Balance charging lithium cells - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: April 24, 2019, 10:29:38 pm
content: In pretty much every balance charger circuit I can find, they just use some braking resistors to discharge the cells that are charged to a higher voltage. wouldn't it be just easier to create multiple isolated supply lines and charge each cell to it's desired voltage, e.g: 4.2V and then stop charging all together? it should be more efficient too?

author: Miti
author_rank: Super Contributor
timestamp: April 25, 2019, 01:30:02 am
content: Quote from: OM222O on April 24, 2019, 10:29:38 pm...and way more complicated. Probably you're playing with RC balanced chargers. Technically, if the cells are well matched, you'll have to balance them only once, after that they should be pretty much charging/discharging in sync without much "braking". However, if the cells aren't perfectly matched, the circuit that uses shunt resistors, if properly designed, will give you a better capacity matching than the circuit that you proposed.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 25, 2019, 01:46:43 am
content: how come? with a voltage reference and an op amp, you can create a very exact cut off voltageyou create the desired voltage, e.g: 4.2 or 4.1 volts and use one comparator per cell to deactivate the charging circuit and since they are all referenced to the same point, then the only error would be the input offset of your comparator, which you can select to be very small, even less than 1mvalso there are plenty of DC-DC isolators available in through hole packages which do boost / buck conversion as well! they are limited to low currents (500mA max I believe) but it wouldn't be much more difficult to make small converters on your PCB eitheryou can also create small constant current / constant voltage charging circuit which adds more complexity than just charging with constant voltage and cutting off at some point, but it's presumably much better in keeping the batteries happy campers

author: digsys
author_rank: Unknown
timestamp: April 25, 2019, 02:03:45 am
content: For serious applications, we use ICs like this -https://www.analog.com/media/en/dsp-documentation/evaluation-kit-manuals/dc2064afa.pdfThere are a few manufacturers / designs out there already, and it very popular in EVsI NEVER have it permanently connected, always as a plug in - and only when the BMS says I need to. They are very efficient, also cascade able.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 25, 2019, 02:25:23 am
content: very interesting ... but it seems like a very specialty item?the method I proposed can be done with jelly bean parts howeveror another way would be to have 1 constant current constant voltage charger with it's cut off point set to a voltage reference IC, and use mosfets to effectively swtich in / out different batteries. this way the offset would also be the same, so the cutoff point is exactly the sameit would just take longer to charge the batteries which is not ideal

author: digsys
author_rank: Unknown
timestamp: April 25, 2019, 02:57:58 am
content: Quote from: OM222OPossibly correct, but it gives the OP another method / ideas. This IC (and others) do most the work, so all you need is some Si switchesand the inductor. Some users have trialed off-the-shelf ones from Digikey etc with pretty good results. No need to design it yourself.

author: mikerj
author_rank: Super Contributor
timestamp: April 25, 2019, 09:07:10 am
content: Quote from: OM222O on April 24, 2019, 10:29:38 pmHow would multiple isolated charging circuits be simpler than a balance circuit (comparator, transistor and a few resistors) per cell? It would be both more complex and considerably more expensive. It's also likely to be less efficient if cells are well matched.

author: David Hess
author_rank: Super Contributor
timestamp: April 25, 2019, 10:06:10 pm
content: An alternative I have though about is using an operational amplifier with a simple class-B output stage to force the voltage between two adjacent cells to the midpoint during charging. This could also be suitable for a bank of supercapacitors.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 25, 2019, 10:57:48 pm
content: Quote from: David Hess on April 25, 2019, 10:06:10 pmthat is actually a simpler approach, although still less efficient, but not by farwould you be interested in developing a prototype PCB?

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2019, 02:38:28 am
content: Quote from: OM222O on April 25, 2019, 10:57:48 pmYou are welcome to the idea. It is just one of those things I have been considering. It is awfully simple for what it does.It has the same efficiency as resistive loading but if only two cells are present, only one control circuit is required for the two power transistors. Power transistor dissipation can be limited with a resistor in series with the collector or emitter.With a micropower low input bias current operational amplifier, the circuit could be left in parallel and powered by the two cells continuously since when they are balanced, the quiescent current could be lower than the self discharge. That brings up the problem of whether it is a good idea to enforce balancing under all conditions.For a version which is not always attached to the battery, it would be relatively easy to add a disconnect to disable balancing and leave the output stage in a high impedance state. Some operational amplifiers have this built in.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 26, 2019, 07:51:08 am
content: why would this be as efficient as using a braking resistor? I'm not sure I understood what you said correctly, but what I understood is using an op amp and an output stage which forces the voltage to the midpoint between cells (i.e: taking charge from the battery with higher charge and forcing it to the battery with lower charge). this should be more efficient than just wasting it all as heat?

