LiPo Battery on Holding/Float Charge - Page 1
Forum section: electronics / general

author: imk
author_rank: Regular Contributor
timestamp: June 28, 2021, 02:47:31 pm
content: Hello,Are there any issues keeping a LiPo battery on a holding charge of 4v?Basically if i connect a modest load of 40ma or so to a LiPo and then connect the output of a Buck Convertor set at 4v to the LiPo plus the Load are there any issues.The situation is that i want to run something 24/7 but in case of power failure the device will still work from LiPo power.I know there are many Lead Acid battery emergency light systems that work like this but how do LiPo's tolerate it?imk

author: Nauris
author_rank: Regular Contributor
timestamp: June 28, 2021, 03:10:17 pm
content: Quote from: imk on June 28, 2021, 02:47:31 pmI don't think there is any problem with that, maybe 4 V is a bit too high, you should check it does not draw any current after few week at that voltage, even microamperes are cause for concern.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 28, 2021, 07:24:10 pm
content: Quote from: evb149 on June 28, 2021, 04:01:39 pmYep. Well, it would largely depend on how you implement this though. Doing this with a lead-acid battery is much simpler if you want something safe and durable.What you should usually not do with LiPo is constantly trickle-charge them. Never. You should at least use a safe charging circuit. Barely monitoring the cell's voltage, delivering a charging current if it falls below a given threshold (you mentioned 4V) is a pretty bad idea.A better approach is to set a much lower threshold, and "isolate" the battery from the load while external power is on. Basically, you'd let the battery self-discharge and only recharge it if the cell's voltage drops below 3.7V, or even lower. Downside is, in the worst case (power loss while the battery is at the threshold), you won't have the battery's full capacity. But that will limit the number of charge cycles to a decent number.In any case, absolutely avoid constantly charging LiPo batteries with a small current. If said current is low enough relative to the battery's capacity, it may end up degrading even if the cell's voltage has not exceeded the max rated voltage (usually 4.2V). I have actually done something like this years ago with my first LiPo-based backup system, and I was using a proper charger IC though! But with a very low charging current. That would result in the circuit getting in charging cycles way too often, and the battery would end up inflating. A lot. It didn't explode, but that's still scary to see.

author: thm_w
author_rank: Super Contributor
timestamp: June 28, 2021, 09:27:03 pm
content: Quote from: SiliconWizard on June 28, 2021, 07:24:10 pmA large number of phones and laptops do this though. They don't have enough surge capacity from the AC power supply so they pull from the battery, then top it up shortly after.Not saying its ideal for the life of the battery, but I don't see how its as disastrous as you claim.edit:https://www.researchgate.net/publication/308854873_Cycle_life_evaluation_of_lithium_cells_subjected_to_micro-cyclesQuoteSomewhere from 3.8 to 3.9V should be good, ~50% charge, will vary slightly depending on the cell type. If you want lifespan of the cell only.

author: amyk
author_rank: Super Contributor
timestamp: June 28, 2021, 11:56:04 pm
content: It's fine. The cell won't draw any current except its tiny self-discharge, and 4V is somewhere around 80% for a 4.2V cell. What kills lions quickly is keeping them at 4.2V for extended periods of time, and more so if they're also kept warm.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 29, 2021, 07:24:31 am
content: It's OK. You can get some more lifetime out of it, especially in hot environments, if you use a tad lower voltage. 3.8V would be great but then you lose some capacity (i.e., have 60% to empty instead of 80%).Remember the circuit needs current limiting and safety circuit preventing charging an overdischarged cell, if that ever happens.Also make sure the regulator can't fail short overcharging the cell.

