Battery balancing - big cells  - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 18, 2018, 11:22:14 am
content: Hi,I want to add battery to solar system as prices already drop to level when it can be cheaper to have battery than buy electricity from grid at nightMain goal is to keep cost as low as possible so simple passive balancing/monitoring like provide LTC6811 looks goods and little decrease in charging efficiency is not a big problemI wan to ask for experiences and advices with this systems as I will use big cells with 58Ah each

author: MosherIV
author_rank: Super Contributor
timestamp: April 18, 2018, 07:57:38 pm
content: QuoteDo you mean cells or do you mean a module?A quick search did not find any 58Ah cells but did turn up modules.Yes, you MUST keep the cells balanced. This simply means keeping the cell voltages equal.Normally, you get a Battery Managment System to do this for you.Designing your own is not that easy, there are many parameters and effects to account for.Eg cell voltage droop and recovery, when you draw current from the battery the voltage will drop as it drains but when the current draw stops, the voltage will recover a little. This must be account for.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 19, 2018, 03:12:12 am
content: Even Tesla uses resistive balancers. If the battery pack is in any good shape, the balancing rarely even comes into play.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 20, 2018, 05:19:44 pm
content: Quote from: MosherIV on April 18, 2018, 07:57:38 pmYou are confusing balancing, which is (theoretically) only for maximizing energy storage capability of the aging pack, with cell-level monitoring, which is (again theoretically) for safety and protection. I could consider accepting the answer that the latter is a "must", but the former is clearly not.Talking about "keeping the cell voltages equal" is nonsense. What is equal, anyway? Within 0.0001V? Within 0.1V? Even in a perfectly balanced pack, voltages will be significantly different over the operating range. For example, if the pack is balanced at 100% SoC as usual, then there are going to be significant voltage differences (>0.1V) at, say, 5% SoC, under considerable (over 0.5C, for example) load.If this balancing is a must, why it's not universally used? Why most power tools don't do it?That's about balancing. Then, even the cell-level monitoring is not actually a hard requirement; definitely not a generic "must". Companies such as BOSCH do not do it on many of their 6s and smaller li-ion packs.Though, for such big cells (or paralleled cell groups - doesn't matter as long as the parallel connection is securely fixed during manufacture of the pack), cell-level monitoring could be important, but even more than the cell size, it depends on the number of cells connected in series. The more you have cells in series, the less relative contribution to the total voltage from any single cell. For example, almost no one uses cell-level monitoring on 2s packs, for which there seems to be total 100% industry expert agreement that it can be considered as a single 7.2V cell and managed as such. Above 2s, the viewpoints start to spread.Now, you had one thing right - designing a BMS is not easy. It feels like an easy task, but the devil is in the implementation details. This can be easily seen by analyzing existing BMS's. Every single one I have seen has been faulty by design, and possibly dangerous. The most typical issues are: 1) high quiescent currents, deep discharging and killing the cells in typical use cases that the designer still didn't anticipate, 2) lack ofreallynecessary protections due to the limited lithium ion chemistry knowledge of the designers; instead, all kinds of fancy "we-think-we-need-this" features are implemented, 3) stuck on balancing resistors, either just killing the cell, or worse, when combined with not-designed-for-worst-case thermal misdesign, heating up the adjacent cells over the thermal runaway onset temperature (about 150 degC for LCO).I have designed my own, as well, and sold it in fairly small numbers for suitable clients for specific purposes, and I'm quite darn sure I still have some bugs there, as well. In any case, the correct order to design a BMS is:1) Make sure it doesn't heat up or catch fire in any possible use or misuse scenario - do the thermal analysis assuming it's coupled with the cell, then thermally insulated for little cooling effect2) Make sure you have correctly learned and identified all critical safety parameters of the cell chemistry. Unfortunately, forum posts and easy-to-digest fake information sites such as Battery University are not enough, you need to attend some courses or read enough research material on actual cell chemistry.3) Implement monitoring on these critical safety parameters and react by shutting down any operations if these are exceeded. Make sure you have this completely working before implementing anything else4) Implement non-critical "comfort" features such as balancing. Make sure these do not prevent point 3) from working.Last but not least, modern li-ion cells from the brand manufacturers are very well protected on chemical and physical level. This is necessary, since the industry experience has shown that the BMS's do not protect reliably due to so much misdesign and misuse; and even if correctly designed, there are limits on what they can do.

author: woodchips
author_rank: Frequent Contributor
timestamp: April 20, 2018, 05:39:22 pm
content: Since it is stationary why not be sensible and use lead acid batteries? Get one of those cells up to thousands of Ah!

author: MosherIV
author_rank: Super Contributor
timestamp: April 20, 2018, 10:09:31 pm
content: QuoteOk, I admit I did over simplify on my statement. Let clarify.Balancing is a MUST when many cells (say >5) are in series to prolong life.Why? To avoid the potential of reverse biasing single cells in the stack.In extremely large stacks it is essential to make sure that all cells are closely matched in charge and discharge rates. Further more they must be kept balanced (equal in voltage) to stop one cell discharging before others and therefore being reverse biased. Reverse biasing leads to over discharging the cell and the voltage drops to 0V.Once 1 cell goes to 0V, it brings all other cells around it down for the same reason.How balanced do they need to be? I am not sure I can say for commercial reasons.I currently work for a battery company developing 400V to 700V systems.QuoteDepends on the application and cost of the packs.Power tools are commodity, so are made to the lowest price possible. So long as the packs last the 1 year warranty period, after that it is expected that the user bears the cost of replacement.The high power, high voltage systems I work on have warranty periods of around 5 years, so it is worth making sure they have mechanisms to prolong life. The extra expense of the measuring and balancing circuits is cost effective in this application.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 21, 2018, 12:34:28 am
content: Quote from: woodchips on April 20, 2018, 05:39:22 pmLead acid is not that good for regular cycling. LiFePO4 is the way to go for regular cycling.

