
author_rank: Super Contributor
timestamp: April 04, 2016, 06:35:17 pm
content: Quote from: linux-works on April 04, 2016, 06:23:27 pmOr let the Arduino to adjust the mosfet for both voltage and current.You can always check my website for how real chargers do it:http://www.lygte-info.dk/info/indexBatteriesAndChargers%20UK.html(I record current and voltage, but do usual not look at the electronic).

author: Siwastaja
author_rank: Super Contributor
timestamp: April 04, 2016, 08:21:31 pm
content: While having poor quality and safety, and while being overspecified, those RC hobby "lipo" batteries are still optimized for very high discharge currents. With high discharging currents also come somewhat higher charging currents (construction has different ratio of active surface area vs. mass so that reactions can happen at a faster rate), so I'm pretty sure 1C charge is just fine. Just don't trust their claims about 2C, 3C, even 4C charging capability.Just implement a properly working CC-CV supply. Look for instructions how to build a proper lab supply. No magic going on li-ion charging. There are no "algorithm" things beyond this basic level. Talking about complex algorithms is marketing bluff.Just make sure both current and voltage limits are precise and actually working robustly. Implementing a "stopping" feature is an optional bonus, mostly to tell user that the battery is full. Nothing wrong in leaving a battery in charger set at 4.15V and manually removing it later. (Given that your supply works without glitches!)You can implement sense wires to compensate for voltage drop in power wires, but it's not really that important; the CV phase (i.e., the low current towards the end) also minimizes any voltage drop in the wires. Sense wires are more important in high-power discharge when you want to measure the cell capacity under heavy load.

author: linux-works
author_rank: Super Contributor
timestamp: April 04, 2016, 09:04:21 pm
content: sounds good.one thing that is puzzling me, though. I might have to take a photo or have a diagram to show what I mean, but let me see if words can describe what is going on.I'm using the ads1115 a/d module:http://www.amazon.com/Qunqi-ADS1115-Precised-Digital-Converter/dp/B014KID8ZQand I have it in differential mode, so that's 2 channels instead of 4. connect a pair of a/d leads to each end of the series R that is my current sensor (.1ohms). read voltage and do math scaling (calibration) and you get current. that works and stays within 1ma out of a few hundred compared to my fluke bench meter. so far, so good.trouble comes in when I try to measure voltage. should be simple, right? its not. I use the other 2 a/d ports (as a diff pair since the whole module is either diff or SE mode) to connect to the battery + lead and ground. that SHOULD just give me the battery voltage, right? and what I get is a linear scaled value that matches the voltage curve but its values are not accurate. they seem to be accurate when I calibrate them for a given current setting (say, 500ma, for my larger copter batteries). if I measure the voltage with my bench meter and compare to the a/d value, then apply a scale fix-up factor to make 4.2v on my meter match 4.2v on the a/d, this won't work when I set the charge current to be different (say, 100ma).what am I missing, here? loading of the a/d module? I would assume its high z and that won't be an issue.I'm talking to the a/d module properly; as I get good numbers from the current pair (ports a2,a3) and I get relative-looking values from the a0,a1 pair when I graph them, but their values are never accurate.its almost like a need a different fix-up mult factor for each current setting. and that makes things crazy, since the current will VARY (go down) as the charge time moves forward. I'd need calibration tables or functions that involve the current reading and that makes me think that I'm on the wrong track, somehow.so, the connections are:---- (0.1 ohm) ----+---- battery ----the charger board supplies its approx 4.2v across that series combo, I clamp one a/d port pair on the 2 ends of the .1 and 2 more ports around the battery (and yes, one port lead connects to the other, so you either run a pair of wires to the same point or you bridge the connection and run just one; is that a problem?)why would my measurement across the battery differ when I'm a/d measuring it vs using my volt meter (doing both at the same time; maybe THAT could be a problem?)