author: tooki
author_rank: Super Contributor
timestamp: June 29, 2021, 07:40:26 am
content: Quote from: imk on June 28, 2021, 02:47:31 pmDO NOT do this.You can’t even keep them on a tiny trickle, it damages the battery.Just use a Li-Ion charger IC that will do exactly what you want. In a project I’m just finishing up, the TI bq25895 charger handles this. It supplies the load with power from the AC adapter (which can be USB) when connected, from battery when it’s not. A microcontroller programs the charging parameters via I2C.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 29, 2021, 07:47:57 am
content: Quote from: tooki on June 29, 2021, 07:40:26 amMaybe you could elaborate why you think so.

author: tooki
author_rank: Super Contributor
timestamp: June 29, 2021, 07:50:34 am
content: Quote from: Siwastaja on June 29, 2021, 07:47:57 amThe next sentence says why.Even a small trickle charge is expressly warned against by the battery manufacturers.At 4V, you’d have far more than a small trickle charge.Sorry, brain fart, strike that part.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 29, 2021, 08:38:36 am
content: I asked because I didn't understand what you mean by the next sentence.Never heard about such recommendation from the manufacturers. Maybe you can give an example so we can see what's it all about.I don't understand the sentence "At 4V, you’d have far more than a small trickle charge".You'd need to define "trickle charge". AFAIK, it usually means holding a cell at fully charged voltage (or over it) so that charging current equals self-discharge current at that voltage. This is mostly relevant with the Pb, NiCd and NiMH chemistries, holding the cell with significantly over the 100% SoC open-circuit voltage. This also causes significantly more current to flow than what would flow at 100% SoC open-circuit voltage. These chemistries are not significantly damaged by charging to over 100% equivalent voltages with limited current and this allows balancing series strings.Thisis what I think trickle charging usually means.If you mean this by trickle, then in LCO, NCA or NMC li-ion, firstly, 4.0V is not at or over 100% SoC, it's actually around 80% SoC, and secondly, self-dischagrge is typically around 3%/year. Average charge rate would be this 3%/year and would stress the battery equally compared to just charging the missing 3% once each year.Because lithium-ion cannot accept overcharging currents unlike many earlier chemistries, the term "trickle charging" is not typically used. I.e.; li-ion cells are not to be trickle charged.But holding at 4.0V is not trickle charging.I can see the confusion if some manufacturer has forbidden trickle charging. What they mean is: do not overcharge expecting the battery to shunt excess charge like Pb or NiCd or NiMH.Float charging li-ion at anything less than say 4.15V is regularly done whenever needed. Back-and-forth cycling with a hysteresis is another option but I'm not sure it's any better.Large ripple currents are a consideration, though.

author: tooki
author_rank: Super Contributor
timestamp: June 29, 2021, 07:13:10 pm
content: Quote from: Siwastaja on June 29, 2021, 08:38:36 amYep, you’re right, I had a brain fart there. I’m right in the middle of final exams, stressed and sleep deprived, and got myself mixed up.Quote from: Siwastaja on June 29, 2021, 08:38:36 amTake a look at the datasheet of the Samsung INR18650-29E batteries my current project uses. (Its specs are still fresh in my mind since I just used the datasheet last week to calculate my charger parameters.)Page 9 says:QuoteI will concede that this is contradicted by a subsequent statement on p.10 that says:QuoteBut since it seems they won’t warrant them in that situation, there must besomedownside.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 29, 2021, 08:25:03 pm
content: Quote from: tooki on June 29, 2021, 07:40:26 amYes, but there's still something to be careful about. (Got bitten by this once.)Even if you're using a proper Li-ion charger IC, setting too low a charging current, even if that looks right for the application in question, is often not a good idea either. It's probably partly dependent on the charger IC itself. But for very low charging currents (relative to the battery's capacity), the IC may have troubles properly detecting the end-of-charge condition. I ran into this using a Microchip charger IC (do not remember the exact ref, but I could dig it up.)Now some ICs do include an internal time-out, so the charge would stop after a given amount of time even if the IC doesn't detect an EOC condition. I don't quite remember, but I think the IC I used didn't have a time-out.The reason EOC can be hard to detect for low charging currents is simple. Most charger ICs roughly use the following algorithm:1 - If Vcell < Vmin => source a predefined constant current < charging current (to "prepare" the cell);2 - If Vcell < Vmax (usually 4.2V for LiPo) => source constant current = charging current;3 - When Vcell reaches Vmax => switch to constant voltage mode @Vmax, and monitor current to the cell until it goes below (charging current)/10;4 - Stop charging => charger output in Hi-Z.The condition in step 3 is one reason EOC detection can fail for very low charging currents. Since the threshold is usually (charging current)/10, it might be below the usable range for the internal current monitoring circuitry if charging current is set too low. Something to keep in mind...And, as tooki also said, I will never emphasize enough how careful one should be with Li-ion/LiPo batteries. Better safe than sorry. They forgive very little.