author: Gregg
author_rank: Super Contributor
timestamp: April 21, 2018, 12:56:39 am
content: When dealing with larger capacity batteries, lead acid still gives the best bang for the buck. All of the big data center UPS units and all of the telco terminals use lead acid batteries. The secret to long life is maintenance, proper sizing and proper charging. If lead acid batteries cannot get a full charge on a daily basis, or if they are discharged too deeply, their life and capacity will diminish. Lead acid battery banks do not require separate cell monitoring or balancing; there are lots of UPS systems with 500 volt battery banks without individual cell monitoring or balancing.Rule of thumb with lead acid is not to discharge them below 1.75 volts per cell and to recharge them as soon as possible. A minor downside is the initial voltage drop upon loading lead acid batteries; google coup de fouet.In any case some research before spending lots of money is always prudent.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 21, 2018, 01:32:13 am
content: Quote from: Gregg on April 21, 2018, 12:56:39 amThose don't cycle the batteries by any significant amount every day or even every week. Keeping the batteries at full charge until needed for backup power is about the ideal use case for lead acid. Contrast that to lithium which prefers to stay around half charge.

author: ikrase
author_rank: Regular Contributor
timestamp: April 21, 2018, 06:52:55 am
content: I am actually somewhat suprised and frustrated by the lack of really good BMS options for medium and large cells. Closest would be the Electrodacus system.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 21, 2018, 07:00:11 am
content: Quote from: ikrase on April 21, 2018, 06:52:55 amThey're pretty easy to find for common 4.2V lithium and 3.6V LiFePO4. What's tricky is LiMn which is fully charged at around 4.1V, most commonly in the form of Nissan Leaf battery modules. I ended up developing my own BMS using a PIC for my 4S pack.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 21, 2018, 12:09:59 pm
content: Quote from: woodchips on April 20, 2018, 05:39:22 pmBecause lead acid battery capacity per actual kWh has been far more expensive than lithium ion for many years now, especially the complete lifetime cost, at least by factor 2-3.The fact that some large backup packs still use (or were using a few years ago; this is changing as we speak) lead acid is not because the lead acid itself makes much sense, but because of tradition and supply chain. Some big things change slowly: something needs to be cost-effective and reliable for many years if not a full decade before being accepted as a replacement for a traditional low-tech usage.Li-ion has been considered a luxury thing, so that the pack constructor / maintainer company can add 500-1000% price over the bare cell price, while the equivalent lead-acid constructor / maintainer only adds, say, 200-500%. IMHO, this has already been changed in the recent years.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 21, 2018, 12:11:38 pm
content: Quote from: MosherIV on April 20, 2018, 10:09:31 pmI suggest you reread my reply, or study elsewhere. Cell voltage monitoring (specifically, per-cell Low Voltage Cutoff) is for this purpose. Balancing is not, and it cannot even address this problem at all!

author: Yansi
author_rank: Super Contributor
timestamp: April 21, 2018, 12:25:42 pm
content: Quote from: NiHaoMike on April 21, 2018, 07:00:11 amWhats so difficult on a 4.1V LiMn battery? :-O There is a metric shitton of fully programmable BMS ICs on the market.To the OP: I have no personal experience with LTC6411, but it is a successor of the LTC6804 which I have a lot of (not the greatest) experience. We did use these for a couple of larger 300 to 400V LiIon battery packs. The LTC6804 did not behave as it should have a lot of times, we got several BMS burned for no apparent reason (did work at the time of installation absolutely fine, only to find later it fucked itself for no clear reason). The LTC6804 are discontinued and I think my experience with these just tells the reason. As the LTC6811 should be compatible if I remember correctly, I guess it is a corrected revisit of the 6804 and the 6811 should be more or less fine.However we switched recently to using the new 16cell BQ series chip from texas instruments. So we'll see how these will do in the near future.

author: MosherIV
author_rank: Super Contributor
timestamp: April 21, 2018, 07:13:13 pm
content: QuoteYes, you are right that there is a min cell voltage cut off BUT you do not want that to shut down the battery pack during operation because one cell is lower voltage than the rest. That is why balancing is a MUST for systems where you have many cells in series.If you do not beleive that cell balancing is necessary, ok.I am try to explain why it is necessary in a civil curtious manner.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 21, 2018, 07:18:13 pm
content: Quote from: Yansi on April 21, 2018, 12:25:42 pmHard to find a reasonably priced module for a 4S pack because there's little demand. The PIC is doing a lot of other things with power management so I needed it in my design anyways.

author: Yansi
author_rank: Super Contributor
timestamp: April 21, 2018, 11:48:51 pm
content: Well finding an off the shelf module for them may be hard for sure.

author: woodchips
author_rank: Frequent Contributor
timestamp: April 22, 2018, 09:09:37 am
content: Ok, so where are Li-ion batteries being cheaper than lead acid obtained?Just done a quick check at a distributors prices and lead acid is far cheaper. Cheapest Li-ion batteries were DeWalt 54V 6Ah strangely.A forklift battery must be the cheapest lead acid, and they have a life measured in years, mine must be 30 years old, down to 1/5 of rated capacity though.

author: ikrase
author_rank: Regular Contributor
timestamp: April 22, 2018, 12:55:13 pm
content: The initial cost is still much higher.

author: MosherIV
author_rank: Super Contributor
timestamp: April 22, 2018, 01:10:31 pm
content: QuoteHi, someone else has already pointed out that we are in the transition period, where LiPo are still at a premium price.It will also depend on application. Unlike SLA, I doubt that you will be able to buy raw modules like a SLA batteries, any Li based battery module will require some kind of BMS. Think of laptop batteries, they all come as a pre sealed module with the BMS embedded.The company I work for is already developing replacement modules for SLA batteries in electric pallet trucks. They are complete modules and the end user has no access to the batteries inside. The manufacturer of the electric pallet truck is driving this change over. The volume price of LiPo is now slightly cheaper than SLA. Ironically, for pallet/forklift the weight of SLA is an advantage, the LiPo modules need ballast weight in the housing.The need for the BMS is because when Li battery is abused, they vent gas which is highly dangerous so the cells must be protected from abuse.It will be a while before Li battery modules are available as COTS items that you can just order.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 22, 2018, 02:08:27 pm
content: Quote from: woodchips on April 22, 2018, 09:09:37 amCheaper *after accounting for cycle life*.

