Lithium Ion Cell Balancing - Page 1
Forum section: electronics / projects

author: SenSeS
author_rank: Contributor
timestamp: October 13, 2015, 01:55:41 pm
content: I’m making a Lithium Ion charger circuit and successfully used a Texas Instruments bq2057c to charge 2 Lithium Ion cells in parallel (4.2V). Now I want to do the same with the cells in series (8.4V). I read that when charging multiple cells in series you have to do cell balancing to take into account the difference in charge speed between the 2 cells to avoid gradually reducing the battery’s capacity.I bought the bq2057c in the first place because in the datasheet it said that it could charge 1 or 2 cells in series. Indeed, you can set the threshold level to 8.4V and the IC will charge the cells to 8.4V. The IC doesn’t balance the cells though so it might very well be that one cell is at 4.1V and the other is at 4.3V which in the long run will ruin the battery’s capacity. I don’t even know why anyone would want to charge 2 cells and not do cell balancing and therefore why this feature of charging to 8.4V is there. Maybe there is something I’m missing. Is it common to use multiple cells in series and not use cell balancing?I could use an IC that has cell balancing, or I could use 2 single cell ICs and use them to charge my cells individually. Which one is best, and what is the most common? What I don’t like about the cell balancing ICs (e.g. bq29330) is that they always come in packages with a high pin count. I can’t seem to find one with low pin count. Do they exist?

author: KL27x
author_rank: Super Contributor
timestamp: October 13, 2015, 08:41:43 pm
content: If you try it you might find balancing is very rarely a significant benefit. IME it usually occurs to a significant degree only after I have over discharged a battery to the point of damage.

author: nctnico
author_rank: Super Contributor
timestamp: October 13, 2015, 08:51:43 pm
content: You still need balancing in order not to overcharge cells. With 2 cells in series it is not much of a problem but with more cells in series you need at least protection against an overvoltage situation. Overcharging a Li-ion cell can go horribly wrong (call the fire department).

author: KL27x
author_rank: Super Contributor
timestamp: October 13, 2015, 08:56:03 pm
content: IME, up to 4S, no not at all. Charge all cells fully before assembly. Put fly leads on all cells for testing and balanc8ng. And years later find out it is really not necessary. That is my experience.

author: eas
author_rank: Frequent Contributor
timestamp: October 14, 2015, 04:17:36 am
content: Quote from: KL27x on October 13, 2015, 08:56:03 pmWhat is your sample size?If this is just a one-off and the OP is happy to check the cells and, if necessary, rebalance them manually from time to time, then doing as you say is probably fine.Cell balancing is one of those things that isn't that important when everything is going right, but its cheap insurance against the possibility things get out of wack, because once they get a little out, they'll probably get worse. Most likely, you'll have the pack die prematurely, but you might end up with a YouTube worthy event.Actually, we should be ask the OP: which lithium-ion chemistry, specifically?

author: SenSeS
author_rank: Contributor
timestamp: October 14, 2015, 08:31:17 am
content: Thanks for all the answers! Am I right in assuming then that for hobbyist projects it's probably fine not to do load balancing but that for real applications always cell balancing is used? Because I can't imagine a company running the risk of burning someone's house down, even if it happens only in 1 out of 10000 products.@eas: I'm not sure what kind of batteries I have at the moment. I found them in a USB battery that I took apart and they have "HLD 18650 220" written on them but I don't think this tells us something about the chemistry. I'll probably be using a Lithium Polymer in my final product because it allows for a variety of shapes. But again those can be made in various chemistries right? Is there a preferred/most common battery chemistry and what is the difference with regards to charging? I'm using an IC so I'm guessing not much, except for getting the appropriate IC?

author: IanB
author_rank: Super Contributor
timestamp: October 14, 2015, 08:46:21 am
content: You should balance the cells manually by charging each of them to exactly the same voltage before joining them into a series configuration. Lithium ion cells have an almost 100% perfect charge retention so once balanced they will tend to stay balanced as long as they remain in series and both cells pass exactly the same current at all times. Both cells should of course be very closely matched by type, lot number and a capacity test before being joined up in this way.There are a few kinds of lithium ion chemistry and some of them are less energetic in failure than others. However, the common kind found in cell phones and laptop batteries is not of the "safe" kind and can catch fire quite energetically if abused.