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2019, 01:14:57 pm
content: Quote from: OM222O on April 26, 2019, 07:51:08 amThey are both doing the same thing. With the operational amplifier, the transistor is being used as a variable resistance and dissipating the same power that the resistor would have to maintain balance. Power is power no matter how it is dissipated.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 26, 2019, 03:52:47 pm
content: Quote from: David Hess on April 26, 2019, 01:14:57 pmI didn't mean dissipation, I meant using the op amp to transfer charge between the batteries until they are balanced, therefore very little (almost zero) dissipation. Similar to what the ICs that digsys mentioned (check the app note / datasheet on how they balance the batteries).

author: Siwastaja
author_rank: Super Contributor
timestamp: April 26, 2019, 05:13:10 pm
content: What you describe is called "active" or "redistributive" balancing. While widely discussed on academical (and hobbyist) level, very rarely used in the real world. Simply because: li-ion cells have almost negligible self-discharge rates, and thedifferencesbetween self-discharges between the cells (which determines the balancing need) are even lower.Think about this: a typical cell self-discharges around 2%/year. Let's say it's neighbor self-discharges by 2.1%/year; the difference to balance out is 0.1%/year. At, say, 10Ah capacity, this is 10mAh per year. Assuming an example pack of 10 series cells at 10Ah, this would be around 0.37Wh wasted in heat - in a year! Approximating at $0.15/kWh, the cost for this electricity would be around $0.00005, per year.Now, passive balancing BMS cost per cell is about $0.50, and well designed redistributive is about $2.00 - assuming both types are cost optimized for mass market. (This is based on my own designs of both types, so I have a general idea of what it costs. Sadly, the differences are actually even bigger, since redistributive balancers are expensive niches.)Saving $0.00005 per year, the payback in a 10s pack (10 * $1.50) would happen in 300000 years. Of course, assuming that everything else, such as quiescent current, stays the same, which is a ridiculous assumption.In larger packs, the numbers are less ridiculous. Yet, it's highly improbable that an active system ever makes sense, even in large systems. They are used in very specialized conditions. Another chemistry in the future might require the re-evaluation of redistributive balancing; li-ion is just too good.Of course, if this pack is in any electric vehicle, the extra consumption of carrying a few dozen grams of extra electronics with you makes the bottom line negative.Factor in any cost for developing the more complex system, and it falls apart completely.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 26, 2019, 06:13:57 pm
content: I haven't done any actual testing with batteries, but when it comes to capacitors (especially super capacitors) they need to be balanced every single time as they have different capacities and discharge and discharge at different rates and become unbalanced very quickly. the cost of that would add up very quickly in the waste heat (especially given the main goal is charging at insanely high currents. e.g: 5 or 6 amps!). So I would assume batteries would be the same and discharge at different rates (regardless of self discharge). especially the recycled ones that are used in most DIY power wall type applications where the batteries might not even be the same model (e.g: mixing samsung cells with LG ones). you don't want to be throwing out tons of your input power simply because one cell has been charged to its limit.I agree in a single pack that wouldn't make much sense but if you scale it to a power wall type project, waste energy scales just as much so efficiency is important.

author: digsys
author_rank: Unknown
timestamp: April 27, 2019, 12:02:23 am
content: Totally concur with Siwa.. on his comments, that's why ours is a plug-in maintenance option, NOT embedded in the pack. It is only ever used IF the BMS saysit needs it. Race batteries do get stresseda heck of a lot morethan any other use scenario. To complete an event, we may go well past "ideal" operation.Also, NEVER leave ANY active device (capable) on a battery pack. It can be very dangerous, should it fail, and transport options are FAR more restricted thana pack of raw lithiums. We've seen the after effects of "shunt" failures.Super caps are another ball game. I have been working on a different implementation of super-cap usage the last couple years. Hope to have results in a fewmonths. It will do away with the very wasteful balancing problem.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 27, 2019, 12:13:13 am
content: so you mean once the batteries are balanced initially there is no need for balancing them ever again?then why do balance chargers even exist as opposed to a CC-CV charger with different cut off voltages for different packs?