author: mtdoc
author_rank: Super Contributor
timestamp: April 22, 2018, 03:57:42 pm
content: A few relevant points:While it is true that when one accounts for total cycle life, lithium chemistry batteries are cost competitive or better in daily cycle applications, deep cycle lead acid batteriess still have one key advantage: they are very robust and will tolerate extremes of temperature and charge/discharge that will kill lithium batteries. This gives them an advantage in some home solar power applications where temperature extremes may occur and power/weight is not a big issue.If you choose to go with lithium, large, prismatic LiFePO4 cells with capacities of 100-200 Ah are the battery of choice for serious DIY home systems IMHO. Yes some people are making large DIY “Tesla power wall “ style packs out of the ubiquitous small 18650 LiPo cells, but that is still a niche, labor intensive, unproven approach with risk of catastrophic failure.The large LiFePO4 prismatic cell approach was pioneered by DIY EV builders some time ago and in the last few years is becoming very popular for home PV and other renewable energy systems.The issue of cell balancing large capacity lithium battery packs has been a topic of debate for some time. As the OP notes it is difficult or impossible to find BMS systems up to the task.The good news is that it turns out that contrary to conventional wisdom, for these system aBMS is not neededand may actually make things more prone to failure.The answer is to bottom balance the cells.Bottom balancing has become the method of choice. You can read more about the rationale and conceptHEREandHERE.

author: Yansi
author_rank: Super Contributor
timestamp: April 22, 2018, 04:33:34 pm
content: QuoteProof? Why Lithium based chemistry is the choice number one for the coldest of temperatures?QuoteAgain, any proof of such statement? Stacking cylindrical Li-Ion based cells with NCA chemistry can yield the highest energy storage density, both volumetric and by weight when properly done. Stacking cylindrical cells is much safer and easier.It seems like you are talking out of your ass with these ones. Otherwise you have my approval on the rest.

author: mtdoc
author_rank: Super Contributor
timestamp: April 22, 2018, 05:54:37 pm
content: Quote from: Yansi on April 22, 2018, 04:33:34 pmIt is the experience of many eBike and EV builders that charging Lithium batteries below freezing temperature causes irreversible damage. I personally damaged my eBike LiFePO4 pack that way. This does not happen to lead acid batteries.There is a reason why the Auto companies now building EVs are almost all including some active temperature management to prevent both low and high temperature extremes. My Volt will automatically turn on the ICE to warm the battery pack when charging in very low temperatures. Why do that if it is not an issue?Nissan's well publicized failure to provide active cooling to the Leaf led to many premature battery pack failures.If you want datahere is some. I'm sure much more can be found with a search.QuoteProof?I suspect your confusion is with the difference between performance at low, non freezing temperatures versus theirrobustness- which means their ability to withstand very low temperature charging or very high temperature discharging without sustainingirreversible damage. THAT is what I was referring too and why I specifically used the term "robust".The fact -based on many decades of experience - is that deep cycle flooded lead acid batteries can withstand abuse in both extremes of temperature and charging/discharging without suffering irreversible damage. Period. Full stop. The same simply cannot be said of lithium batteries. Why else all the concern about cell monitoring, balancing, and use of BMS for lithium and not for LA?That said - Lithium batteries do have many advantages which is why they are replacing LA in most applications.QuoteHighest energy storage density is not always the most important design consideration. In particular that is not true for home renewable energy applications. This is why using large prismatic LiFePO4 cells is the best solution there. Their energy storage density is not quite as good as other lithium chemistries but their other advantages (including safety) outweigh that.It is a fact that stacking large numbers of 18650 or similar cylindrical cells and utilizing electronic BMS introduces more failure points. BMS is not easy to do correctly. Experience has shown that this can result in catastrophic failures. Do I really need to point out the examples of this?Early in this thread Siwastaja, who knows his shit when it comes to lithium battery chemistry, said as much:Quote from: Siwastaja on April 20, 2018, 05:19:44 pmOf course it can be done safely - but at high cost and complexity. That price is worth it in commercial EVs where energy density is so critical. But not so much home RE system or some DIY EV applications.

author: MosherIV
author_rank: Super Contributor
timestamp: April 22, 2018, 06:14:04 pm
content: QuoteI was intrigued to see what 'bottom balancing' is.Turns out that the guy is destroying cells because his system is NOT balancing, ie trying to keep all the cells in a series stack all at the same voltage (ie all at the same SOC) and therefore some cells would get reverse biased and be destroyed.Firstly, all cells in a large series stack should be monitored and the pack shut down if any cell reaches a minimum cell voltage. Yes, you do not get the max capacity but it stops cells from being destroyed.His system is not using closely matched cells. It really helps in series stacks if all the cell charge and discharge at the same rate, even if it is only at the start of life. Again, this is where cell balancing helps.His system is using the full capacity range of the cells. In EV systems, this is unusual because EV application normally require a minimum sevice life. The main strategy to acheive long life is to limit the max and min SOC to 20% for min and 80% for max. By doing this, again it avoids bringing cells down to the point where they get reverse biased.Fyi, for those that have not read the article - top balancing is bring all cells in a series stack up to 100% SOC or a known Voltage which is known to be 100%.Bottom balancing is bring all the cells in a series stack to the voltage that is 0% SOC