author: eas
author_rank: Frequent Contributor
timestamp: October 14, 2015, 06:16:18 pm
content: So, first off, I want to make a caveat that despite the interest in LiIon battery re-use expressed implied by my sig, I don't consider myself anything more than an unevenly informed layman on the topic of LiIon batteries.I asked about the chemistry because some are indeed safer than others. LiFEPO4 has 1/2 the energy density of the LiION cells typically used in consumer electronics, but is much less prone to thermal runaway from being discharged too quickly (something that can end up happening due to internal damage due to over charge/discharge). Historically consumer electronics batteries have typically been Lithium Cobalt Oxide, which have high energy density but also are prone to spectacular failure. Power tools have used Lithium Manganese Oxide, which is safer than Lithium-Cobalt, and supports high discharge rates, but has rather low energy-density. More recently, some consumer electronics have been using Lithium Nickle Cobalt Aluminum Oxide for highest energy density.Yours are probably Lithium Cobalt Oxide, and your choices for one-off purchase of pouch cells ("LiPoly") will probably be limited to the same + LiFePo4.There are two major risks when using cells in series.The first is that any imbalance in capacity, state of charge or internal resistance between cells can lead to a cell dropping below a safe internal voltage, which can lead to damage. That damage can reduce the capacity of that cell and increase internal resistance, increasing the likelihood of further damage in the future. It can also result in formation of bits of metallic copper which can lead to spontaneous overdischarge, thermal runaway and spectacular failure.The second is that differences in capacity and state of charge can result in overcharge of a cell. An overcharged cells can suffer from reduced capacity and increased internal resistance, which, again, can lead to further damage during future charge/discharge cycles.Over time, this can result in some really f'ed up situations. I took apart a bunch of Dell packs where a majority of the cells had reversed polarities, with others at nearly full charge. There were also packs that seemed to be at a safe discharge voltage, but actually contained two series cells near full charge, and one at a reversed polarity. Now, these packs had cell balancing circuits, but something was clearly defective. They were also intact, but it may that I just got the ones that didn't burn upTo avoid these problems, designers use a combination of approaches. To start they use cells with closely matched capacities and internal resistance. To that is added top-balancing cells during charging, cutting off discharge if the discharge rate is too high, and cutting off discharge if one of the cells drops below threshold voltage.In your case, getting a pouch cell pack that already includes two matched cells in a series is a good start. Confirm that they are balanced, and adjust them if they aren't. Then, limit your discharge to a carefully chose threshold voltage. To choose the threshold, find the manufacturers minimum discharge voltage and internal resistance specification and tolerance. Then figure out your maximum current draw and calculate the internal resistive loss at your max load, and at the limits of the IR spec tolerances. Use that to figure out your possible error in predicting the voltage of an individual cell based on the voltage of the series under load if internal resistance. Add your own safety margin. Adjust your discharge threshold upward to account for this.Doing so should start you out in the best possible position, and keep you safely away from the conditions that are likely to result in a death-spiral.Someone with actual experience probably has better advice though, but at the very least, I hope I've provided more context for understanding the tradeoffs.

author: mikerj
author_rank: Super Contributor
timestamp: October 14, 2015, 09:55:01 pm
content: Quote from: SenSeS on October 13, 2015, 01:55:41 pmYes, very common indeed. Pretty much every laptop/notebook has three or four LiIon cells in series (usually 3S2P or 4S2P) and no cell balancing is used.Quote from: nctnico on October 13, 2015, 08:51:43 pmNo you don't. You simply stop charging as soon as any cell reaches the termination voltage (and stop discharging when any cell reaches the lower voltage threshold), which of course means your battery slowly gets unbalanced. But that's exactly how notebook charging works and you can get through quite a good number of cycles if your cells are well matched to start with.

author: eas
author_rank: Frequent Contributor
timestamp: October 14, 2015, 11:50:42 pm
content: Quote from: mikerj on October 14, 2015, 09:55:01 pmI don't think so.I have in front of me a 1 quart ziplock bag filled with battery management PCBs pulled out of old laptop batteries. I'll pick one at random...It looks like it is from a Dell pack and the PCB has a copyright date of 2005. It has various chips on it, including two with markings that start "bq." BQ stands for Benchmarq Microelectronics, a company that made, among other things, battery management ICs. Those products have long been part of TI's portfolio though. Even now, TIs latest battery management ICs have part numbers starting with "bq"One of the chips is a "fuel gauge," the other is abq29311battery protection IC. Click the link. Look at the buletted feature list. What's the fourth item? What to make of that information?While you are thinking about that, I'll pick another board. This one isn't a random pick, because i looked for one of more recent vintage. In this case, the silkscreen is 2009, but my notes, while not as good as I'd like, suggest that, according the the pack's battery management system, it was manufactured in the middle of 2010. This one also has two "bq" ICs. One of them is abq29312, like the bq29311 I found on the first PCB I looked at, it is a battery protection IC, and not surprisingly, it has a very similar feature set.For those not willing to look for themselves, one of the features is, wait for it... "integrated cell balancing control!!" Now you might argue that the mere presence of that feature is not guarantee that the packs actually make use of it, and you'd have a point. I'm not going to bother to trace out to see if the necessary pins are connected to control a bypass element in order to provide more evidence that laptop packs do in fact use cell balancing, but I'll suggest a look at TIs offerings. An awful lot of them, including some new introductions, support cell balancing, suggesting that it is an important feature, even in cost-sensitive mass-market electronics. But I think it is more telling that some of them don't, and yet cost sensitive manufacturers chose chips, or more likely, packaging that went to the extra expense of including the extra pins needed to control cell balancing.Do you have other information we should be considering?

author: Stuart Coyle
author_rank: Contributor
timestamp: October 15, 2015, 12:30:32 am
content: I have used this series of chips for charging two Li cells and it works fine. I think that you won't want the bq2057c but the bq2057t or w variants to charge to 8.2V or 8.4V.Oh and I found out the magic smoke way that when they say the supply voltage max is 18V they really mean it.I guess that it really depends on your use case wether you want charge balancing of the cells.