author: Seekonk
author_rank: Super Contributor
timestamp: April 05, 2016, 04:22:34 am
content: Quote from: linux-works on April 03, 2016, 01:50:47 pmI concede. Your hands are bigger than Donald Trump's. After a 50 year career in electronics, Idid get out at the right time. One should remember that the EEVblog board is where no one seemsto be able to wire a LM317 successfully. And how many have a stock of SMD components, I don't.It is crappy board material and the only location with any mechanical strength is at the chips pin.All this is moot anyway because there isn't much reason to change the C/10 end charge rate.The point I was trying to make might have been lost. Every one of these boards should have aninput resistor. The chip is pure linear IR loss. Charging at 1A will result in 1W dissipatedby the chip (1A X (5.2V - 4.2V)). A .5 to .75 ohm in series with the input will remove 3/4 ofthe heat dissipation of the chip with no loss in performance. That chip does get hot. Try itonce with a resistor and compare. You will always add a resistor. Then you can stick that boardin a tight location and not worry about heat.The reason people buy these boards is they have a lot of old Li batteries. Probably not a good ideato charge them at high rates if below voltage. Never seen anything written where a charge rate of C/2or C/4 does any damage (end C/10 remains the same). Any added input resistance above .5 ohm willlimit the maximum charge current of 1A on the factory board. We are not always pressed for time andit is nice to know you can just walk away and not worry about a battery charging unattended. A switchadding an additional 1-2 ohms for a reduced charge rate seems like a useful addition.

author: BravoV
author_rank: Super Contributor
timestamp: April 05, 2016, 05:09:14 am
content: What about this Panasonic flow chart ? Is it too complicated or too overkill ?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 05, 2016, 06:34:38 am
content: I have always felt that the Panasonic chart is a bit complex, but whatever. Problem with extra complexity is that, even if it's there to increase robustness against problems, it may do the opposite because every extra feature is a potential pitfall. You should never "blindly" implement complex algorithms without understanding what every step does and why it's really there.Minimum generic algorithm is:Deny charging if starting voltage < 3.0V (copper dissolution due to overdischarge abuse which makes the cell dangerous)Deny charging if temperature < 0 degC or >50 degC (risk of lithium plating at temperatures too low.)Charge at 0.5C, i.e., 1A for a 2Ah cell. (CC) (risk of lithium plating at too high charging current)Charge to 4.20V, taper current to keep the cell at 4.20V. (CV) (risk of lithium plating, electrolyte decomposition and LCO lattice breakdown (thermal runaway) at too high voltage)Stop when current has tapered down to C/20, or using a timer from the start of the CV phase (for example, 10-30 minutes). Exact value is not important. C/10 would leave the cell to somewhere about 96-99%.With lab supply, you do the limit checking manually first.Of course you can implement a timer to shut down charging. It's rare, but a cell may have an internal problem so that it draws current without charging up and finishing, and it might pose a safety issue, especially if it goes on for a long time. Now, a simply timer as suggested by Panasonic will not solve this potential safety problem (the cell still gets a lot of charge in to do unwanted reactions); it will only make the small risk a bit smaller again. In the end, we all trust in cell-level safety technology even when we are talking about battery management and safety algorithms a lot.Optional step is a condition step before actual charging; it's not required, but it helps to recover some borderline cells that would otherwise considered "dead". Conditioning works when the starting voltage is below about 3.0V, but still above about 2.0V (or maybe a bit less). You charge at C/100 max until you are within the normal limits. The only reason to do this, however, is to try save the day after a broken load has tried to destroy a cell, and this shouldn't normally ever happen and either signifies a dead cell (conditioning won't work) or a broken-by-design load (you should fix it instead of recovering the cells). Toss any cells below about 1.5V without even thinking about it. They have too much dissolved copper in the electrolyte and are dangerous.