author: tooki
author_rank: Super Contributor
timestamp: June 29, 2021, 10:49:33 pm
content: I know the bq25895 has a built in timer, among other things. Given how accurate its measurements are, I’m fairly confident it could measure its low currents, too. (Since I’m using 3 cells in parallel, the precharge and cutoff current is not right at the edge anyway.)

author: Siwastaja
author_rank: Super Contributor
timestamp: June 30, 2021, 06:47:32 am
content: I have worked with Samsung cells yes, especially the 29E as it was and probably still is cheaply available from secondary grey sources, as I'm a small-scale (prototyping scale, let's say, some thousands of cells have gone through) battery pack manufacturer. Quite frankly, the public datasheet isn't very usable, IMHO. This is typical to li-ion cells. Also typical to Korean or Japanese products is the language barrier from the engineers to the English datasheet. Numbers are fine, but text around the numbers can be pretty weird.I'm sure if you are a proper (multi-million, I'm not) customer and sign the NDAs, you will be able to get the "actual" datasheet, more data, and engineering assistance from Samsung.So it's not surprising they contradict themselves, and you need to apply critical thinking. In the end, "Continuous charging under appropriate voltage does not cause any loss of characteristics" is quite clear, and matches with what I have seen from the literature and own experience. Nothing happens because current doesn't flow. Li-ion is quite close to an ideal voltage source with a series resistor.I can see the point of adding a timer to increase safety, but let's be frank about it, it doesn't help a lot. If your charger design has a tendency of failing (for example, blowing pass regulator MOSFET short circuit), it can fail during that time, and unless the timer controls a secondary, redundant MOSFET, it won't help the slightest. The problem is, normal charging times can be long, especially CV phases when fully charging per manufacturer recommendations, so the failure event is likely to happen during charging well before the timer expires saving the day. It's like if a motorboat or jet ski had a 5-hour delay in the dead man's switch.Now one point in the safety cutout timercouldbe that if the cell is damaged so that it leaks charge (i.e., self-discharging well beyond normal amount, heating up as a result) then the timeout finally limits the amount of excess charge put into the cell by the charger. But this has two issues,(1) A dumb timeout will still allow quite a lot of charge to be put in, for example for a C/2 charger and 5-hour timeout this would be 250%. In order to actually limit the excess charge, you'd need a lot more complex system; i.e., initial SoC assumption based on voltage triggering charge integrator reset, then integrating current over time, and emergency stop if, for example, 110% is exceeded.(2) A timeout can do nothing if there are loads connected to the very same battery while charging, a common use case in cellphones and laptop computers. How are you going to differentiate between charge lost inside the cell, or used up by the intended load? So again we would need cell current sensing, accurate charge integration, capacity tracking, integrator reset logic and overall failure detection logic. Which can't give false positives because then no one uses your product.(edit: 3 of 2): If the cell leaks charge excessively anyway, the cell voltage would drop, and then the user would just restart the charging by unplugging and plugging in the battery, again delivering full timer worth of charge into it. For such imaginary faulty cell, this would be dangerous. So then we would need a battery ID and memory to flag the battery "do not charge".This is a lot of engineering to be done right and not make things worse. There is no easy off-the-shelf IC solution for all this. Manufacturers never seem to require such levels of BMS measures against protecting from cell damage so clearly they think their passive cell safety is enough to handle this and the BMS is required just to prevent