author: KL27x
author_rank: Super Contributor
timestamp: October 15, 2015, 01:59:23 am
content: QuoteEas, I was playing with Li ion cells back when qualified EEs thought it was too dangerous to even attempt. Ask the question "how do I charge a little ion cell" on a forum like this (since this one wasn't born yet) and the only response you would get is "don't do it. You'll burn your house down."Just because laptop cells can go out of balance doesn't mean all li ion batteries need balancing. Some laptops are poorly managed and/or foolishly (cunningly) use batteries with too low a discharge rate to begin with and/or push too close to the empty line. This is intentional. If you use enough battery for the job and protect from over discharge, and you use matched and fully charged (initially balanced) cells, you might find this out for yourself. Laptop batteries that die prematurely are usually not from a failure of proper engneering so much as they are lasting exactly as long as the designers have chosen.

author: TerraHertz
author_rank: Super Contributor
timestamp: October 15, 2015, 05:53:12 am
content: Quote from: KL27x on October 15, 2015, 01:59:23 amWhich without charge balancing, will not be as long as the batteries *could* have lasted. This is called planned obsolescence. Also there will be outliers, in which greater initial cell miss-matching will result in some packs entering death-spirals, possibly terminating in fires or at least pack failure much sooner than expected.I get the feeling that because *you* don't use charge balancing in series-connected packs (which you get away with because you can monitor and manually balance when needed) you are arguing that no one should include balancing in commercial designs.Seems like faulty reasoning to me.A three cell series design I'm starting definitely will include cell balancing as well as over-under charge/discharge protection. And the circuitry will be in the device, not the battery pack. It's ridiculous the way that amount of circuitry is included in throwaway battery packs, imo.

author: KL27x
author_rank: Super Contributor
timestamp: October 15, 2015, 06:40:06 am
content: I don't get away with anything. Monitoring the manual header of battery packs has shown me that I don't need to balance cells. Under the conditions I have stated. I am satisfied to where I no longer wire this header in to muti cell packs. Unbalance is mainly the direct result of cell damage. If you want balanced cells and longevity you will avoid the damage in the first place. Do that and the packs stay remarkably balanced.Planned obsolescence is a dirty word. The preferred term is product life cycle, and there is nothing morally wrong with it. But to achieve desired results in this case takes more than to omit balancing. That won't cause significant premature failure unless the previously stated conditions are intentionally abused. Or if a smart controller simply shuts things down arbitrarily.This is my opinion, based on real world testing and observation. Your opinion may be more popular. And it is backed up with information that is easily searched on the web. But do you know 5he ultimate source and context of that information? Ask yourself. Am I simply repeating something I have heard 50 other people say, or am I citing fact?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 15, 2015, 07:44:21 am
content: Quote from: SenSeS on October 13, 2015, 01:55:41 pmThis is the wrong explanation.In series, all cells charge at exactly the same current.Percentual charging speed may differ due to the capacity difference, but balancing can't help in that, unless the balancing current is HUGE, balancing is redistributive (non-dissipative) and happens during both charge and discharge. This is mostly subject of academic discussion and not of practical benefit, as the complexity of this approach exceeds any benefits by at least an order of magnitude.Only thing that balancing does is to prevent slow drift caused bydifferences in self-discharge rates, which would otherwise reduce the pack capacity (with cell-level monitoring) or cause safety problems (without cell-level monitoring). However, proper quality li-ion cells have very small self-discharge rates to begin with, and differences thereof are even smaller.It is industry standard to put 2 identical high-quality cells (from proper manufacturers) in series, balance them once (i.e., they are both fully charged separately first), and then treat the pack as a single 7.2V cell. This practice is, or has been used by many many large electronic companies such as digital camera / camcorder manufacturers, power tool manufacturers etc. BOSCH goes as far as putting 6 cells in series without tap monitoring or balancing, but usually this practice stops at 2 cells.I disassembled a used BOSCH power tool 4s battery pack constructed from Samsung INR18650-13Q cells. Battery had increased its internal resistance too much for power tool use, and capacity seemed also smaller. The pack was 5 years old. All four cells were still in very good balance (measured to +/- 0.01V).Balancing is really unnecessary if the cells are good. But monitoring is highly suggested, or necessary. Monitoring means that you measure every cell voltage separately, and stop charge/discharge according to single cell voltage limits. If you do this, balancing is only an optional extra which only prevents very, very gradual loss of pack capacity, which might never even happen. Balancing is then a "nice-to-have" feature which is relatively easy to implement when you already have access to all cell voltages and can measure them. That's why it's almost always implemented in commercial battery management solutions. It's notcompletelyunnecessary, after all; it can help retain a bit more capacity in some cases.The really important caveat in both monitoring and balancing is that you don't want to introduce an extra source for leakage to the cells, so that your monitoring circuit would actually unbalance the cells. Be careful when designing.

author: mikerj
author_rank: Super Contributor
timestamp: October 15, 2015, 05:04:12 pm
content: Quote from: eas on October 14, 2015, 11:50:42 pmI know so. I have taken apart literally dozens of notebook battery packs and almost all of them failed by having one cell pair wildly out of balance with the rest (e.g. 3 cell pairs at 4.2v, one pair at 3.4v). Provided the safety fuse hasn't been blown, balancing the cells will restore most of the lost capacity.It's possible newer packs have balancing built in, but the old ones absolutely 100% do not.