author: digsys
author_rank: Unknown
timestamp: April 27, 2019, 12:29:29 am
content: There are many answers to THAT question. I have to run very soon, so I'll do a couple -We receive "hand matched" cells from manufacturers, who are our sponsors. ALL production processes have tolerances, and there is NO WAY they can affordto batch match every single cell (we're talking about millions of cells). They have no idea what pack sizes are being made anyway. So they ship them toequipment / vehicle manufactures. They ALSO do not have the time to "hand match" every cell ! so a simple / cheap balancer is installed.In our case, we STILL spend the time to run 3x FULL EXTRA balance cycles on our already "specially" balanced batch, and always find 1-2 out.BUT, once they are matched well, AND (big AND), the cells are run within limits, they rarely ever need it again. At the very least not under 1-2 yrs, and thenit may be minor. Outside of "proper" operation conditions, which people often PUSH, all bets are off. On top of that, people are often "re purposing" cells etc.In short, it is a cheap / easy way to have them start life as best as possible. more later

author: Siwastaja
author_rank: Super Contributor
timestamp: April 27, 2019, 06:18:11 am
content: Quote from: OM222O on April 26, 2019, 06:13:57 pmSupercaps and li-ion cells are completely different beasts. Don't assume any similarity.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 28, 2019, 12:26:30 pm
content: Quote from: OM222O on April 27, 2019, 12:13:13 amNo one said that. This being said, sometimes this is the case.I'd guesstimate thatmostof the commercial multi-cell li-ion packs do not use either balancing or even cell-level monitoring. At least considerable number; including well known brands, without any problems.QuoteSometimes the leakage differences are large enough to shorten the product lifetime. My previous example number set isn't such a case; it would do fine without balancing. But assume 10 times worse leakage difference; this is still possible in the real world. Now, if the battery unbalances at about 1% per year, it starts to be a real problem at about 5 years - so balancing helps. But even then, the "payback for redistributive balancing" would still be 30000 years!So, lithium ion cells tend to bealmostgood enough to require no in-system balancing whatsoever. This creates the awkward "it depends" situation, necessitating real system level understanding.In reality, sometimes balancing is needed, sometimes not. When balancing is used, it doesso little workthat it's important to optimize for:* Reliability* Low costinstead of "efficiency", or a fancy feature set. I have seen so many utterly failed li-ion balancer products that started from a complex list of feature requirements, even though in reality, they are irrelevant.So when the balancer is needed, it needs to reliably sit there, and do it's minuscule yet important job when needed, and only when needed - and correctly.

author: KL27x
author_rank: Super Contributor
timestamp: April 28, 2019, 09:30:54 pm
content: QuoteIn my opinion, the balancing is rarely necessary for increasing cell life, regarding longevity or number of cycles. But it can very slightly increase the cell capacity right after a full charge. When I say slightly, I mean very slightly, and this applies mostly if the battery is used immediately after charging it. So balancing would be important for something like an EV, where you have massive weight/space in the batteries. Or a cell phone, which it going to be on 24/7 and sucking battery right after you unplug it. (Not that a single cell battery ever needs to be balanced, of course).But in many other uses, it is not going to do a whole lot of improving. Say for something you recharge then put away. And it may be more effective just to buy batteries with a larger capacity to start with and/or adding more and/or larger batteries.This is because my experience is that the bad/weak cells start to self-discharge faster, but it doesn't appear to be constant/linear. The bad cells seems to quickly SD from 4.20 down to say 4.00-4.08. So even if you go through the hoops to get all cells in a series to 4.20, the bad cells lose that last bit of full charge relatively fast, anyway. So in balancing them, you are not necessarily fixing something that is constantly drifting farther out of whack until it is causing some major problem. Those individual cell voltage discrepancies will reappear on the very next charge. You will have to re-balance them almost just as much on every subsequent charge cycle until you replace the weak links, and you are doing that only to reclaim a relatively small increase in the total charge.This is somewhat limited in sample size. I'm probably over-extrapolating and oversimplifying and jumping to some conclusions. People are still designing products that occasionally catch on fire. But I wonder how much of that is simply due to manufacturing faults in the cells, themselves.edit: Hmm, I guess the way to describe it is that assuming there IS a creep over time issue that is actually corrected by the occasional cell balancing, this issue might be riding on top of a larger discrepancy that is not getting "fixed" in the same way.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 28, 2019, 11:05:47 pm
content: if what you are saying is correct, then one should be much more concerned with the health of their pack, rather than it's balance. I.e: measuring individual cell voltages and comparing how much they discharge and warn the user if one or a few of them are degrading much faster than the othersthis will prevent some major failure and the batteries are cheap enough to replace anyways!