author: BravoV
author_rank: Super Contributor
timestamp: April 05, 2016, 07:16:22 am
content: Quote from: Siwastaja on April 05, 2016, 06:34:38 amRegarding this optional step, just curious, say the cell's temperature never have been beyond nominal/safe range, if it experienced even once at > 2.0V and < 3.0V region, is it considered "partially" damaged ?Curious how deep the damages (like capacity reduction) if this condition happened frequently.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 05, 2016, 07:53:54 am
content: Quote from: BravoV on April 05, 2016, 07:16:22 amIt's considered safe and non-damaged, i.e., major players like Panasonic, Samsung all describe the process of slow charge in their charging flowcharts in some way. Practically, there is no harm done at all. But that's only the borderline condition where the cell is only slightly overdischarged and then conditioned. In practical overdischarge situation, the cell easily goes below 2.0V after which real damage is done and it should not be charged at all. I don't exactly remember these voltage limits, some recommendations go a bit below the 2V limit but you shouldn't push your luck. I'm limiting the conditioning step between about 2.5 to 3.2V myself in my products, just to be sure.At low voltages, copper (which exists as a current collector in the negative electrode) starts to dissolve in the electrolyte, and later it will be all over the place causing increased self discharge or, worse, internal shorting. Above about 2V, the amount will be small enough not to be harmful, so it's considered safe.I don't know if repeated slight overdischarges (let's say, OCV getting down to 2.0V) and conditioning steps are really harmful or not. In my opinion, conditioning step is like hitting yourself with a hammer in the head and then taking some aspirin. I strongly recommend that the battery system is correctly designed so that overdischarge never happens. To my knowledge, a fully discharged cell (which is in good condition to begin with) doesn't "self-dicharge" into overdischarge region, so the only way to do that is by failure-by-design external circuitry. Today, you can easily implement any monitoring solution with quiescent current far below 1 µA, and you can easily implement a safety margin so that you only discharge the cell to, say, 1%, which would, on a 2000mAh cell, give you 1%*2000mAh=20mAh of charge, which would last for 2 years driving 1µA.For the record, I tested fully discharging typical LCO and NCA off-the-shelf cells. I made sure they are fully discharged by CV discharging them to the datasheet voltage endpoints (2.80V for LCO, 2.50V for NCA) with a stopping current of C/10, where datasheet specifies C/5. After 500 days of storage in room temp, the LCO cell was at 3.43V OCV, NCA was at 3.30V OCV. Both samples showed no capacity fade and no measurable resistance rise. Neither required any conditioning because the voltage was over the lower limit with a large margin. So overdischarging never happened, even when i stretched the datasheet conditions to discharge them further, and stored them well over a year after the fact.

author: Skimask
author_rank: Super Contributor
timestamp: April 05, 2016, 02:02:05 pm
content: Quote from: Siwastaja on April 04, 2016, 02:31:15 pmOn this point, I'll agree completely. At the same time, it's a lot fewer characters to type, and a bit less of a tongue twister in everyday talk.Good reading above nonetheless. I printed out that flowchart above. Overkill? Probably, and it sounds like the majority agree. Promotes a bit of extra safety? Can't deny that.I found a couple of older laptop packs laying around the shop. Took them apart (because that's what you do, right?). All 18650 cells, 12 total.These have been laying around for at least a couple of years if not more. 10 out 12 batteries still had 2.9v open circuit on them. Ran them thru a light charge/discharge cycle. Looks like they're good enough for playing around with.When put on the meter, 2 of the cells (a pair in one of the packs) started out at about 2.1v and gradually came up to 2.7v over about 5 minutes with the meter connected. Let them sit overnight. In the morning, they both still had ~2.7v on them. Ran them thru the same light charge/discharge cycle. After that, they're acting just the same as the other 10 cells.What's the deal with that? The initial low 2.1v, then coming up to 2.7v while connected to a DVM? (P.S. the DVM in question is a BK2709B, nothing weird here...)