author: mtdoc
author_rank: Super Contributor
timestamp: April 22, 2018, 06:34:36 pm
content: Quote from: MosherIV on April 22, 2018, 06:14:04 pmYour are not understanding the concept of bottom balancing correctly. It does not mean trying to keep all the cells at the same voltage. The whole point of bottom balancing is to not try to keep the cells at the same voltage. They are only at the same voltage once - that is a the "bottom".He is not "destroying cells". In fact the opposite. When he penned that article (2009) the concept of bottom balancing was new. It has since become widely adopted and found by many to be the best way to avoid destroying cells. You can easily do some research online of its use by DIY EV and home RE users to confirm this. The second link I provided is a start.QuoteWith bottom balancing, you also still have a low voltage cut-off employed to prevent any cell from reaching a low SOC failure point.QuoteNo he is not using the "full capacity" He is simply defining 0% SOC as the point were the bottom balance is. You are not understanding how the concept of bottom balancing is employed. In fact it is used in a way that makes a more conservative used of the middle part of a cells capacity - specifically to ensure longer life. In the case of home RE applications, after the cells are balanced at the "bottom" the systems charge controller utilizes it's charge voltage setting and the inverters low voltage cut-off settings to ensure this. Ideally one utilizes a current-shunt to monitor AH in and out as well.QuoteNot really that simple, though I can understand why you would think that based on the terminology used by the articles I linked. One can choose any voltage one wants to bottom balance at. The manufacturer's recommended "max discharge voltage" is sometimes chosen - (e.g. 2.5V for Calb LiFePO4 cells), though a more conservative approach is to chose a voltage a few tenths above that. After that bottom balance is done and the battery bank charged, the inverter's low voltage cut-off is set to ensure that cell voltages and pack SOC never drop below whatever level you desire - generally well above 0% SOC. 20& SOC would be typical.

author: nctnico
author_rank: Super Contributor
timestamp: April 22, 2018, 07:16:37 pm
content: Quote from: mtdoc on April 22, 2018, 05:54:37 pmA BMS of any Li-ion battery pack should prevent charging the pack at too low temperatures. One of the problems however is that most of these packs are made in China and have low quality when it comes to the BMS functions.QuoteLead-Acid batteries need some care during charge and discharge. If you get the charging and discharging right then an LA battery will last very long and don't get problems due to sulphation.

author: MosherIV
author_rank: Super Contributor
timestamp: April 22, 2018, 07:48:21 pm
content: QuoteI think I understand what is meant by 'bottom balancing'.I am just pointing out that the circumstances that drove the inventor to come up with the idea are dubious.Basically, it sounds like he was driving around an EV without any BMS to protect the cells.

author: mtdoc
author_rank: Super Contributor
timestamp: April 22, 2018, 07:49:47 pm
content: Quote from: nctnico on April 22, 2018, 07:16:37 pmYes, I agree, theyshould. The problems, as we all know, is that electronics are subject to failure and poor design. LA batteries require no such protection.QuoteYes. The main problem that LA suffers from is sulphation if they are left to sit at a low SOC or are not regularly brought up to 100% SOC and "equalized". I think it's that historic "top balancing" approach used in LA that causes so many problems with people trying to implement a similar approach with lithium batteries.In contrast one of the great benefits of lithium is the ability to let them sit at middle of the SOC curve for indefinite periods without damage.

author: mtdoc
author_rank: Super Contributor
timestamp: April 22, 2018, 07:54:42 pm
content: Quote from: MosherIV on April 22, 2018, 07:48:21 pmOK, I read your post to imply that your were blaming bottom balancing. I see that you were referring to his description of the failures that led him to change to a bottom balancing method.QuoteNot exactly. If I understand correctly, he was using the typical current shunting type of BMS employed by many.

author: KL27x
author_rank: Super Contributor
timestamp: April 23, 2018, 08:14:13 am
content: QuoteGuy balances cells at the top with a conventional BMS. Claims this kills his cells because the weaker cells run low, first. And were thusly being overdischarged. (What!?)Guy balances cells at the bottom and lets them be unbalanced at the top and claims eureka. Problem solved. (What?!)What the heck am I missing?If you monitor each cell and simply shut down the battery before that weakest cell reaches the critical lower limit, problem solved. By balancing the cells at the top (fully charging them), yeah, you can't run the stronger cells all the way down to 3.0V (or w/e the cutoff is). But you will get the exact same amp hour capacity (the capacity of the weakest cell) and a HIGHER energy capacity, since your cumulative series voltage is going to be slightly higher than when balancing them at the bottom. If there's a good reason to balance them at the bottom, the linked page doesn't provide the first clue.Sound like this is a fairly intelligent car enthusiast with not so good grasp of the problem.When I read the emphatic posts that BMS is necessary for safety and/or efficiency of high S cells, I was expecting some fancy system of taking power from good cells and giving it to lower voltage cells (or perhaps switching out the dead cell and running the battery at (X-1)S output, where applicable) so that the battery could run beyond the amp hour capacity of the weakest cell. This bottom balancing solution is just bull shit. It doesn't provide any supporting evidence that balancing is a must for safety and longevity reasons; technically, it is individual cell monitoring and shutdowns on both high and low end which is imperative. Top end balancing does qualify as individual cell monitoring on the top end.... bottom balancing should qualify as a way to handle just the lower end... because you HAVE to monitor individual cell voltage in order to balance cells, but you do not HAVE to balance cells to prevent damage to the battery. Perhaps balancing is just the wrong word to use, here.Short: you have to monitor cell voltage at both top and bottom of the range. It can be helpful to balance the cells in order to guarantee maximum battery capacity (capacity of the weakest cell). Balancing cells at the top doesn't mean you set your low end cutoff voltage of the battery to the min for a single cell times S anymore than you could balance them at the bottom and then blindly charge the battery to 4.2V*S.