author: KL27x
author_rank: Super Contributor
timestamp: April 29, 2019, 12:02:33 am
content: I'd put it thatIF one is more concerned about pack life than capacity per charge, it MIGHT be in your best interest to only balance the cells every X charge cycles rather than on every single charge cycle. But I don't know for sure if that is the case, even if this is correct under at least some specific set of circumstances.This also assumes to some extent that the battery might be maintained by cell replacement rather than fully replaced. Like if your parameters say the device must run for X time for the battery to still be considered good, the one that is balanced every time might meet that for a higher number of cycles but in that time suffer slightly more decay/wear on the good cells... but no one cares, because at that point the entire battery will be replaced, anyway. The one that is not resistor-bled-overcycled as often might drop below that X limit sooner due to that weak cell not being corrected on every cycle.... but it might be in better shape after some maintenance including replacing individual cells.

author: jbb
author_rank: Super Contributor
timestamp: April 29, 2019, 04:07:05 am
content: It may not happen very often, but if you get a small self discharge imbalance in a series pack then you can lose huge amounts of capacity. I’ve had this happen to a couple of infrequently used packs; you take a few hundred mAh out and one cell cuts off with undervoltage. Then you change it and another cell cuts off with overvoltage.For your own use, you could probably ignore it and see what happens.For mass production, you’d run the risk of customer dissatisfaction and needing to replace packs.In any case, LiIon and LiPo packs should probably include under and over voltage protection for every series cell. Because fires are bad.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 29, 2019, 03:24:33 pm
content: Quote from: jbb on April 29, 2019, 04:07:05 amOn the Internet forums, everybody "knows" this is always required, otherwise a fire ensues.In reality, BOSCH products do not go up in flames, despite not having any series cell monitoring whatsoever.Most fire incidents are due to cell manufacturing defects, typically in single-cell applications.But, your distinction between monitoring and balancing is a correct and an important one. Cell voltage monitoring may be important for both safety and functionality. Balancing is a secondary improvement which may or may not be used with cell voltage monitoring, to prevent pack capacity reduction caused by imbalance (where OVP and UVP events are caused by different cells). Balancing rarely "protects" the pack.

author: paulca
author_rank: Super Contributor
timestamp: April 29, 2019, 04:00:09 pm
content: Something I have looked into before. It came down to, yes, you can do non-resistive balance charging but it's (1) much more complicated and (2) takes much longer.However, I wanted to point out that those who believe that you don't need to balance charge very often obviously do not use multi-cell packs in the real world very often. EVERY time you charge a multi-cell pack you MUST use a balancer. On a good pack you might only see 100mV difference during charge or discharge, however with a straight 12.6V charge across a 3 cell pack you could end up with 200mV inbalance and charge a single cell to 4.35V or higher easily. If you are high current charging the pack they tend to explode in flames at that voltage.My experience is in high current LiPos discharged at 10C and charged at 2C, 100mV imbalance is not uncommon and depending on use they can be much, much higher. Consider that LiPo alarms or other voltage monitors tend to take their running power form cell 1. It's also not completely uncommon to pull 8.4V off cell 1 and 2 on a 3S or 4S pack to power radio gear, although generally frowned upon, it's still done.Resistive balancing is loseful. The total amount of loss would need to be calculated. It would be interesting to see how much power is actually lost using a set of LiOns such as Adam Welch's solar shed setup.

author: thm_w
author_rank: Super Contributor
timestamp: April 29, 2019, 09:20:45 pm
content: Quote from: paulca on April 29, 2019, 04:00:09 pmBut some real world packs do not use a balance circuit, as I was told by Siwastaja. Even in microchips multi-cell 18650 charger app-note they left it out.I just checked a two cell pack here I have been using for a few months (numerous charge and discharge): 3mV difference in cell voltages (3.880 3.877V). Of course the capacities are very close, I can't remember the specifics but I think within 50mAh.High current charging for sure may be a different story, this is only charged at 1A max.