author: dmwahl
author_rank: Contributor
timestamp: April 12, 2016, 06:52:15 pm
content: Quote from: evb149 on April 03, 2016, 03:41:04 amA lot of the "documentation" on not float charging or not storing at high SOC is forum chatter where myths or partial truths get perpetuated until they're seen as fact. High storage temperature is a far stronger contributor to capacity fade than high storage SOC. Lowering the SOC while at high temperature will reduce the rate of capacity fade, but the dominant factor is still temperature.https://hal.archives-ouvertes.fr/hal-00876555/documentis one good paper I've found on the topic, although there are plenty of others. I'd encourage anyone making general claims such as above to provide their sources, which shouldn't be difficult if it's so well documented.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 12, 2016, 08:08:37 pm
content: Quote from: dmwahl on April 12, 2016, 06:52:15 pmThat paper is about LFP chemistry which is practically unused expect for some niche applications. It's very different due to lower voltage.If we look at the modern world (and foreseeable near future), NCA and NMC are the relevant chemistries. LFP was a commercial failure, and LCO is really getting obsolete now.On NCA and NMC, there is a correlation between SoC and calendar fading, although it's different from what people generally think it is - storing at 90% or 80% is not the answer, if you want toreallyincrease keeping properties, you need to go as low as 40-60% (source:http://www.a3ps.at/site/sites/default/files/downloads/evs28/papers/A3-01.pdfFigure 6 for example); my own research showssomeimprovement at about 80% on some cells, but it really starts getting better at 50%. This kinda sucks. However, more damage is done during charging at 100%, so still charging only to 90% or 80% makes sense, but mostly due to preventing cycle damage, not due to calendar fading.You are right that tempereture is definitely important, agreeably perhaps more important than SoC, at least on some cells.If the cells are really going to bestored, i.e., not used during storage, then it's very easy to just discharge them to low SoCs; temperatures may not be as easy (or economical) to control. You are pretty much stuck at room temperature if you don't want to invest money in refrigeration. And room temp may be rather high in certain parts of the world.For example, in my study, the worst performer of calendar fading, a power cell from LG, was capacity fading at 4.9%/year at 23 degC when fully charged. Stored at 3.60V, the fade was down to 1.9%/year in the same conditions. On the other hand, in a refrigerator, at +4degC, the fully charged cell faded at 1.3%/year, so the temperature is very important but so is the SoC.But there are huge differences between the cells. The best sample was a cheap Samsung LCO cell that didn't fade measurably when stored at room temp fully charged for 500 days, and even when stored at elevated temperatures, fully charged, only faded 1.1%/year during this time. The LG power cell faded 8.4%/year in the exact same test case.

author: Skimask
author_rank: Super Contributor
timestamp: April 14, 2016, 03:51:13 am
content: Excellent reading linked above.Gonna awhile before I can get around to putting the 18650 cells I've recovered thus far to good use. So far, all 36 cells, after a charge and sitting for at least 24 hours, are within 30mV of each other (1 at 4.161v, 1 at 4.139v, the rest at 4.144v +/- .002).Running each of them thru a couple of light charge/discharge cycle to make sure they all have a decent amount of capacity left and are useful,Then one final charge, and discharge at about .2C down to 3.7v, and throwing them in the fridge. Close enough for me...

author: Siwastaja
author_rank: Super Contributor
timestamp: April 14, 2016, 07:24:13 am
content: Quote from: Skimask on April 14, 2016, 03:51:13 amNothing to add to your other points, sounds good, but if you are going to do this anyway, then go a bit lower. 0.2C discharge down to 3.7V would leave you at about 60% SoC, and I'm sure that if you do it to 3.6V or even 3.55V instead, you can get some real calendar life benefit. Nothing ground breaking, but why not?It may actually happen that the cells stored as you describe may live very well for a decade or two! (Or may not, but it's possible.)

author: Skimask
author_rank: Super Contributor
timestamp: May 03, 2016, 02:35:01 am
content: Following up on this a bit...I'm working on a dual rail converter circuit, basically beating up on the chips by working them past the datasheet limits for current, etc. Getting them hot, watching them shut down, cool off, and restart themselves, etc. At the same time, I'm also beating up a few older 18650's that are powering the PCB. I'm using one of those ebay TP4056 PCBs for 'protection'. Working out well enough so far. It's cutting off the battery at around 2.6v or so. And shuts off the charge at about 4.2v. A bit low and a bit high for each for my tastes, but it works.Question is...in general, what conditions cause the cells to self-destruct, eg. vent, explode, melt-down, etc. I'm not talking about just "not taking a charge anymore" or "not holding a charge anymore"... I'm talking about the causes of violent self-destruction.I'm surfing the misinformation intarwebs right now looking for something conclusive and factual, rather than the normal "well, this happened to me one time, so it must happen to everybody all the time" anecdotal information.