author: paulca
author_rank: Super Contributor
timestamp: April 23, 2018, 09:40:42 am
content: Quote from: Siwastaja on April 20, 2018, 05:19:44 pmWhile I doubt this very much it might come down to terminology.You MUST balance a lithium pack in series in order to charge it safely. You can do that with individual cell protection with resistive dischargers per cell, but this will limit you charging current.If you don't balance it during charge then when it reaches 12.6V for a 3S on the charger you might have cell voltages like 5V, 4V, 3.6V and if you are pushing high current into it, you will have a potential explosion.Usually however laptops and power tool packs just let the cells unbalance, then monitor the peak cell voltage and end the charge when one hits the 4.2V. This causes the pack to degrade over time and have a relatively short life span. Usually you will find that the pack becomes substandard and fails while 5 out of the 6 cells are fine, but without proper balancing the pack is dead. This is why they are so good for recycling into power walls.I would not consider most commercial BMS boards to be much more than bare minimum to prevent fire, but are in no way the best way to manage the battery, usually the opposite.

author: MosherIV
author_rank: Super Contributor
timestamp: April 23, 2018, 06:57:53 pm
content: QuoteNothing, I came to the same conclusion.QuoteThis is known as Active Balancing.It has been tried, but the losses in the transfer circuitry and the complexity of the circuitry made it not worth while.QuoteBalancing is not a safety issue.Balancing (of cell voltage) is a good strategy to improve longevity.Balancing during the usage phase is basically matching the discharge rate of stronger cells to the weak ones.As I said before, it is best to start with closely matched cells. However, cells will age at different rates, so you need some way to catter for this.As you pointed out, shutting off the pack when the weakest cell reaches min V is one way to do it.QuoteYes.What some people may not be aware of here is that for systems that have high S do not usually charge cell individually, they charge the stack in series. When a cell in the stack reaches man V, charging MUST be stopped. So yes, high S packs must be balanced during charging. Or be shut down when a cell reaches max V and the stack remians unbalanced, and gets more unbalanced over time.

author: nctnico
author_rank: Super Contributor
timestamp: April 23, 2018, 08:06:15 pm
content: Quote from: MosherIV on April 23, 2018, 06:57:53 pmAgreed. And being involved in battery packs myself I think that balancing a fully charged cell is better than a nearly discharged cell. One of the problems is that it is much more unlikely a pack gets discharged enough to do the balancing at all. Also the voltage of a cell (usually a group of cells in parallel) may drop below the shutdown voltage which means the BMS has to shut down and cannot complete the balancing operation. On the other hand most packs will be fully charged when they are charged so basically the balancing will happen for each charge/discharge cycle.Ofcourse the BMS has to shut down charging and discharging when the cell voltage limits are exceeded (usually these are not the absolute minimum and maximum voltage for the cells but have some margin to prolong battery life).

author: KL27x
author_rank: Super Contributor
timestamp: April 23, 2018, 08:40:19 pm
content: QuoteThis is where terminology comes in. I disagree that a battery must be balanced for safety reasons. As long as you stop charge/discharge when the FIRST cell out of the series reaches the limit, there is no safety issue. There MIGHT be a capacity issue, IF the weakest cell is not getting fully charged, or if the weakest cell is not reaching the lower threshold, first. The former is the most likely problem, IME, which will reduce the capacity of the battery. A weak cell can be a little leaky to where it does not fully charge by the time the rest of the pack gets there. This is the big reason you want to balance them at the top, aside from the slightly higher energy density. On the flip side, the weak cell is pretty much always going to hit the lower threshold, first. It is almost impossible for the pack to get unbalanced in a way that this does not occur. So balancing them at the bottom is simply a bad strategy, AFAIC.QuoteWhat you do not understand is that in order to balance cells, individual cell voltage must be monitored, somehow. The resistive bleeders that kick in at X voltage (PER INDIVIDUAL CELL) is a form of individual cell voltage monitoring, IMO. Those bleed resistors kick in at a specific voltage. Whether you kick in the bleeder resistor or you stop the charging when this happens (the latter is technically safer, or at least it is simpler/easier to implement in a way that will always be safe), you have some kind of voltage detection on individual (stacks of parallel connected) cells, in either case, whether you are balancing the cells or not.I reiterate. Inidividual cell voltage monitoring is what is necessary for safe charge/discharge. It is necessary at both the top and the bottom. Cell balancing is for maximizing the capacity of the battery, and it defacto requires individual cell voltage monitoring. And it's best done at the top.

author: nctnico
author_rank: Super Contributor
timestamp: April 23, 2018, 09:03:14 pm
content: Sidenote: Texas Instruments has a whole range of cell monitoring / balancing chips which do all the work for you based on field proven algorithms. There is not much sense in trying to roll your own balancing system. TI is putting a lot of research into these chips but you have to configure them properly to match the cell specs ofcourse.

author: C
author_rank: Super Contributor
timestamp: April 23, 2018, 09:19:04 pm
content: I think that some are missing that battery voltage is not equal to charge.A battery is not a simple thing.There is a internal series resistance that must be taken into account.Any current in or out of battery changes voltage you can monitor.You have a choice of allowing for Max-Min resistance change in measured voltageOrMeasuring internal resistance.To compute internal resistance requires measuring Current & Voltage over a short time with different current load.The internal battery charge will change little over short time, so internal voltage will change little.But note that many things can change internal resistance.c

author: KL27x
author_rank: Super Contributor
timestamp: April 23, 2018, 09:25:59 pm
content: QuoteNot exactly. But cell voltage of a Li ion battery is a function of charge, and the amount of stored charge is a function of voltage. There is no case where a given cell can have a higher stored charge and a lower voltage. As the voltage goes up, the stored charge goes up. And vice versa, as the stored charge goes up, the voltage goes up. Period. Li ion ARE very simple compared to NiMh, where this is not the case.

author: nctnico
author_rank: Super Contributor
timestamp: April 23, 2018, 09:41:49 pm
content: Quote from: KL27x on April 23, 2018, 09:25:59 pmDefinitely not. You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.