externally induced over/undervoltage events (i.e., stopping charge and discharge based on voltage), and for a few other oddities, like preventing charging of overdischarged cell, and preventing charging at cold temperatures. And finally,temperature sensingof the cell is an easy way to deal with cell problems (the 18650 cells do have the integrated PTC for this very reason; an external extra sensor connected to your micro won't cost a much extra).All in all, for a typical float/holding charge application, do implement the following* Force turn-off if cell voltage < 3.2V (assuming low load currents and no significant peak currents. Adjust this down for larger peak loads like 2.8V at C/2)* Refuse turn-on if cell voltage < 2.5V (replacement of the cell is needed)* Refuse charging if cell temperature < 0 degC* Make sure your regulator circuit doesn't fail in any way generating excess voltage (pass element short circuit being most typical). It's recommended to add another pass element (two back-to-back MOSFETs typically) controlled by a small overvoltage/undervoltage protection IC. This way, protection is actually redundant and not dependent on some massive Benchmarq/TI monster chip no one understands.* Always remember the classic ceramic fuse, nothing beats it in fire protection in case all semiconductors fail short.* Thermal fuse coupled to the cell won't hurt if you want to do this seriously but usually designers won't bother.

author: magic
author_rank: Super Contributor
timestamp: July 02, 2021, 08:35:05 am
content: Quote from: SiliconWizard on June 29, 2021, 08:25:03 pmPerhaps it could also be that self discharge at 4.2V exceeds the CC/10 threshold if CC is set too low.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 02, 2021, 02:25:04 pm
content: Quote from: magic on July 02, 2021, 08:35:05 amNo, only going to happen with a completely dead/destroyed cell and then it could happen at any voltage really.Self discharge at 4.2V on a good cell is approx 1-10%/year so put in C rate, approx. C/1000000 to C/100000.This may be one of the reasons for the recommended "charging timeout" feature, though, although I think it's insufficient if you don't flag the cell as dead and refuse using it again (and then false positives would be disastrous).One otherwise-broken-by-design-as-well TI/Benchmarq IC I used in error had this great idea of having gazillion of pins to configure things yet they shared the setting of CV ending current and "overdischarged cell revival current". You couldn't even set the revival current low enough to be safe, without setting the CV cutoff to some abysmally strange value like C/100. So with something sane like C/10 stopping, overdischarged cell precharge became also C/10, basically way beyond what any cell manufacturer recommends. And you couldn'tdisablethe damn feature. This chip was full of self-destruction features and traps and actually using it is the only caseeverwhere I managed to overcharge cells unintentionally, charging a few packs, two of which already at customers, to 4.5V per cell. No fires, no problems, by the way. The cells worked fine and provided nice extra capacity for the few customers before the recall. Having never seriously failed a from-scratch li-ion management design, this experience is clear cut proof to me that I'm never going to put any BQ anything chip in any of my designs again, period. (Many of them are OK, I guess.)

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 02, 2021, 04:01:47 pm
content: Quote from: magic on July 02, 2021, 08:35:05 amThat can be a factor too, although it's pretty unlikely, unless your charging current is freaking extremely low, to a point of making no sense.But Li-ion battery chemistry is not that simple, and for low charging currents (for a given battery), you can start observing some weird non-linear behavior.That's why I recommend charging currents in a reasonable range. Of course refer to datasheets, but that's commonly between C/4 and C/2. Some batteries of course can widthstand > C, but that's still most often reducing battery life significantly.