author: amyk
author_rank: Super Contributor
timestamp: October 16, 2015, 08:34:19 am
content: That is more likely to be the outcome of a brief internal short or other defect that lead to one cell suddenly going out of balance, and the balancing circuit can only correct small imbalances.

author: Rick Law
author_rank: Super Contributor
timestamp: October 16, 2015, 07:43:09 pm
content: Quote from: mikerj on October 15, 2015, 05:04:12 pmHelp me understand. Wouldn't the pack go wildly out of balance again in a short time? The same reason that caused one of the three pairs to discharge more than the other two pairs would be right there still.

author: TerraHertz
author_rank: Super Contributor
timestamp: October 17, 2015, 01:45:00 pm
content: Interesting thread. Let's see if I've grasped the main points:* Monitoring of LiIon series pack individual cell voltages is essential, to allow stopping charge or discharge when the worst cell reaches its limit.* LiIon cells have extremely low self-discharge when good.* Allowing a LiIon cell to go outside voltage limits damages the cell, including increasing self-discharge and reduced charge recovery.* Balancing in LiIon is really only coping with significant cell differences in self-discharge. Which of course accumulate over time regardless of charge/discharge cycles when no balancing is present.* Poorly designed balancers can make LiIon balance worse, by mis-matched quiescent current.* Commercial LiIon series battery packs typically omit balancing, and most 'failed' battery packs are due to cells being greatly out of balance. Re-balancing them mostly restores such packs.Anything wrong or missing there?I think if designing a battery LiIon management system based on spare functionality of a micro used for other things, I'd still try to include balancing. For one thing it would give you tracking of the adjustments and their effectiveness. And that would give you advance warning of a cell starting to degrade.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 17, 2015, 02:11:16 pm
content: I will add my personal observations, which apply only to pouch type LiIon batteries used in radio control model airplanes. This is a demanding application with very high current discharges (typically 20-50 C). Batteries are often used to a fairly significant discharge with end state cell voltages of 3.9 or even 3.7 volts. There are wide variations in price which might be reflections of initial build quality, but my observations reflect all points on the price scale.1. Cells can, and do, become unbalanced. Usually they will last a few dozen charge cycles without balancing, but this does vary. A careful rebalancing will then last for more charge cycles, but always less than the first time around, and less each time.2. Chargers sold into this market typically do what is called a balancing charge. This alternates a few seconds of charging with a few second interval where high voltage cells are discharged through a load. This has the effect of letting higher capacity cells "catch up" with lower capacity cells. The discharge rate is often only 50 mA, so it cannot cope with a badly out of balance pack of cells. Balancing devices are available which do this selective discharge process for hours to balance more severely out of whack packs.3. Letting the magic smoke out is spectacular, and something to be avoided. Usually it occurs due to physical damage in a crash. Once it has happened to me for an undiagnosed reason during charging. Rather than take any chances I retired the charger. The event is common enough that a. Products to contain the effects of an unintended fire are widely marketed and used in the RC community. b. Several people in the community have lost a home, or garage or other structure due to battery incidents.Lithium technology batteries are not evils that should be avoided at all costs, but they do require respect.

author: akis
author_rank: Frequent Contributor
timestamp: October 17, 2015, 04:05:17 pm
content: Quote from: CatalinaWOW on October 17, 2015, 02:11:16 pmFully agree with everything you said.balancing is tricky with all the extra wires you need, unless you have some sort of enclosure with multi contact connectors and no moving parts. Otherwise you have thin wires directly connected to the cells no fuses no protection nothing, recipe for disaster.

author: KL27x
author_rank: Super Contributor
timestamp: October 17, 2015, 07:14:23 pm
content: My oldest li ion batteries are 18650 cells that were salvaged out of my personal 1995ish Dell laptop. I dont recall finding any problem with the cells, but it wouldnt power the laptop even for a second. The ESR may have been increased too high for their original purpose. Who knows. But they ran a 3S laptop for a full 45 minutes with wifi on and playing a DVD. Later, 4 of these cells made it into one half of a 4S pack I used to run a cordless drill. I paired these up with another set of 4 matched cells to make the 8 cell battery. This drill was used for maybe 6 years before I eventually upgraded to a real li ion drill. There was a header 9nl the pack for monitoring and balancing, but it never needed it. Then I dismantled that pack to recombine into anew 8 cell 2S pack (2 of each cell in each stage to run my Owen oscilloscope, which is where they currently reside.These particular cells have never been balanced. And they have never been balanced charged. Plugged into a CC/CV psu set to the sum total voltage, only. And they are still fine. Some of the earliest common knowledge i could find about li ion batteries came from a website called battery university. One nugget said that li ion batteries were doomed for death the day they were made with a typical life of about 3 years. The suggestion was that there was a shelf life, which is now largely known to be false information. Probably patent misinformation (or blind regurgitation of misinformation) intended to promote/prolong consumer acceptance of shortened product life cycle.The only time I have ever personally seen a battery go out of balance is when overdrawn or when acvidentally incorrectly charged. I would think in a laptop, a cell that is out of balance is already the beginning of the end and that it should be replaced. If you are eking out maximum cell capacity and using near max discharge/C of your cells, then balancing would be very important. You would be staving off issues from the beginning. If you are concerned about l9ngevity, you should preferentially aim for plenty extra C to avoid ESR/decay issues and avoid deep discharge. This would be much more important than cell balancing, IMO. That is my working hypothesis. In an app like RC aircraft, balancing cells should be more important due to the requirements.