author: Nitin25
author_rank: Newbie
timestamp: April 30, 2019, 07:01:46 am
content: So which logic you have used to sense individual cell voltage.how to sense effectively individual cell voltage with microcontroller if there are more number of cells in series. As microcontroller works only 5 volt.

author: paulca
author_rank: Super Contributor
timestamp: April 30, 2019, 07:13:02 am
content: Quote from: thm_w on April 29, 2019, 09:20:45 pmYes, but this is cheating you. What will happen is as soon as the first cell reaches 3.0V the pack disconnects. As soon as a single cell reaches 4.20V the charger disconnects.This is exactly why laptop packs without balance circuits have such a short life span. It's obviously for one reason. It makes companies more money.How many laptop packs have been taken apart by cell refurbishers to find one single parallel cell set at 2.9V but the rest are absolutely fine, yet the pack is unservicable.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 30, 2019, 11:14:14 am
content: there seems to be different schools of thought here like alwaysAs I mentioned earlier the packs that I plan on making are DIY packs made from recycled laptop packs which as pointed out earlier, only have one or two bad cells but the rest are fine.so there will be no guarantee of capacities being equal or even the cells having exactly the same chemistry.A few pointed out that if a cell is not charging like the rest it would only mean that they are band and will fail soon so replacing them is best, but I'm still not sure how a pack can remain balanced when cells have different capacitiesyes, I know I compared them to super caps which are different, but the main reason that they charge to different voltages and need balancing every time, is exactly because they have different capacities. maybe I should do some real world tests after I build the packs and do 10 full charge / discharge cycles using both methods and compare the voltage levels of each cell at the end of each charging / discharging cycle to see after the initial balancing, if the cells drift again or remain close together.

author: paulca
author_rank: Super Contributor
timestamp: April 30, 2019, 01:12:08 pm
content: Generally, as a rule of thumb, do not mix different capacity cells in series... or parallel for that matter.If you watch through some of this guys videos he provides links to software designed for what you are doing. Basically you capacity test all your batteries, enter them into the software and it gives you the best way to lay them out in parallel and series. He also shows various ways he has used to balance them. His latest videos show an elaborate system where each parallel cell pack has a BMS itself and these chain together over I2C (or similar) to communicate with monitoring software.

author: paulca
author_rank: Super Contributor
timestamp: April 30, 2019, 01:14:49 pm
content: Also, just a polite reminder that Lithium cells are genuinely dangerous. Unprotected cells can get pretty upset if overcharged or shorted. The metal 18650s can still swell, rupture and spray flames. Just be careful and don't charge them unsupervised unless you are totally sure of your setup.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 30, 2019, 01:21:11 pm
content: the protection is already taken care of. I'm just not sure about balancing them or not and if I decide to go down that route I want to minimize losses

author: Siwastaja
author_rank: Super Contributor
timestamp: April 30, 2019, 01:47:29 pm
content: Quote from: OM222O on April 30, 2019, 11:14:14 amThis is the classical difference between hobbyists consuming "for hobbyists" products (RC toys), and li-ion battery analysis, pack construction and management professionals, such as myself.The mantra of "absolutely needing balancing" is ubiquitous on forums, yet hilarious if you have ever looked at real-world products. Apparently the world's biggest and most well known companies have no idea what they are doing. HobbyKing School of Engineering knows better!Of course, if your target is just a hobby, then it might be wise to listen to what other hobbyists say, and use the designated hobby products; they have a wide audience testing the products.If you are actuallydesigninga product, go with the real-world information from the industry. It's a long road anyway. Completely ignore any RC hobbyist information, Battery University, and similar sources. Peer reviewed scientific studies are better, although slow and inefficient way to learn. If someone is talking about "li-ion and LiPo", it's a good telltale sign already they have no idea what they are talking about.Even better, do teardowns of commercial and industrial li-ion packs. You'll seemanydifferent approaches!As said earlier, non-balanced, even non-monitored packs are absolutely ubiquitous in consumer and industrial products; especially in 2s and 3s, but fairly commonplace up to 6s. I have measured a few BOSCH power tool packs that, after End-Of-Life of years of hard use and highly increased DCR, are still in balance within 10mV at top, using Samsung SDI cells. They do not include any electrical connections to the series taps. It's highly possible some units are worse, and do imbalance, but it seems they still don't go up in flames. OTOH, we have seen quite a few incidents of flames and early destruction of cells in laptop packs which do include balancing.In general, if you don't know what you are doing and need to design a safe system, I recommend you doneitherkind of a system. Keep learning, and in the meantime, use something off-the-shelf that everybody uses already; it's a good way to hobby around, but having used a battery analysis toy from HobbyKing to measure a Chinese toy battery from the same place doesn't magically make you an expert on battery tech.