author: Zeranin
author_rank: Regular Contributor
timestamp: May 03, 2016, 06:53:00 am
content: Quote from: Skimask on April 03, 2016, 02:50:14 amYou raise good points, and I have had similar thoughts in the past. One point that I believe you are getting at, is that the rule ‘terminate charge when current has dropped to 1/10th of inititial charging current’ will result in a different final state of charge, depending on the charging current. Specifically, a lower current charger using this algorithm will give the cell a higher final state-of-charge, compared to a higher current charger. Presumably all chargers aim to provide the same final state-of-charge, therefore this charging algorithm cannot in general be exactly correct, and will need to varied slightly depending on the charge rate relative to the capacity of the battery. Agreed.The final state-of-charge is judged from the cell voltage, after the cell has been removed from the charger for a few hours. That is the ‘bottom line’ in determining whether any charging algorithm has correctly charged the battery to a full state of charge.As much as anything, the common charging rule as described is predicated upon reaching full state of charge within a reasonable time. In principle (within reason) one could use a lower charging voltage, but wait longer until the current had dropped to 5% or 1% or whatever, but in practice the consumer won’t stand for waiting that long, so the charging voltage is set to a voltage that would overcharge the battery is left on for a long period of time, and disconnected while the current is still falling, to compensate, at least that is my understanding.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 03, 2016, 08:38:52 am
content: Quote from: Skimask on May 03, 2016, 02:35:01 amThe most prominent reason, by far, is poor cell-level design and/or manufacturing quality, from the lack of cell safety features (shutdown separator etc.), to outright manufacturing defects such as contaminants, electrode alignment problems...Slight electrical abuse may increase the risk of catastrophic failure on high quality cells, too, but the risk is still very small, because it's miniscule to begin with.For non-quality cells, I don't like speculating, because you never know. Some may go off without any abuse at all, some may be surprisingly good, until you buy more from another batch. This is why if you are concerned about safetyat all, you absolutely must buy Samsung, Panasonic, etc. Or take the very real risk.Now, to the abuse side:Thermal abuse is the worst. Thermal runaway onset temperature depends on the chemistry, but all modern high-capacity li-ion chemistries are at about 160 deg C. AFAIK, they can't lower this no matter what safety features they design in. This (about 150-170 degC) is the hard limit for any li-ion brand, if it uses LCO or NCA chemistry. Of course, real damage happens way before this limit. Competeting chemistries with higher onset temperatures (LMO and LFP) are unsatisfactory for other reasons. Thermal abuse point is really important, because if youreallyfail your electronics side, it can heat up even the best quality, safe cell over it's onset temperature! Design proper fusing, and don't design in high-power resistors dissipating several watts for balancing.Mechanical abuse; it depends. Low-quality cells have high chances of blowing up if you penetrate the cell with a nail or crush it. Higher quality cells are not immune, either, but the risk is smaller. 18650 is relatively safe due to the strong steel casing.Overcharging; depends a lot. High-quality cells are unlikely to blow up even if you severely overcharge them (like connecting to 12V for hours!), but it can happen. Going to about 4.30V should be safe (while shortening the life), expect for the poorest cells that are on the brink of exploding anyway.Too high charging current: Comparable to overcharging, but partially different mechanism. High-quality cells are unlikely to really blow up. Same thing for charging at temperatures too low.Charging after severe overdischarge; again, depends. Severe overdischarge causes copper dissolution, and copper ions can build up dendrites which cause internal shorting, similar to lithium plating. Any cell below about 1.5V-2.0V should be discarded. High-quality cells are rather immune, because they are designed to be immune against internal shorts.Having said all this, you should still avoid abusing even the highest quality cells. No cell is completely immune to abuse, there always are increased risks.