author: amyk
author_rank: Super Contributor
timestamp: October 17, 2015, 11:16:11 pm
content: Another possibility is that 20 years ago, 18650s were built to much better standards than those today.Quote from: KL27x on October 17, 2015, 07:14:23 pmThere's some very interesting discussion on that here, including the quote about shelf life:https://en.wikipedia.org/wiki/Talk%3ALithium-ion_batteryhttps://en.wikipedia.org/wiki/Talk:Lithium-ion_battery/Archive_1...and this, which just leaves me speechless.QuoteCould that be about balancing?

author: richcj10
author_rank: Unknown
timestamp: October 18, 2015, 02:43:08 am
content: You really should ballance cells. Circuits ( w/ uP) that do this are really easy to put together.Sent from my SM-N910T using Tapatalk

author: KL27x
author_rank: Super Contributor
timestamp: October 18, 2015, 02:53:31 am
content: I dunno why people repeat this like a parrot without adding any new information.Just browsed the wiki for the first time. Noticed it makes the declaration that balancing is not necessary after a battery is balanced. I did not find a source.

author: akis
author_rank: Frequent Contributor
timestamp: October 18, 2015, 07:50:32 am
content: I have had 3 and 6 cell batteries at large capacities (can crank a car engine) and observed how they were being charged, the individual cells were many times falling out of line and the charger had to "balance" them, ie discharge the stronger cells until they matched the weaker cells, then start charging again. If you do get a weaker cell your total voltage will be less than nominal, but as the charger will not be aware you will end up overcharging some cells which is a no-no for LiIon baterries as is over-discharging (they last less)

author: KL27x
author_rank: Super Contributor
timestamp: October 18, 2015, 06:41:33 pm
content: QuoteI almost always observed some cells at slightly different voltages when partially discharged. Talking on the order of a hundredth of a volt or two. But seems to me that balancing them in this state could be counterproductive. I would leave it alone and the cells would all get back to 4.2V when charged. No need to discharge the stronger cell, else it will now not fully charge. Six of one, half a dozen of the other? I suppose that method of balancing would ensure that the weakest link/cell is fully charged. But now the cells might not all be exactly 4.2V at full charge. I'm sure individual monitoring and charging is a good idea in a battery that strong, anyhow, but this could be adding complexity in a simpler lower C project?

author: akis
author_rank: Frequent Contributor
timestamp: October 18, 2015, 07:14:09 pm
content: The great problem with LiIon batteries, especially the large capacities those capable of cranking car engines, is that they need some pretty hefty conductors as leads, and there are no switches, no fuses, nothing. Add to that a 4 pin or 7 pin conductor for the balancing. Now you are holding a live bomb in your hands, with no fuse, no capacity for self limiting, if you accidentally short something, I have instantly vaporised 2mm wide tracks on PCBs, there is no "kill switch" nothing to make you feel safe while you are working on them.As to charging the cells are brought down or up with a great accuracy typically 0.1V is more than enough to trigger the balancer circuit or the discharge cutoff onsome devices that use them. Except model airplanes that must fly no matter what, there the batteries have no such circuits.