author: KL27x
author_rank: Super Contributor
timestamp: April 23, 2018, 09:49:26 pm
content: QuoteNot completely flat. I maintain my statement is accurate. A hyperbola approaches flat, but it's still a function no matter how ridiculously flat it gets. Is it practical to measure the Y axis and to calculate the X? Not necessarily, but that doesn't mean it's not a function. The Fvd on a diode is a function of current, even though for many purposes we can practically consider it static (or flat).QuoteYou have not thought this all the way through, C. This a red herring. Computing the internal resistance allows you to calculate the energy available at a given load. But it is not necessary to include that into cutoff limits. On the low end, the cell with higher internal resistance will sag in output more than the others. And you will still safely cut off the output when it reaches the lower limit. Maybe it cuts out a little earlier than it could have, but at least it's not a safety issue. On the charging top end, the current is very low and not a factor in fully charging a cell. It reached the proper voltage or it does not. It also is not a safety issue. On a battery charger it is a nice feature to have. It is nice to know. In practice, when the battery doesn't last long as it needs to, this is all you need to know and you will replace the high internal resistance cells. Increased internal resistance is one of the main things that goes south in properly used cells. In very low current applications compared to battery capacity, the drop is going to be neglible, and the battery works fine and doesn't drop significantly in capacity due to cutout; so you never see the problem. In higher current:C application, this problem largely takes care of itself, as stated, but you could include compensation if you want to run the batteries to the ragged edge (provided the decreased output voltage under load is not the limiting factor). Well, I suppose this brings temp sensors into light. Higher internal resistance also increases the battery temp of that cell under given draw. Yes, temp sensors are part and parcel if you are talking about large batteries and large currents and duty cycles.

author: mtdoc
author_rank: Super Contributor
timestamp: April 23, 2018, 11:30:20 pm
content: What is missing in the above discussion is that the OP was asking about balancing very big lithium cells (most certainly LiFePO4). Many here are extrapolating their experience with small Lipo packs.Of course a high quality BMS which monitors individual cell voltages at the top and bottom can be a good thing - as long as it is reliable.It is not easy to purchase or build an adequate, safe, reliable and effective BMS for a 20 kWh bank of prismatic LiFePO4 cells.What many in the DIY EV and RE world have found fromfirst hand experiencewith large, expensive LiFePO4 banks is that bottom balancing alone is sufficient and probably better than the available BMS options. These are people who have spent many thousands of dollars on their battery packs. IOWthey have skin in the gameand are not simply opining on an internet forum.When the idea of bottom balancing large LiFePO4 packs was first thrown around on the internet 9 years ago, I was skeptical too - as I believe were most people who follow this stuff in the EV and RE world were. But after following the debate for years and watching as people whose opinion I trust shared their experience and seeing that this is now replacing the conventional wisdom for these applications, I'm convinced. Many people have destroyed their expensive LiFePO4 battery banks relying on traditional top balancing methods. I have yet to hear of anyone not employing the bottom balance method successfully. Of course it does take more knowledge and understanding by the end user than the brainless "plug and play" BMS approach that most consumers expect.Feel free to be skeptical -but before writing the idea off and extrapolating your experience with small Lipo packs (i.e. not very large LiFePO4 battery banks) spend more than 5 minutes reading one guys early take on it.And if you have a better concrete answer for the OP as to were to find a high quality, affordable BMS system for very large cells, please share. The OP mentioned 58Ah cells which are almost certainly LiFePO4 prismatic cells.

author: KL27x
author_rank: Super Contributor
timestamp: April 24, 2018, 02:09:00 am
content: ^This is the take you linked, dude.If it is better, this guy didn't give any clue why it is better to balance at the bottom, nor how you even do it. When the lowest cell hits the floor, you cutoff the battery and you drain the others? Then you charge until the first cell hits the max voltage and stop charging?You say experience and investment and blah. But I don't care how much money someone want so spend and burn. If they can't explain what/why/how, and they are the dudes linked to for information, this is the reason people are burning up batteries. There is a clear lack of understanding compounded by dumbasses.In every field there are dumbasses who are deemed experts. They learn one way to do something, lacking clear grasp of physics and science and engineering and abstract thought. And they create a new language based around faulty conclusions and incomplete understanding of the big picture.Being the first person to stumble backwards onto a practical working solution or improvement through thousands of dollars of trial and error doesn't make you not an idiot. Accepting incorrect explanations and faulty terminology just ensures that people continue to blow up batteries.Creating new language and vague explanations for things we already have words for is the hallmark of the charlatanism that accompanies every major industry. We don't need to accept that, here.

author: mtdoc
author_rank: Super Contributor
timestamp: April 24, 2018, 02:39:43 am
content: So you have no solution the OP, then?. And you have no experience with daily cycling of large LiFePO4 battery banks?Hmm, who’s opinion to trust, yours or the many people, including several EEs who do have that experience over several years.?It’s always revealing when someone exhibits such strong emotional responses to something new to them.Quote from: KL27x on April 24, 2018, 02:09:00 amOr deem themselves experts. Yes, clearly true.

author: KL27x
author_rank: Super Contributor
timestamp: April 24, 2018, 02:43:37 am
content: Your link explains nothing. Explanations that are incomplete.2+ bangledesh = chocolate chip icecream.We are supposed to accept this as the holy grail of battery management.I'm not an expert at using batteries. And no one has to be an expert in anything to understand this link is complete bull shit. W/e bottom balancing is, it might be sliced bread, but this web page is bull shit.

author: mtdoc
author_rank: Super Contributor
timestamp: April 24, 2018, 02:45:47 am
content: Quote from: nctnico on April 23, 2018, 09:41:49 pmYes, absolutely right. I believe this is part of the reason why traditional “ top balancing” of series strings of large LiFePO4 so often fails.