author: paulca
author_rank: Super Contributor
timestamp: April 30, 2019, 02:08:31 pm
content: Quote from: Siwastaja on April 30, 2019, 01:47:29 pmSo if I show you batteries out of balance I would clearly be making that information up and we are both hallucinating as clearly they don't go out of balance in your world. Laptop packs don't fail either because of cell imbalance.Also, "Hobby" batteries are most often used in different ways, surprisingly far more intensely. You show me a power drill that will pull 100A, 200A peak and drain it's battery pack in under 7 minutes. Then show me the person who expects that pack, cooled, charged and flying in the air gain in under 30 minutes.I'm sure that if you buy expensive matched, low resistance, batch matched cells for £4 each and you charge them at low rates and discharge them at low rates and seal them in a can preventing anyone accessing them, they MIGHT remain in balance for years. However not all of us live in the world where we want to spend £65 on a new battery pack.On your advice I will now use "CHARGE" mode at 2C rate and not "BALANCE CHARGE" mode when I charge my LiPos. As you state I am completely safe doing this. So I will. I assume it's okay to do this while my daughter sleeps upstairs and leave them unattended, yes?Also, we had the conversation before on LiIon versis LiPo, we agreed I think that while it has nothing to do with the chemistry, that marketing and branded has polluted that completely (LiPo just means pouch cell these days) and there is little to now way to tell, asides a datasheet what the actual chemistry is in both cases. In consumer grade batteries, typically LiPo = High current, fast chargeable pouch cell. LiIon is generally a low current, hard cased, slow charge battery. Neither really are those chemistries/technologies anymore, but they definitely NOT the same type of cell either.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 30, 2019, 02:14:32 pm
content: Quote from: OM222O on April 30, 2019, 11:14:14 amThey will be balanced only at one point - typically at "top". Everywhere else, they'll be out of balance, until you are at top again - necessitating per-cell* cutoffs for charge (OVP) and discharge (UVP). But theydoremain balanced (at that same point) because same current runs through every cell, and they all charge and discharge the same number of Coulombs. (Coulombic efficiency being around 99.99%, with little difference between the cells.)So when we talk about "balance", what we really mean is balanced at one point.*Cell as in series element. Multiple parallel cells count as a single cellNow, to came back to your question about redistributive balancing; if youreallyhave a pack where capacity differences are large, you would (theoretically) want "dynamic balancing" - balancing current which is high enough to make large balance differencesduringdischarge cycle. I.e.:Cell 1: 100AhCell 2: 90AhCell 3: 110Ah"Normal balancing": all are at 4.20V at the same time. Pack capacity is 90Ah, when Cell2 hits 2.80V (or whatever you use)."Dynamic redistributive balancing": All are at 4.20V at the same time. While discharging, charge is moved from Cell3 to Cell2. Pack capacity is 100Ah. All cells hit 2.80V at the same time.Now, unless you discharges are super slow, you'll quickly see you need enormous balancing power to achieve this. Making the cost higher (easily at least around $5-10/cell, even if optimized for cost in masses), also weight (transformer magnetics). So, such systems are mostly of academic interest, they almost never make economic sense;evenin second-life applications.The largest completely repurposed pack I have built was a 85V, 250Ah (around 21kWh) pack for an EV, some cells were damaged by overdischarge (by a redistributive, expensive, broken-by-design BMS, by the way!), meaning I had bigger-than-normal differences on leakages. Still, 20mA balancing current (resistive) was enough to keep the system in balance. In any case, I carefully tried to match the capacities, just by measuring all the cell capacities beforehand and working up combinations where the capacities match as closely as possible. This is the preferred way instead of dynamic redistributive balancing: you'll have much lower-cost system, with less extra weight and complexity. So, instead of 90, 100, 110Ah, you would have 99, 100 and 101Ah, resulting in a 99Ah pack.If you are building a large second-life pack out of small cells, such matching is fairly easy to do. Remember that if you parallel different cell brands/types/conditions, you risk problems with current sharing, so may need to derate both charge and discharge current.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 30, 2019, 02:16:42 pm
content: Quote from: paulca on April 30, 2019, 02:08:31 pmThat's not at all what I wrote; a complete strawman. I have developed several li-ion balancing BMS systems;includingthe redistributive ones asked by the OP, because, yes, balancing is often needed. Please reread carefully; if you are unable to understand, it will be your issue, not mine. Thank you.