author: tooki
author_rank: Super Contributor
timestamp: July 02, 2021, 05:12:28 pm
content: Quote from: Siwastaja on July 02, 2021, 02:25:04 pmAre you sure about that? The cell datasheet I attached above lists a max precharge current of 150mA, with 1C=2750mA, which is just above C/20. And the same datasheet recommends no less than C/20 termination current.. So very, very similar values, and definitely not “way, way beyond” a typical recommendation.Quote from: Siwastaja on July 02, 2021, 02:25:04 pmFWIW, the bq25895 I’m using is working great. It defaults to values that are conservative for an 18650, but offers a gazillion parameters to set via I2C — including separate precharge and termination currents.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 03, 2021, 09:46:32 am
content: Quote from: tooki on July 02, 2021, 05:12:28 pmYes I have read through that and I have decided I don't just trust Samsung on this. Read it again, carefully, and you'll see the text allows charging a cell at 0.0V or even -99.9V at 150mA forever with no timeout. This does not match the precharge as usually described. There almost always is some minimum voltage requirement which is more than zero, and also a timeout required in case the cell does not reach some "normal charge can begin" threshold like 3.0V here (which is a sensible value). Problem with high currents is localized heat generation at the site of the dendrite. And C/20 is surprisingly lot IMHO.After all, the whole point of the "precharge" feature is to see, carefully, with very low charging current, if copper dissolution has happened within the cell, and if it has caused internal shorts by copper dendrites piercing the separator; possibly even forcing these dendrites back to the copper current collector, at least to some extent. I.e.; see if the cell is damaged; try to recover; bail out if impossible. The only thing that makes this proceduresafeis correct settings and thresholds. You do detect the presence of the dendrites by the fact that the cell voltage does not rise while charging (slowly) within some sensible time (like 30 minutes).Or maybe Samsung has carefully calculated/simulated/tested that their 29E cell, completely overdischarged to any voltage, including negative voltages,cantake continuous 150mA forever without issues, just heating up some and dissipating that heat. But I wouldn't base a charger logic on this because others have stricter rules, and quite frankly, the fact youcando something does not mean you necessarilyshould, this includes not running at the maximum values.QuoteGood to hear that.Personally, I don't just bother with "precharge", at least not an aggressive one. It's relevant only in case of BMS or cell failure, trying to "save" a possibly slightly damaged cell. Primarily, the BMS should be fixed so that overdischarging doesn't happen in the first place. I refuse to charge cells below say 2.5V, which is the "classic" way.Maybe I limit charging current below say 3.3V but I won't call that precharge, but just because cells show significantly higher DC ESR near 0% SoC it's a good idea to charge more slowly to generate less heating power.

author: tooki
author_rank: Super Contributor
timestamp: July 03, 2021, 03:38:12 pm
content: Quote from: Siwastaja on July 03, 2021, 09:46:32 amIt took me a few readings of your claim to actually figure out what you meant: your wording makes it sound like it's allowing acharge current of 150mA at -99.9V, whereas you meant "the text allows charging a cell whose voltage is 0.0V or -99/9V with a charging current of 150mA…", correct?Anyhow, I think that's a rather contrived interpretation, since the datasheet expressly warns not to ever discharge below 2.5V.What is true is that they do not define whatshouldto be done with a cell that nonetheless has fallen below 2.5V. But there is no ambiguity that <2.5V is NOT an allowed condition. Since -99.9V<2.5V, and 0V<2.5V, they clearly aren't "allowing" charging in that state.A sensible interpretation of the datasheet is this: precharge current of up to 150mA from 2.5V to 3V, then regular CC charging up to 4.20V, then constant voltage to C/10.Quote from: Siwastaja on July 03, 2021, 09:46:32 amCorrect. But your assertions only exist by ignoring the maximum values given elsewhere in the datasheet. The datasheet says to use 3V as the 0% SOC, and 2.5V is given as theabsolute maximumdischarge voltage, not as recommended. 3V is clearlynotzero or a negative voltage.Quote from: Siwastaja on July 03, 2021, 09:46:32 amOf course. (Again, the 3V specified 0% SOC is greater than 2.5V, so they're clearly telling the designer to design around 3V, not 2.5V.) The datasheet also specifies maximum discharge currents for <3V and <2.5V, which is presumably to limit the chances of a cell going below 2.5V to begin with.Of course, the (separate) protection IC should be selected to enforce the absolute maximums as a last resort.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 03, 2021, 04:55:36 pm