author: jt
author_rank: Regular Contributor
timestamp: October 19, 2015, 02:55:34 am
content: So this thread got me interested if the Tesla Model S does cell balancing on all those 18650s.Someone did a bit of a tear down of the battery and posted here:http://www.teslamotorsclub.com/showthread.php/34934-Pics-Info-Inside-the-battery-packFrom what I understand the battery is arranged as follows:The series combination of six sets ofnparallel cells per module; 16 modules for ~4.1*6*16 = 393.6V max charge. I'm not sure whatnis.Further down the linked page, an image of their battery controller is posted. There is one controller board for each module. Each controller board has a header labeled cells 0:6 and what appears to be six identical front-end circuits. In each front-end circuit it looks like there is a parallel string of four 158 ohm 0805 resistors with a transistor (ex. Q1) attached. If this board does do balancing, I'm guessing the transistor switches in the resistor network to dump excess charge as heat, in a manner described as the "Passive Cell Balancing Method" in the following linked .pdf.http://americansolarchallenge.org/ASC/wp-content/uploads/2013/01/SAE_2001-01-0959.pdfOr perhaps the circuit doesn't do balancing and just monitors the cell voltages. Perhaps becausenis large, the cell performance is averaged-out to a point where imbalance does not occur on a significant level.What do you guys think? I don't have a lot of reverse-engineering practice.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 19, 2015, 06:26:30 am
content: Quote from: KL27x on October 17, 2015, 07:14:23 pmIn my experience, apart from the cells that were damaged by overcharge or overdischarge, the usual failure mode is increase in internal resistance until it no longer works in the intended application.At one point, I cobbled together a 3S8P pack out of several "bad" laptop packs and actually ended up with something that would run a laptop for a reasonable time. The charge metering was way off whack, so I later swapped it for a "mostly good" pack (nearly brand new, problem was a broken latch that was easily fixed) and put the hacked pack in my Instek oscilloscope that I converted to be able to run on battery power. Now I have a scope that will easily run over 15 hours on a charge, though it weighs about as much as an old fashioned "boob tube" scope and the metal chassis means it can't safely be used for high voltage isolated measurements.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 19, 2015, 06:55:19 am
content: Quote from: TerraHertz on October 17, 2015, 01:45:00 pmExtremelywell understood. You'll have a bright future in designing li-ion battery management systems, by understanding the basics right.Increased self-discharge may be the most typical failure mode. So a laptop pack which doesn't work at all due to unbalance and which can be fixed by manual rebalancing, has a cell that is consideredfailed. It's definitely notnormal, but it does happen.Sometimes the self-discharge increases a bit over the "failure" level, but the cell can still be used if balancing circuitry is provided. But it makes more sense to replace failed cells than to design and implement strong enough balancing which would cause significant waste of energy.But laptop battery management might not have balancing at all, or it may be designed forverysmall levels of unbalance, i.e., the implementation is a combination of small balancing current and a dumb balancing algorithm which cannot utilize time very well.QuoteI'd include balancing, too, because it seems so trivial to do once you already have monitoring. However, it's not completely trivial - you want to verify your balancing algorithm so that it works as intended (more "critical" code to work with), and you may have thermal design issues. Many people have opted to install a power transistor and power resistor to do the balancing in mere seconds during the CV phase of charging, by bypassing the charging current completely to hold some cells at their top voltage. IMO, this is stupid - if anything goes wrong, you have just generated a big problem, which can cause excessive heating and/or massive unbalance in very short time. In my Battery Management System, I balance at 40 mA - using two spare microcontroller IO pins with small SMD resistors. With small batteries (a few Ah), balancing current this small will be enough even when done at the end of charging. My BMS uses this small current even in large packs (up to hundreds of Ah), so a smarter balancing algorithm is used, so that it can memorize the amount of unbalance that was present at the end-of-charge, and do the actual slow dissipation process all the time, all the time during charge and discharge cycles. For example, a competitor uses 500 mA balancing current for typically less than a minute at the end-of-charge, while I use 40 mA for hours, if needed. Competitor needs an expensive through-hole power resistor, power transistor and a self-resetting thermal fuse for protection, while I only need two 0603 resistors in BOM. tldr; by all means include balancing, but keep it small and simple.

author: Warhawk
author_rank: Frequent Contributor
timestamp: October 19, 2015, 06:57:20 am
content: I am an R/C models enthusiasts and I totally agree with KL27x. In general, my 3s2p and 3s1p LiIon power packs have never had any balancing connector. I got rid of them this year, after 5 years of service. I probed them occasionally and there was not any significant voltage difference.Myexperience: LiIon does not need balancing.I also have a pack of 4s1p LiFePo4 (A123) - same experience as LiIon. The pack has a balancing connector so I use it for charging at home.LiPols are a bit different. I use a balancing charging even on the field, but when a charger starts balancing my pack It is a signal for me that I need a new pack soon.My rules of thumb:- Always discharge not more than 80% of capacity- Never try to get over "the knee" on a discharge curve- The battery temperature is ok if you can comfortably hold it in your hand after the flight- Charge 1C (1.5 - 2C on the field)- When you buy a crap battery, there is not any tool which may help them (balancer)- There is nothing better than standard CC/CV chargerWhen LiPol cells started be affordable for R/C hobbyists I remember that there was a lot of discussion and very innovative ideas on the model forums. However, as far as I know everybody went back to CC/CV, small balancer and personal responsibility.I also experienced a very poor implementation of safety switches in low power battery pack (e.g. my Futaba power pack) and the first thing I do is stripping them out of the system.In laptop industry, I've also seen a battery monitor based on ATmega, which constantly drawn a current only from a one cell (cost reasons). It also killed the battery pack.Here are a charger and balancer I used couple years ago:http://www.zajic.cz/omezovac/omezovac.htmhttp://www.zajic.cz/nablion/nablion.htm(use translator from Czech to English or just have a look at the circuit diagram).

author: akis
author_rank: Frequent Contributor
timestamp: October 19, 2015, 06:58:25 am
content: Quote from: jt on October 19, 2015, 02:55:34 amI think that the sleeping giant has awakened (VW) and soon we will see major changes in the alternate energy car industry.Somehwat less seriously, "the truth is out there", Fox would tell you that engines running on water or thin air have long been invented/copied from Alien designs but the inventors have all be assassinated...

author: Siwastaja
author_rank: Super Contributor
timestamp: October 19, 2015, 07:19:26 am
content: Quote from: jt on October 19, 2015, 02:55:34 amCorrectly analyzed, I think. I reached the same conclusion when I saw the teardown photos - they use standard dissipative balancing with relatively small balancing currents (around 100 mA, I guess). This is not surprising, as they need to have a cost-competitive product that needs to work, and they shouldn't need to satisfy purely academic interests by doing something that is not needed in real life, such as redistributive balancing.If you scale it down to laptop pack level, similar balancing current would be about 2-5 mA.Parallel cells are always treated as one bigger cell. It makes sense to theoretize that paralleling large number of cells help average out any slightly differing self discharge.