author: mtdoc
author_rank: Super Contributor
timestamp: April 24, 2018, 03:02:41 am
content: Quote from: KL27x on April 24, 2018, 02:43:37 amWhy the obsession with that one link? I never claimed it was the be all end all of the bottom balancing concept. I only posted it because it was the first one that I saw in 2009 that introduced the concept. It has since been flushed our as a valid method of cycling large LiFePO4 battery banks (where no good BMS solution exists). As I’ve said, there are now many people (some EEs) who have been successfully using this method over several years now. I know one personally and have seen his system in action.This method has proven sound. If it offends your delicate lithium battery care sensibility, fine. Why the rage?You seem to admit you have no experience with it or other methods of daily cycling of large LiFePO4 battery banks. What solution do you have to offer the OP?

author: KL27x
author_rank: Super Contributor
timestamp: April 24, 2018, 03:06:07 am
content: QuoteIt's almost as revealing as people linking webpages that make no sense but look sorta official.This web page is nothing but misinformation. It takes something which is actually very simple to discuss and understand and explain. Yet it manages to obfuscate the subject while hinting at enlightenment.I've said it before. There is a reason we watch li ion batteries explode in the news, so much. It's the same reason there's so much intentional misinformation and anti-education. It's because batteries are a billion dollar a year industry, and the less we know the more they make.QuoteThe rage is at you, specifically, repeatedly stating things without backing them up. You have used this link as your support, but this emporer is wearing no clothes. Please explain what the hell you are claiming as facts in things called words, and we will gladly listen.I fully respect your right to feed your family. You have disclosed your work in the industry. If you are a knowing shill, I respect that. If you're an unwitting shill, I feel sorry for you. There are many people on this forum who understand how batteries work, and your attempt at obfuscation is offensive. Can you explain this bottom balancing thing, technically? Or do you just sell something and point to experience and money invested as why I should buy it? What department do you work in, anyway? In what capacity are you involved in designing BMS? Why you talk around it instead of talking about it?

author: mtdoc
author_rank: Super Contributor
timestamp: April 24, 2018, 03:54:04 am
content: Quote from: KL27x on April 24, 2018, 03:06:07 amExample? I suggested a solution for the OPs question with links to others who agree. That’s all. You’ve offered the OP nothing (while backing up nothing you’ve said).QuoteNonsense. It was one of two links i offered to explain the concept of bottom balancing. It was not meant to prove anything.QuoteThe only “fact” that I’ve claimed is the fact that many people have been successfully using bottom balancing to cycle and maintain large LiFePO4 battery banks. A simple internet search will confirm that.You’ve offerred only your opinion about something you'd never heard of until this thread.QuoteMany on this forum know my profession and it has nothing to do with this topic. I do have several years experience maintaining large LA battery banks for solar PV systems (as a non-professional) and LiFePO4 for ebikes. I am only an electronics hobbyist with a strong interest in Home RE systems, ebikes and EVs. I’ve never claimed otherwise. You need to get a grip..Once again since you seem to have completely misread my posts:I suggested to the OP that a BMS system is not required to maintain large LiFePO4 battery bank - that there was a method called bottom balancing that many are using successfully. (Easily verified)There is nothing inherently wrong with BMS for large capacity LiFePO4 battery banks- it’s just that affordable and reliable ones do not seem to exist. The top balancing current shunting systems that many have used have often failed.(also easily verified).Bottom balancing is just an alternative method that has proven valid.You don’t have to agree (neither did I when I first heard about it) but demanding proof and spewing nonsense about me and accusations of “shilling” ( again WTF? Selling what exactly?) while offering no answers to the OP youself does not look so good..

author: KL27x
author_rank: Super Contributor
timestamp: April 24, 2018, 04:19:29 am
content: I'm sorry. I was wrong. I should have just walked away after my first response to this link. Please accept my apology. I was confusing balancing (initial balancing) and active balancing. And confusing posters. And I had poor impulse control.This explains another guys reason why bottom balancing is better. Still don't get his explanation.A bunch of people following the leader in something that works doesn't make any of them understand why it works or able to explain it.As he says, you have to avoid over charging, as well as over discharging. And completely removing any cell monitoring from the bottom would just shift that burden to the top end, would it not?Perhaps it's a simple matter of LiFePO4 are harder to damage by overcharging and easier to damage by overdischarging than Li ion. Or maybe it's harder to detect the lower threshold by voltage monitoring. (Pretty stupid easy with Li ion cells). But this is just speculation. Perhaps there is someone out there that can explain why the hell they are doing it this way. If not, just say it works better and I don't know why. Instead of showing fancy graphs and using circular logic.Another speculation, despite all the arguments that coulomb counting is the only way to do it, is that by bottom balancing it might be possible to detect at an earlier point in time when the battery is getting low, and in an EEV you don't want to be suddenly stranded in traffic.I an do a lot of speculation. The guys that are driving these vehicles and playing with these batteries could actually give reasons. If they could only articulate them.

author: C
author_rank: Super Contributor
timestamp: April 24, 2018, 05:06:18 am
content: KL27x's quoteQuoteLooks to me that "You have not thought this all the way through"Yes a low voltage cutoff works for safety, but what is the result?Simple examplepanasonic ncr18650bhttps://na.industrial.panasonic.com/sites/default/pidsa/files/ncr18650b.pdfIf you pick a protect the cell from low voltage at 3 volts,you are stopping the output at around2250 mAh for higher current case for use of about 69% of capacity.3150 mAh for a lower current case for a use of about 96% capacity.And this is from a graph that is not complete, this is just different discharge rates.If you pick a lower voltage and then you have to hope that your voltage sense is up to task and some other variable does not go out of range.Quote from: mtdoc on April 24, 2018, 02:45:47 amEven this is not complete.To use a coulomb counter you need it calibrated to the specific pack & it needs to be re-calibrated as the pack changes.One way to do this calibration is to sense the rate of voltage change with a constant current as you get close to lower limit.You are sensing the curve.But again if you have a series pack, each battery in series will have a different capacity and change over time.So use little of battery an pick very safe top and bottom voltages where any other change can not take you out of safe zone.Or get more facts by measuring them an making a better decision allowing more use of packs capacity.C

author: mtdoc
author_rank: Super Contributor
timestamp: April 24, 2018, 05:25:47 am
content: Quote from: KL27x on April 24, 2018, 04:19:29 amAccepted and kudos for acknowledging the error.QuoteHave a look atthe second linkI posted in my first post in this thread. If you read through the entire thread in that link I think it will make more sense. There are a couple of engineer posters there who do a better job explaining it than I likely could. Again, it’s no panacea but it has been shown to be a practical method that works for these large prismatic LiFePO4 cells.