author: paulca
author_rank: Super Contributor
timestamp: April 30, 2019, 02:35:02 pm
content: Quote from: Siwastaja on April 30, 2019, 02:16:42 pmIt just seemed from how brutually you attacked "hobbiests" on their understanding of lithium batteries that they don't go out of balance. You seemed to suggest that professionally developed products don't even consider balancing as there is no need to.I frequently (after a sunny week) run a laptop from RC LiPos charged off a solar panel to use up excess solar power from a battery only system. They are always balance charged, but as they are unprotected high current packs (I don't know if you can protect packs capable of 100A constant). I run a LiPo alarm on the pack which cycles through cell voltages so I can literally watch the pack unbalance. Within about 5 minutes of running the laptop they will go out of balance by a few tens of mV. By the time the pack is flat the total output voltage is greater than 3.0V per cell, but one cell will set off the alarm as it's below 3.0V per cell.In fairness the lipo alarm (a voltage sensor, LED display and smoke alarm sounder) is running off cell 1. But without that they still go out of balance and cells 2 and 3 are not equal either. Maybe this could come down to cheap hobby lipos. However I have seen similar tests with expensive LiPo packs from top name brands. Maybe if you approach LG or Samsung and make a commerical order for exactly matched and batched cells you might get better, I don't know.Once thing to consider and you might experience this in an EV, when tightly packed into a pack the middle cells are exposed to higher heat and this, I believe changes their internal resistance which under high load will cause imbalance to build up pretty quickly. Although, if you are dealing with 18650s this is probably less of a concern as their total pack density is poor compared to tightly packed LiPo flat "slab" cells shrink wrapped into packs.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 30, 2019, 02:53:31 pm
content: Quote from: paulca on April 30, 2019, 02:35:02 pmSorry 'bout that, I can see my poor choice of words...Quote... very hard to see how you would make this interpretation, though. Your claim, instead, was absolutely clear: balancing is absolutely necessary, always. This is untrue.QuoteI see. You are mixing up two things here (see my previous reply). Having different cell voltages outside the "balance point" (typically top), doesn't mean the battery should be called "unbalanced". Try charging it upwithoutbalance charger, and you'll see the voltages match again! It was (top) balanced all the time! Now, do it 1000 times (and/or wait for 5 years) and you may be developing an actual imbalance; forthis, balancing is needed. It would never happen during one cycle, unless the battery is dying in your hands. Now, if you started to balance during this discharge, you would be out-of balance from the top, and would need to balance again in opposite direction during charge! This would be a massive waste.To avoid confusion, it's better to talk about capacitymismatchedpack, and not use the term "balance" to describe what you are seeing.Quote.Yes - necessitating either cell-levelmonitoring(Low-Voltage Cutoff), or safety margin (i.e., use 3.10V instead of manufacturer-specified 2.50V or whatever). The smaller the number of cells in series, the more a single cell contributes to the total voltage; hence, 2s untapped packs areverycommon, and over 6s, all packs tend to use cell-level monitoring (highly recommended unless you have a very good reason not to). Neither case still requiresbalancing.QuoteThis is true. Higher temperature increases leakage (self-discharge) current, causing slow imbalancing. Large systems are difficult to thermally couple, again necessitating balancing. Luckily, large packs are expensiveanyway, and tend to have some complexity in any case, so adding a simple cell-level management system with simple resistive balancing tends to become a no-brainer.Note that still this is a phenomenon that happens slowly (during years) if not handled. Don't mix it up with varying voltagesduringa cycle; it will be back "in balance" again, even without balancing. In fact, trying to balance "mid-cycle" causes imbalance; hence balancers typically work with full battery, during the end of charge cycle, or, if they are smart enough (such as one of my BMS design was), they sample the balance at the end of charge, then can do the actual dissipation work at any state. This was my trick of reducing the balancing current from the 500 mA of the competitors down to 40mA, yet still do the same work (or even more), making thermal design and safety easier to achieve. Yes, I saw a report of stuck-on 500mA balancer resistor burning a car. I was actually able tofusethe cell connections with 100mA fuses.