content: Quote from: tooki on July 03, 2021, 03:38:12 pmYes but if you always follow the standard discharge ending condition there will never be a need for the precharge!The precharge is a conditioning step for overdischarged, possibly damaged cells. The problem with the Samsung instruction is that it lacks the initial acceptance voltage. I have seen values like 1.0V or 1.5V before.Quote... so you think they are implicitly saying (as follows from following other criteria) the initial acceptance voltage is 2.50V. This is an interesting interpretation, and a good one which I would like to agree with. But I'm almost sure theydon'tmean that (see later on my message about a contradiction); it's nearly the same as not doing precharge at all. If youdouse an initial 2.5V threshold for acceptance, this 150mA procedure is much safer and I have nothing against it.QuoteYes but the need for precharging usually arises from ignoring the given values. It's a well-known "rescue tactic" that varies from forbidden, to "dodgy but acceptable with a careful algorithm", to something considered a part of "normal operation".I like the middle one, of those three opinions about it.There is one interesting corner case though: very low-current discharge. This is because when a manufacturer comes up with the discharge end voltage, they do that on some certain discharge current, very often C/5. Samsung 29E datasheet, unsurprisingly, forgot to document this vital piece of information* as well. Now if you use a significantly lower current than what they did, the ESR * I drop over the cell is lower, you can discharge more capacity out of it before reaching 2.50 under-load voltage, and you end up at lower SoC% (per manufacturer definitions 0% being with the given conditions, this will be a negative percentage!), and lower open-circuit voltage. This will lower the amount of voltage bounce-back after the discharge. While a cell discharged at C/5 until 2.50V bounces back somewhere around 3.3V, one discharged at C/50 until 2.50V is slightly overdischarged and only bounces back to, let's say e.g., 2.90V.*) Again, I'm sure you can get the actual datasheet from Samsung if you commit to buy ten million cells.Now if you use thesamedischarge cutoff value (2.5V) as the acceptance limit to enable charging, there will be no problems. But if you use a higher acceptance limit like 3.0V, there might be: your gadget discharged the cell until end, and now it refuses to charge. This effect could be seen in some laptop batteries of early 2000's; they refused to start charging and you would need to inject a bit of charge into the cells using a lab supply, then it would start. It was a design issue because it wasthe laptop itselfwhich discharged the cells too far to begin with; OTOH, the cells were fine so maybe it was the charging side which had too high of acceptance limit. Maybe it was a "discharge to 2.5V, require 3.0V" issue with a misestimated bounceback.Thecorrectway to solve this problem at the root, IMO, is to make the low-voltage cutoff limit dynamic and based on current. This is often done for high discharge applications, but you should think about this for very low current applications as well, for example only discharging down to 3.2V and change that limit temporarily to 2.5V under short load peaks, for example when a radio sends a packet with high power.QuoteCertain open-circuit voltage corresponds to certain SoC% and that's defined by the actual cell chemistry and characterized by the manufacturer or the customer due to lack of manufacturer data availability. I have characterized the 29E and OCV for 0% SoC for this cell is roughly around 3.35V to 3.45V, I don't remember exactly. Edit: looked up that 29E discharged to 2.50V with C/10 cutoff bounced back to 3.304V OCV. With C/5 cutoff that would be a tad higher.I guess you are referring to this: "Cell voltage in SOC 0% of Pack should be Min. 3.0V."My interpretation is that after discharge is stopped i.e.load is removed, each cell in the pack should always readat least3.0V. Which is a very sensible requirement. It doesn't mean 0% SoC is at 3.0V.But that aside, don't you see the contradiction to what you said? If this condition is satisfied,there never is need for the precharge, as normal charging can commence already at 3.0V according to the very same datasheet! This IMHO confirms my interpretation that the precharging as described is a rescue procedure when other datasheet conditions have been already violated (in this case voltage < 3.0V). So if they assume this 3.0V is already violated (it's not a recommendation, it says "Requirement"), what