author: KL27x
author_rank: Super Contributor
timestamp: April 24, 2018, 07:04:17 am
content: Thanks. I was hit in the head pretty hard as a kid. That's my latest excuse for being an occasional ass.On the forum linked, it seems when referring to top balancing, they are actually talking about active BMS system (specifically a "Vampire" board) but bottom balancing they are referring to initial balancing just once with no active measures. Same thing on the first web page, which was part of my initial confusion where he was talking about active balancing on top, then going to claim bottom balancing was better. (I thought he was talking about active bottom balancing). I was assuming there was cell voltage monitoring in both cases.The things that stand out to me which sound accurate and appropriate are:QuoteandQuoteThese seem to be pretty obvious issues. Of course you can't rely on a "Vampire board" to save you from overdischarging your battery if it has nothing to do with this. And relying on the minimum voltage which your vehicle's motor controller will accept before it stops functioning? It seems perfectly reasonable that a manufacturer of an EEV will design the motor controller to accept a minimum voltage which MIGHT still be safe for the S's and chemistry of the expected battery. It's not up to the motor controller to limit your mileage, arbitrarily. Unless it talks to a battery management system on the battery, of course it doesn't do this job.If bottom balancing and simply running until the motor controller shuts down work for you, then great. I don't understand why you want to run out of battery in an EV, though. Seems like you'd be stranded. I would rather know when the battery is dangerously low and have the option to kill a cell if I really wanted to.Then there is a lot of explanation that is less realistic.Take a complaint that the Vampire board allows overcharging of cells.QuoteThis is specific to the Vampire board and the specific battery charger and battery being charged, firstly. If 1 amp shunt isn't enough to prevent overcharge, that is potentially bad design of the Vampire board or it is maybe user error, using the board for something out of its specification. But he is probably not aware that his battery/charger is going to hit the constant voltage phase well before the first cell reaches max voltage, and the current will have probably dropped well below 50A by that time. But maybe 1 A isn't enough. This is a fault of the overall system and/or user error.There is also the repeated notion that a cell dies because of reverse polarity, when good cells push current through a dead cell. This is news to me. With regular li ion, the cell doesn't need any help to be killed by overdischarge. You have to actively stop using them before this happens whether they are all balanced or not. So I know LiFePO4 are more forgiving, but I would personally be doing something a little more, here, that just balancing them on the bottom and running them dry.QuoteThis explains what happens on the top end. So after bottom balancing, you have to cut charging when the weakest cell reaches full charge (or your target goal, 90% or whatnot). Perhaps this is easier to implement than monitoring voltage at the bottom for many applications. In charging a batt in your home under controlled conditions that can't leave you stranded. I can see the advantage, and you could even simply set a charger to a conservative voltage and then physically do a voltage check on your cells (assuming you have access to all the terminals).QuoteSame amp hours, slightly lower energy capacity, though.But I really like the idea of bottom balancing, esp if this is enough to do the job on LiFePO4 batts. Esp because the weakest cell ironically will take the longest to charge when actively top balancing, at least with Li ion. IME, it's not ever a matter of one cell having slightly less capacity than the others because it didn't get as much of the good stuff in there... it has always been found, IME, that one cell is slightly defective when there's a significant difference in capacity. By bottom initial/pack-balancing, you will get the weakest cell to finish charging first or close to first, and this is very simple and elegant and the balance should remain stable for a very long time. But I think eventually, the stronger cells are going to generally charge up a little more and discharge ever so slightly less, and you end up out of balance. But starting from bottom balanced might essentially give you a head start and a longer time between manual pack balancing (compared to top pack-only balancing, of course). I might have to try it, but it is a lot of work to do this.... I have a circuit for draining cells, somewhere.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 24, 2018, 07:54:35 am
content: Quote from: mtdoc on April 23, 2018, 11:30:20 pmI was wrong they are not single cell but two parallel rated at 33Ah flat LiMn2O4But it is not that big difference

author: nctnico
author_rank: Super Contributor
timestamp: April 24, 2018, 12:03:10 pm
content: Quote from: mtdoc on April 24, 2018, 02:45:47 amWithout knowing the BMS architecture that is hard to determine. In the links you provide it seems people have destroyed their batteries due to a BMS which doesn't care about the individual cell voltages. That is a big red herring (ofcourse). I'm also not sure whether using very large cells is a good idea to begin with. The battery packs I've been around (mostly for small motorcycles and small vehicles) all use 18650 cells connected in parallel/series. Tesla does the same and apparantly for a good reason.

author: paulca
author_rank: Super Contributor
timestamp: April 24, 2018, 01:18:28 pm
content: Quote from: nctnico on April 23, 2018, 08:06:15 pmThe chargers I have used balance charge at different stages within a set of conditions. Balancing involves resistive loads and reducing charge current.If the cells are unbalanced, it will balance them to bring them into a tolerance before it will open up the charge current. As an example I put a LiPo on the other night and got 3.1V 2.9V 3.3V cells. Even though I selected 2Amp charge rate it only charged at 0.2A will it balanced the cells to within about 100mV of each other, then it ramped the current up to 2Amp.My current once supports unbalanced charging which omitts this step and leave it till the end.If also has "Fast charge" which ignores cell balanacing completely and just charges at whatever current you ask for until a single cell hits 4.20V when it ends the charge.Finally there is storage charge which balances the cells at 3.7V per cell, either charging or discharging to achieve that.Lithium batteries are best stored at about 60% charge in the fridge.