author: paulca
author_rank: Super Contributor
timestamp: April 30, 2019, 03:08:58 pm
content: I my case my LiPos are all high current discharge packs with no cell protection. I'm not sure you can get cell protection for a 5Ah 30C cell capable of 150A, I'm not sure how you would switch that kind of current without heavy bulky mosfets. I have seen people take the LiPo alarms I have and disconnect the sounder and connect a relay, but that's also bulky and heavy.In their real application the brushless motor speed controller connects to the balance lead and watches cell voltages. When one falls under 3.30V (normally, but configurable) it starts a "soft shutdown" in that it limits your power progressively. This usually gives you 30 seconds to a minute to make a controlled landing. Hard cut off happens at a lower voltage and forces you to do a dead stick landing which can be messy with a plane and very difficult and usually fatal with a heli.If I was to try and test "top balance" I don't think my charger would permit it. In "CHARGE" mode with no balancing it will charge until a single cell his 4.20V and then end the charge, regardless of the other cell voltages. I could I suppose try this and if it is top balanced, the others will be with in 10mV or so. Obviously not on the pack running the cell monitor from cell 1.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 30, 2019, 03:09:59 pm
content: so what I'm getting at here is to test the capacity of each cell, then make the packs so they are as closely matched as possible, then balance at the full charge point (4.2V) and actively monitor cells to protect against over dischargeas you can see from my first post, my main goal was to reduce the waste as well, so can you please share how you managed to reduce the balancing current using resistive balancing?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 30, 2019, 04:39:23 pm
content: Quote from: OM222O on April 30, 2019, 03:09:59 pmYes, matched for same capacity, then balanced so that voltage matches at 4.20V. It won't match perfectly at low end, hence the cell-by-cell low voltage cutoff.So, monitor each cell and stop discharging when any hits LVC (I'd recommend 3.00V as a baseline). Charging is a bit different and depends how well you can control the charger. If you can control the current, do a proper CV phase based on thehighestcell voltage - dynamically adjust current to maintain the highest voltage at 4.20V, stop when your current tapers off (C/20, for example). If you can't do that (i.e., if your charger has fixed CV output voltage), just let the charger do the CV phase based on total pack voltage, and use the cell-level OVP as an extra safety measure, but you'll have to adjust it a bit higher (say, 4.25V) so that it won't cut off prematurely, preventing full CV charge.If you don't need 100% charge, just skip the CV phase completely, and you'll have a battery at around 90% charged, and a simplified procedure.QuoteBy using longer balancingtimecompared to the competitors. Instead of dissipating for a a fewminutesduring the end of the CV phase, like most products do, I just measured the voltages at the end, calculated required shuntingtimesper cell based on imbalance voltage measured - this could be hours, and the balancing could run while discharging, or doing whatever you do. The required amount of balancingchargeis the same, but during a longer time, with lower current. This makes sense, because very seldom you need a lot of balancing at all.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 30, 2019, 05:36:11 pm
content: oh! so it doesn't actually reduce the waste, just makes it over a longer time (presumably for heat dissipation factors ? and safety like the fuses you mentioned?)by that definition I would just choose the normal method (higher current over shorter time) if the balancing is not required that often (lets say once a month or something) as it does not produce a huge amount of wasteI watched the video about the "DIY Powerwall" that was posted and it leaves a lot to be desired (using cheap pre made BMS? 7 segment leds for voltage indication?) and it's pretty rough around the edges but I had a similar idea so might take some inspiration. My end goal is to have my bench running all on DC from this sort of DIY power wall so there won't be massive current draw (100s of amps or so) and they won't be abused, maybe something like 20A would be a worst case scenario (for charging super caps). currently I'm just using switch mode PSUs that you can buy online but they are a bit too noisy for some of the analog projectsit might end up becoming a modular system like he proposes but that's later down the line. it'll also save me quite a bit as I can run it from solar and have free electricity in my work area. Thanks everyone for their advice.