author: Skimask
author_rank: Super Contributor
timestamp: May 03, 2016, 01:38:40 pm
content: 1 - Design/quality/etc - The only cells I've got at the moment are used 18650's out of old laptop packs, all unprotected, only a few that have had <1V on them when I took the packs apart, only a couple that wouldn't come up to any usable voltage under a light charge.2 - Non-quality cells - I'll play games with used cells, but from what I've seen on the intarwebs, I'm not even thinking about buying or using any of the cheap "insert garbage name here"-Fire cells.3 - Thermals - 160C!!! And here I was beginning to worry when the outer wrap was getting up above 50C.4 - Mechanical - Don't see that happening in my situation, but, ya never know.5 - Overcharging (voltage) - Usage those TP4056 boards. They seem to be cutting off right around 4.2V. Good there.6 - Overcharging (current) - Those same TP4056 are limited to 1 amp (by a programming resistor) depending on the USB charger connected, and I'm using an old 5V @ 500mA part. Good there as well. Near the end of the charge, I'm not seeing much above 100mA before cutoff. Good there.7 - Overdischarge - TP4056 cutting off at about 2.5V under light load, about 2.8V under heavy load.Maybe I'm playing it too safe. Who knows. Suffice to say I'll probably NOT have any unintended fires going on around here.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 03, 2016, 04:47:18 pm
content: Quote from: Skimask on May 03, 2016, 01:38:40 pmDon't get me wrong, 160 deg C is kind of "will burst in flames despite all manufacturer safety efforts" limit. Note that hotspotting can happen inside the cell, and case temperature is not the same as internal temperature, even though in 18650 it's close (which is great in 18650, and one of the reasons Tesla states they use them for!). In any case, a lot of damage will happen way below 160 deg C. The shutdown separator will melt, practically disabling the cell (permanently) somewhere near 120-130 deg C; given that thereisa working shutdown separator (poorest Chinese cells may not have it).160 degC thermal runaway onset temperature is kind of low, because when leaving appropriate leeway for shutdown separator to work, the "absolute maximum rating" cannot be much over 90 degC, which leaves little margin for internal heating in hot environments. Just leaving a black laptop battery pack in a car on a sunny day is enough to destroy the cells, in extreme cases.You need to worry if the 18650 steel case is getting over 60 deg C for extended periods of time, or over about 70-80 deg C momentarily. A lot of damage will happen anyway beyond let's say 90 deg C, even though it's unlikely that they go in flames until you are getting nearer to that 160 deg C region.QuoteNah, you are implementing the basic minimum protection, as you should. At least it means you are not destroying the cells. Even though it would be very unlikely that any small abuse beyond normal limits would causecatastrophicfailure such as fire, don't risk it. And destroying working cells makes no sense anyway.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 03, 2016, 04:53:52 pm
content: Quote from: Skimask on May 03, 2016, 01:38:40 pmDid you get this right? It sounds to me that those voltages should be swapped for maximum capacity under heavy load, and good protection under light load.

author: Skimask
author_rank: Super Contributor
timestamp: May 03, 2016, 08:14:25 pm
content: Thermals - Ya, I figure 50C on the outside means (value thrown to the wall here) 70C on the inside. Trying to take images all around the cell to see what's happening.QuoteNo, those numbers are right. Thermal images don't show anything getting hot. Could be a discharge current limit on the DW01 which are present on the TP4056 PCBs I've got. (http://www.ebay.com/itm/10PCS-5V-Micro-USB-1A-18650-Lithium-Battery-Charging-Board-Charger-Module-/201039030988?hash=item2ecedc2acc:g:zHgAAOSw1vlUxZ0Q)See at 20:57 below...Had the battery loaded down pretty good in that test trying to find the "boiling point" of the circuit, and it cut out at 2.984V, which is measured AFTER the TP4056 board, along the "rails" of the protoboard I was using for power distribution. So, yes, maybe as the voltage gets lower and the current gets higher, I'm hitting that limit on the DW01 and it trips.On other "tests" with a much lighter load, the battery would die at about 2.5V, which is the limit for the TP4056.I'll try it again with slightly less load this weekend and see what happens.EDIT: Aha! I was just re-reading the datasheet for the DW01. Says that it uses the voltage drop across the external control MOSFETs to determine current. So, too much current or too high resistance across the FETs, overcurrent detection trips, DW01 engages the shutdown on the FETs. Next time thru, I'll try to put a meter across the FETs and see what that reads.

author: BravoV
author_rank: Super Contributor
timestamp: May 05, 2016, 11:23:15 am
content: Quote from: Skimask on May 03, 2016, 08:14:25 pmI've been using these cheap TP4056 boards for a while, beware of these with DW01 mosfet + 8205A protection ic, the RDSON of DW01 vary so much.Nowdays I used those without these protection IC + Mosfet as its more stable and efficient among boards since its just the single IC that does all the magic. I'm using separate PSU rails soldered on board that is adjusted exactly to minimize the drop out, not thru the USB port.Attached photo of two versions that are common in the market, left boards that are with protection, and the right are just pure bare minimum with single TP4056 ic only and with it's supporting discrete components.If you watch closely, the ones without protection have the shunt resistor 0.4 Ohm as the TP4056 design specification, that alone is enough to measure the current with adequate accuracy if needed. Yeah, the IC's quiescent current is too small that I think can be safely ignored.