makes you think the 2.50V limit is not violated, too?That's also why I don't like the fact this rescue protocol is so poorly documented and simplified, but it needs to be seen as providing some absolute maximums, i.e., never exceed 150mA below 3.0V. In my opinion, more limitations (initial voltage, time before giving up) are needed but these might be available in the full documentation.QuoteNo, you got this backwards. To prevent cell from going below 2.5V (OCV, no-load voltage, or <0% SoC), you need a MINIMUM not maximum discharge current rating! You need enough current going, generating high enough voltage drop over I*ESR, to ensure ending up above 0% SoC given 2.50V cutoff voltage.Samsung does not specify this parameter or any data helping with determining this value in this datasheet; I'm almost sure I have seen other similar-looking Samsung datasheets where the current used to obtain discharge cutoff voltageisspecified. But in this case, own testing is needed. Choosing discharge cutoff at 3.0V is obviously the easiest and safest choice and Will Just Work and satisfy all conditions given in the datasheet. It just greatly limits the available capacity in high discharge current applications.I guess you refer to this:"Shut Down Mode : Under 10uA / Under 3.0V.Under 1uA / Under 2.5V."This refers to their opinion about what is a good quiescent current taken from the cell by the BMS in "shut down mode", not maximum discharge currents when you are discharging to loads on purpose. Here the idea is to ensure the cell is not overdischarged over some (unspecified) calendar time due to the BMS leakage. My suggestion for a BMS designer is to always calculate this yourself leaving the necessary paper trail on the reasoning.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 03, 2021, 05:06:45 pm
content: Quote from: Siwastaja on July 03, 2021, 04:55:36 pmExcept that the battery could get into this condition just by self-discharge. You may object that a battery that got into overdischarged condition by self-discharge only has a good chance of being ruined anyway.But a slightly too low cut-off threshold + self-discharge could get you there relatively fast. To avoid this, you need to set a cut-off voltage with a comfortable margin. And then you're going to lose a bit of capacity. Sure that's a reasonable trade-off to make, but in some applications, engineers like to milk the last drop out of batteries.In any case, all those points are not fully trivial. Only reading datasheets and hoping to get things completely right with LiPo batteries is not a really good idea. Their handling still requires proper knowledge and proper care. Product recalls for risk of fire are not at all uncommon, showing that mistakes are routinely being made.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 03, 2021, 05:13:05 pm
content: Quote from: SiliconWizard on July 03, 2021, 05:06:45 pm... and I will do just that. Especially because according to my tests, self-discharge of good cells becomes completely negligible (something that can't be measuredat allusing conventional lab measures after 1.5 years of wait, even at elevated temperatures) already below 50% SoC.I do accept there can be some corner conditions where some amount of conservative "precharging" is a good idea. A slightly bad cell could get years of extra lifetime out of it while not significantly worsening the safety of the system. Or you could get a few % more battery capacity by being able to run closer to the 0%, reducing your margins. Which is important when energy density is critical for the application. If you can safely discharge to 1% and let the precharge handle accidental -1% and still do that safely, why not do that instead of always leaving 5% of margin just in case. (To be clear, I suggest you do add margins unless you absolutely must maximize the energy density, and in that latter case I also recommend paying for a better voltage reference and charge to 4.25V which is allowed in most datasheets.)QuoteI think with some Google-able Samsung datasheets - which BTW often say "confidential" and show the name of the customer who leaked it - you need to really be a lithium ion battery expert to understand what they areactuallysaying. Despite being quite well-read and real-life experienced on this subject, I often wish I knew more or had better connections inside the industry. I do know someone closely involved in cell manufacturing and design of cell chemistries but I feel awkward calling him every time I have some small question. Luckily I know someone else who can act as a proxy and has the necessary social courage to distract the Guru.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 03, 2021, 06:04:04 pm
content: Quote from: Siwastaja on July 03, 2021, 05:13:05 pmAbsolutely right.