author: SenSeS
author_rank: Contributor
timestamp: October 23, 2015, 03:46:16 pm
content: Thanks for all your replies. Seems like I won't be needing cell balancing then but definitely should monitor each cell separately and switch off charging/discharging when the thresholds are reached. Are there ICs out there that handle all of this for me? Or should I monitor the cells myself with microcontroller pins and take appropriate action? I was wondering since - as already mentioned - this is indeed very critical code and it seems safer to have an IC from a well established brand take care of this.

author: jt
author_rank: Regular Contributor
timestamp: October 24, 2015, 09:57:48 pm
content: I've found that Seiko Instruments Inc. (SII) offers good minimalist solutions if all you need is cell protection. The TI and Linear Tech components can get overly complicated if you don't need the balancing and/or fuel gauging features.http://www.sii-ic.com/en/semicon/products/power-management-ic/lithium-ion-battery-protection-ic/

author: KL27x
author_rank: Super Contributor
timestamp: October 24, 2015, 10:33:02 pm
content: QuoteKilling your batteries in the name of "safety." When li ion was newer technology, it was all too easy to justify a 3-yr lifespan. Who the hell wants a battery that lasts forever? As consumers we do... but we really don't. It would kill the economy. IMO, releasing li ion batteries in "stages" of improvement was more of a deliberate choice than for "cost reasons."I remember when there was a certain phone that started burning people. It was determined that the phone's battery security measures would purposely turn on a high drain mode/shunt when a counterfeit battery was detected. So much for consumer safety.I don't consider myself a conspiracy theorist. Just practical. It's all about $$$.

author: johansen
author_rank: Super Contributor
timestamp: October 24, 2015, 10:48:30 pm
content: so we know that lithium ion batteries have almost perfect amp hour in = amp hours out, but what if they don't, we just haven't bothered to measure it carefully enough.

author: KL27x
author_rank: Super Contributor
timestamp: October 25, 2015, 12:21:17 am
content: Then it won't be almost perfect? And all your batteries will go out of balance, quickly....And you will be able to determine this in just a few days of real world testing.The guys that have done years of testing are coming to the same conclusion, that balancing is not necessary. And we could spend a lifetime on it, but we will never be able to prove that our sample size wasn't simply the 1% of the bell-curve by freak accident. It should be exceedingly fast and easy for someone to do real testing to prove that batteries need to be balanced.

author: johansen
author_rank: Super Contributor
timestamp: October 25, 2015, 08:48:22 pm
content: Quote from: KL27x on October 25, 2015, 12:21:17 amif the battery's colombic efficiency decreases with increasing state of charge, (meaning the batteries are more amp-hour efficient, at lower charge levels) then the batteries will self stabilize, so you spoke too soon.

author: KL27x
author_rank: Super Contributor
timestamp: October 25, 2015, 09:58:37 pm
content: Quote20 years, and no one has bothered to measure it "carefully enough." Then someone will be the first, eventually.QuoteIs this an observation that you have made? I assume you mean coulombic. And if we're on the same page about what "coulombic efficiency" means, I think it drops with decreasing state of charge,if anything. This may be why low voltage cutoff is important. OTOH, damaged cells may lose efficiency. They may continue to suck charge, forever, at 4.2V. That is an observation of mine, not complete speculation.I'm curious to see your reasoning for making this statement. Is this just idle speculation to make a rhetorical argument? Or are you the first guy that has actually measured things "carefully enough?"

author: SenSeS
author_rank: Contributor
timestamp: October 26, 2015, 11:36:54 am
content: While searching for a set of batteries I found that many already have an on-cell safety circuit that protects against overcharge and overdischarge. If I take a battery pack that already has this kind of on-board safety (e.g.http://www.powerstream.com/poly/GM555375.pdf,orhttp://www.farnell.com/datasheets/1441766.pdf) is it ok then not to have any protection circuitry on my PCB? Also, how common is it in consumer electronics to rely on this on-cell protection circuitry rather than using an on-PCB IC for this purpose?

author: Fungus
author_rank: Super Contributor
timestamp: October 26, 2015, 12:04:15 pm
content: Quote from: SenSeS on October 26, 2015, 11:36:54 amYes.Quote from: SenSeS on October 26, 2015, 11:36:54 amYes, but you still need to be able to switch between charging and draining.Quote from: SenSeS on October 26, 2015, 11:36:54 amIn devices where the battery is built-in? I imagine not very common. Attaching it to the battery will be more expensive. Those batteries are mostly for people who build their own devices.Some batteries have the protectoin built right into them - not an add-on cap:eg.The chips on that PCB look suspiciously like the chips on the right of this board:You can get them on eBay for about $0.50 each, eg:http://www.ebay.com/itm/221533778016(Randomly chosen seller).They have a DW01A for battery protection:http://www.hmsemi.com/downfile/DW01A.PDFThe two MOSFETS in the datasheet "Typical application circuit" are provided by anML8205AThe other chip on the board is a charger/switcher TP4056:https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdfYou can pretty much see the entire circuit in that pic (there's only two tracks underneath the board). It's taken straight from the datasheet. R3 sets the charging current.$0.50 each ... USB connector for charging ... charging indicator LEDs ... battery protection ... it's not really worth building your own IMHO.(Well, OK, it doesn't do temperature monitoring or balancing...only good for single-cell systems).Update: You can buy those battery-shaped PCBs separately. This auction has the schematic, and, yep, it's a DW01:http://www.ebay.com/itm/111725058450

author: SenSeS
author_rank: Contributor
timestamp: October 26, 2015, 04:37:44 pm
content: QuoteMy charging IC switches on charging when a power source is plugged into my device. But is it necessary to switch draining off when charging the battery? Can't I keep the load connected at all times to the battery? The on-cell circuit will stop it from draining when a single cell goes below its threshold voltage.