author: Skimask
author_rank: Super Contributor
timestamp: May 05, 2016, 01:54:39 pm
content: The two types are basically the same price on ebay, figured having the DW01 was better than nothing at all.I understand that process variations in the 8205A and DW01 make the over-current protection iffy at best with potential for a wide range of cutout currents.Are you implying NOT to use the type with the DW01 due to the V(drop) across the FET's?Surely these things wouldn't be used in any kind of consumer product, but good enough for the home lab, yes?

author: Skimask
author_rank: Super Contributor
timestamp: May 06, 2016, 05:59:32 am
content: And yet another slightly confusing...thing...Still working on loading down the 18650 cells (old cells, rated at 2200mAh on the package, who knows what their true capacity is at the moment, semi-irrelevant at this point)With about a 1.8amp load on one cell, about .81C, the load circuit ran for 50 minutes before the low voltage limit kicked in on the DW01.I paralleled up 2 fully charged cells, cells that I knew would hold X volts after charging and an overnight rest, and ran the load test again. Same load, about 1.8amps. It ran for 103 minutes before dumping.In theory, the load on each battery went from .81C, to about .4C. I would've figured, based on looking at various discharge graphs, I would've gotten a fair bit more run time on the circuit.Maybe the one battery has significantly less capacity than the other. Don't know. I don't have a proper way to test that. But I did run both batteries, individually, on the same load circuit, and got the same results for each one, 50-51 minutes before dumping.Maybe I have to cut the discharge rate significantly before seeing a measurable increase in run time.Maybe I should just be happy with what I've got and run with it...

author: Siwastaja
author_rank: Super Contributor
timestamp: May 06, 2016, 06:57:00 am
content: If you have 50 minutes of runtime to a constant current load with both cells separately, that should definitely double to 100 minutes when connected in parallel to the same load, or maybe just a little bit longer due to smaller voltage drop at the end of discharge.If you expect significantly larger capacity at lower load, this is a wrong expectation maybe based on lead-acid, where the capacity is radically reduced at high discharge rates. Great thing in li-ion is that capacity is not (much) dependent on load current, you always get (very close to) theoretical full capacity out of it.Capacity may appear to increase considerably on small loads when the higher reference load is too much for the cell (considering the aging-induced increase of DC resistance and discharging temperature), or when the cutoff voltage is higher than necessary.CC-CV discharge always gets the full capacity out of the cell.But your test procedure appears way too indefinite to do precise conclusions about the cells.You can easily measure both coulombic and energy capacity (Ah and Wh) with any resistive load and two multimeters, graphing V & A every few minutes. Yes, you need to concentrate on this for an hour or so, so you want to have a tester if you do it more than a few times.Typical discharge cutoff for LCO laptop cells is approximately 2.8V @ 0.2C discharge.

author: Skimask
author_rank: Super Contributor
timestamp: May 06, 2016, 06:18:54 pm
content: Quote from: Siwastaja on May 06, 2016, 06:57:00 amIt's almost a constant current load. The load is my TPS65131/TPS63001/TPS63002 buck/boost converter board. But with that being said, the conditions are the same between runs...no changes in load resistors on the outputs of the converters, I don't change the wiring around, etc.QuoteCould be. Another look at the discharge curve for capacity vs. discharge rate shows the mAh drawn off the battery converging once it gets down a really low voltage, where if you stop the drawdown higher up, the capacities vary a bit more.QuoteThe only thing definite is that the "procedure" is the same between the runs.QuoteI've been doing it the hard way...3 meters, a stopwatch, and a spreadsheetQuoteI've been running them down until the TP4056/DW01 cuts out, usually about 2.5v, which in the last run was only 6 minutes after the batteries hit 3.000v, for a total of 103 minutes. If I wreck those particular cells, no biggie. They were used in the first place, and I've got to find my own limitations.No real problems or issues overall. Probably digging into it wayyy too deep anyway. As I said earlier, I should just be happy and go with it.

author: amyk
author_rank: Super Contributor
timestamp: May 07, 2016, 01:31:12 am
content: Quote from: Skimask on May 06, 2016, 05:59:32 amThat's 1500mAh, so the cells are around ~68% of their rated capacity. Very well-used but there's still plenty of capacity left.

author: Skimask
author_rank: Super Contributor
timestamp: May 07, 2016, 03:30:58 am
content: Quote from: amyk on May 07, 2016, 01:31:12 amOh ya, they're "vintage" cells, circa 2002 or so.The price was right...zero