author: tooki
author_rank: Super Contributor
timestamp: July 03, 2021, 06:13:48 pm
content: Quote from: Siwastaja on July 03, 2021, 04:55:36 pmI don't know why you're being so adversarial.Could Samsung have been clearer in their datasheet? Absolutely. But I rather dislike how you repeatedly insinuate that I've been deficient in my interpretations of it ("Yes but", "Yes but" ...). My initial comment to you was simply disputing your claim that having the same precharge and termination current is "way, way beyond what any manufacturer recommends", since this datasheet puts them notthatfar apart from each other, disproving that claim. That was all. I am not a battery expert, obviously, but constantly expanding the scope of discussion and bogging it down with minutiae, while actually ignoring the few points I have made, is not exactly priming me to be receptive to what you have to say.There is no "contradiction" in what I said, nor did I get anything "backwards". You literally quoted the two sections here that explain what I'm saying: I interpret their datasheet as saying "design for 0% SOC to be at 3V" and "make sure your doodad doesn't draw more than X µA current, even when turned off, so that you don't deep-discharge the cells, since they must not go below 2.5V".I know what you're talking about regarding minimum discharge current vs end voltage, but that wasn't what we were talking about. The point was thatthisis how a cell might end up deep-dischargeddespite adhering to the 0% SOC ≘ 3V requirement. There's no contradiction because ending up below 3V doesnotnecessitate violating the specs.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 04, 2021, 06:43:43 am
content: That is not my intention but I'm not always very good at expressing my intentions. I enjoy the discussion and truly hope you enjoy it too. I have been "accused" of sounding condescending before so it's likely a true observation; but in reality, I want as many as possible to be able to reach as deep understanding as possible; if that wasn't my intention I would write shorter replies only arguing about disagreements, not providing as much information. Or if I felt so superior I wouldn't bother to reply at all to mere mortals, instead sitting comfortable in an ivory tower.And I'm not sayingyouare contradicting yourself, I'm saying the Samsung datasheet is in contradiction if interpreted like you did even though your interpretation would be very sensible as it is.Because in my view, ending up below 3.0V is either because of a faulty cell or violating the specs, but I do admit there can be some interesting corner case I'm not thinking about. For example, this might be a case of me saying a one-in-thousand heavily-used-but-not-abused-for-3-years "cell is faulty" but Samsung saying "it's still barely acceptable". Maybe that imaginary cell increased its self-discharge at 0% SoC from basically zero to say 2%/month which still isn't much but requires precharging step if the pack designer didn't add margin on bottom.And of course, the fact they are allowing 10µA stillat3.0V in their BMS quiescent current requirement means it can go below 3.0V due to BMS, just sitting on the shelf. But IMO it is just so stupid to first allow slight overdischarge by BMS then allow a recovery precharge. Why not just require the BMS to be better designed not to do that in the first place. Especially because theyrequirethe BMS to have Iq < 1µA below 2.5V, if you can achieve this by design you can equally well achieve that at over 3.0V, too! That would solve the whole issue, they are making it more complex than it needs to be.Also, they allow much higher BMS Iq when in "sleep" mode whatever that is, without defining the criteria for exiting the "sleep" and entering "shut down". Maybe a compliant system is allowed to pull 250µA even at 3.0V when a user switch is in "power on" state or something stupid like this. 250µA would overdischarge an empty cell in no time.In any case, this doesn't matter so much because I think that your interpretation of the precharge step is better than mine and I would like Samsung to more clearly define it like you interpreted, with 2.5V initial acceptance limit. It's just the I don'tbelievethey meant it that way. In fact that would be even stricter I would like, I personally use something like 1.5V, C/50, 1-2 hour timeout to 3.0V, when I allow the preconditioning at all, that is.