author: johansen
author_rank: Super Contributor
timestamp: October 28, 2015, 12:41:32 pm
content: Quote from: KL27x on October 25, 2015, 09:58:37 pmthere you go, that's why your batteries don't need equalizing, they are self equalizing.your observation that Coulombic efficiency drops with decreasing state of charge means that the batteries are not stable. the first one to get slightly more discharged than the other will slowly drift further and further down the state of charge march to zero.if however the cell loses capacity because it got discharged first, then it will return to 4.2 volts first, before the others, which as you say or speculated: is where they may suck charge forever at 4.2 volts.so again, self stabilizing.. in theory, provided you don't continue to over discharge the cell.

author: KL27x
author_rank: Super Contributor
timestamp: October 28, 2015, 09:09:30 pm
content: QuoteNot following. I agree on the first section. The cell that discharged more deeply is because it had a smaller capacity. And now it will take more coulombs to get back to fully charged, than the other, better cels. If you are doing simple sum CC/CV charging, which I am still doing, that cell would not quite get back to full charge and it would get worse on each cycle. The pack would quickly go out of balance. This is why I have stressed that if you do not balance cells, you must avoid deep discharge.To be clear, this is not an observation of mine. I do not know if it is because of decreasing coulombic efficiency, or if the cell just takes some damage at lower voltage which ends up decreasing the overall or upper end coulombic efficiency, but what I know is bad stuff happens when you overdischarge. And the bad cell will quickly die. The rest of the pack may be fine, if caught in time. Furthermore, I do not know if the overdischarge issue is caused solely by permanent damage, or whether there is a "gray area" where there is a temporary decrease in coulombic efficiency that would not decrease cell performance provided it were rebalanced, after the fact. But I am satisfied that if this grey area exists, it's so small as to be not a concern in general. Except for the most specific application that needs to eke out this extra capacity, it would be foolish to chase this storage by utilizing balancing. In my load discharge testing, the vast majority of a li ion cell's energy had been released before the float voltage gets down to even 3.3V.QuoteThis part doesn't match with what I see. The damaged cell takes longer than before to reach 4.2V, in general. Seems like it's leaky. There's maybe some greatly increased internal discharge going on. And at the point where a single cell battery goes to 4.2V (eventually) and continues to suck current forever, it is shot. The battery won't work well at all. I'm sorry, but your reasoning is simply incorrect and is not the case.In case you didn't read all my posts, I measured voltage at each node on occasion. If one cell were discharging to a lower voltage than the others at 90% depletion, I would have been able to observe this. I did observe some differences, but they were minor (and also remained consistent.) But for this reason, as a rule, as I add more cells in series, I would add a little more cushion for the cutoff voltage, to account for individual differences in cell voltage nearing the point of depletion. Nothing major. Maybe something on the order 1 tenth of a volt per cell, provided the cells are matched and balanced to begin with, to be safe. So if you find you can safely discharge a single cell to 3V under load, you might want to cut power at 6.1V for a 2S battery under similar condition. And note this is just an example. Depending on the load vs C, you might need to cut out the voltage at 2.8V on a high draw, or at 3.2V if the draw if very light.For the record, while I am fully satisfied in my own conclusions, I am open to discussion and disagreement. Everything I have done and observed over the years is the reason for my hypothesis, and I'm not going to try one more time, just in case. I have been and continue to use home-spun and home-charged batteries in my daily life for ever a decade. Plenty of other people don't agree, so to them, rigging up a battery and taking some measurements wouldn't be a waste of time. If their observations are in disagreement with my own, I would like to know about it.One additional data point: OWON scope takes a 2S battery. There are only two leads on the battery connection. My 3S li ion drill charger has 2 leads on the battery connection.

author: nukie
author_rank: Frequent Contributor
timestamp: October 29, 2015, 01:05:22 am
content: Quote from: Siwastaja on October 15, 2015, 07:44:21 amStill true for Bosch blue 18V coolpack. 4Ah 10x18650 Samsung cells in 5s2p. Or 1.3Ah 5x18650. No balancingtabs. In fact there is so little electronics in the battery pack. The cells are probably factory matched and they have so much confident that the cells will make it within the warranty period. I have run these packs more than 600 cycles, drained to 3.1v per cell and charged everyday for the past two years the cells still balanced proves that they don't need it.There is a few more power tool manufacturer who doesn't balance charge their cells. I need to Google that.Balancing is much pushed for R/C related pouch cells made cheaply and questionable quality.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 29, 2015, 06:37:33 am
content: Quote from: KL27x on October 28, 2015, 09:09:30 pmAre you sure there isn't a balancing PCB built into the pack? Every pack I have opened had one, although I suppose there are some cheap ones with cut corners...