LiFePO4 balancing. - Page 1
Forum section: electronics / general

author: paulca
author_rank: Super Contributor
timestamp: February 28, 2023, 12:37:21 pm
content: It's been a while since I looked into lithium cell balancing. I already see a few "new" techs thrown at it. Like "active flying capacitor balancers", which looks like a set of capacitors, a PWM circuit and some mosfets which connect the capacitors to each cell in turn. Such that higher cells charge the caps and lower cells discharge them. Efficiency seems to be the issue, certainly when testing extremes which they probably would never be used over.It made me start thinking of active balancing again as I had similar or better ideas for top balancing. Passive resistors are just pants.I figured I could do better than a PWM circuit.An other example I seen "out there" which is more my style was using a completely separate MCU (STM32G030) per cell, running from the cell and connecting back to the mother MCU via opti-couplers, probably just UART. I like it, but it does seem a bit overkill. Also it was designed to clip into one of those 18650 grid arrays, not LiFe bulk cells. And... it doesn't actually do anything special in terms of balancing... just a passive power resistor to cap the top cells. Making it a bit snake oil for Lithium shedphiles.So. My thoughts are to take the two ideas and merge them.A single MCU connected to (initially) cell 1. It sees 2.5-4.2V. Via a 50Ohm resistor it charges a supercap. The MCU spin monitors that cap and waits for it to stabilise. Notes the voltage. Disconnects entirely from cell 1, pauses a few miliseconds and reconnects to cell 2. Again via the 50Ohm resistor the supercap equalises to that cell voltage and it notes the voltage and moves on cell, 3, 4 etc.After a pass of all cells it can now make intelligent decisions about which cells need the most attention and focus there. It can also determine when the cells are "close enough" and put itself into a shutdown state, disconnected from all cells. Waking up periodically to scan the cells, balance if absolutely necessary and return to sleep. Data can be reported back to base via Wifi from an ESP-AT module.I figure as long as the capacitance and surges are managed during disconnects/reconnects and the device is always 100% floating, even if powered from a wall wort it should be doable.

author: paulca
author_rank: Super Contributor
timestamp: February 28, 2023, 12:42:13 pm
content: One challenge which might need addressing separately are the "fail safes". Low/High voltage cut outs for protection. Overcurrent protection (asides a basic fuse!). Soft fusing, cell fusing etc. I doubt a "floating" device can help much here except to monitor and inform any "intelligent" control system something needs done. For me, that is not enough, so a completely separate set of interlocks in the form of cell or pack cut outs ... I can probably source those pre-made, it's just hard to find ones that DONT also try and balance or charge monitor these days.

author: tszaboo
author_rank: Super Contributor
timestamp: February 28, 2023, 03:30:46 pm
content: LiFePO4 has very stable voltage across it SOC, so small measurement errors (and balancing then) leads to imbalance in the pack. So they are usually balanced when the SOC is low, maybe below 10% or when it is almost fully charged. If you do it when it's charged, you can get away with cheaper parts, with some wasted energy, that's why it's usually done that way.Otherwise yeah, why not put a motor on each cell, and a gearbox selects which cell to charge and discharge. I'm sure we can come up with many bad ways of balancing cells.

author: paulca
author_rank: Super Contributor
timestamp: February 28, 2023, 04:11:13 pm
content: Well. For now I am top balancing them manually to 3.65V with a PSU and 5A. They were about 100mV across the pack out of balance at 14.40V pack charge, which is a bit too much. I'll go round them tonight bring each to 3.65V individually until they take less than an amp, then repeat. Then I'll leave them with only the solar charge controller load on them till morning and remember to check them before the sun comes out.I have ordered a simple little passive balancer/monitor. The plan being, if I balance them first, then it shouldn't have much to do. It only has like 100R resistors or something, so shouldn't waste too much until I decide if I need a better solution and if then what that is.I believe I am correct they "can" technically go to 4.1/4.2V, but there is little point past about 3.7V.

author: lastguy
author_rank: Contributor
timestamp: March 08, 2023, 11:28:40 pm
content: Where you get the power source for each cell? it might be easier for 12-16V battery array, how about high volt?if you use isolated power source for each cell, system would have high cost I think?

author: Red Squirrel
author_rank: Super Contributor
timestamp: March 09, 2023, 02:38:39 am
content: I have not gotten to a point where I play with these, considering the cells are very hard to even get here but my philosophy would be that each cell bank (multiple cells in parallel) would essentially have its own isolated charger circuit and be a single module. Then you have multiple of these modules in series to get the total voltage you need. They would have a separate port for the charger which would be powered by a common bus which would be connected to the charger input for the entire device. Ideally you'd want it to have a rather wide input range so you can charge from various sources such as mains or the output of a solar charge controller. This would also be rather modular as you can just keep adding more of these modules in series to get a higher output voltage. Would that idea actually work? The main issue I see is in setups like solar where the batteries are being discharged at same time as being charged. Charging lithium batteries requires to monitor how much current is going in so if there's also current going out, it makes it a bit tricky to do right. I guess in an application where there is also a load, you can have the charging circuitry actually power the load directly, then also charge the batteries out of circuit. Ex: with relays and a huge capacitor bank so there is zero switch over time.

author: paulca
author_rank: Super Contributor
timestamp: March 09, 2023, 07:47:58 am
content: That works. I am upgrading from ~12V pack to a ~24V 8 series pack.Current only flows in net. So if your charge controller is producing 5A into the battery and the load is using 2A then only 3A flows down the conductors to the battery. When the sun goes down and the panel is producing nothing, 2A is flow out of the battery.LiFePO4 100Ah cells are "happy" up to 0.5C charge rate, although 0.2C is meant to be gentler. That's still 20 amps - 50 amps!

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2023, 07:57:51 am
content: Forget all redistributive (some call it "active") balancers, it's academic masturbation and those few who have tried to make it in production have failed by making complex, expensive and unreliable implementations.Especially if a redistributive balance suffers from poor efficiency (any that only can push charge to neighbor cell, so has to go through multiple conversion steps to reach the destination; or any that uses capacitive charge pumps), then it's a total oxymoron. Because the only reason to use redistribution is to save energy.And that gets us to the point. It is analogous to designing a switch-mode converter (instead of resistor) to drive a 1.6V 5mA indicator LED from 3.3V "to save energy". How much energy can you save and is it ever worth the increased cost and complexity?Turns out, the only source of imbalance in li-ion cells is thedifference between self-discharge currents. What a dissipative balancer does, it effectively turns every cell into the pack to have the same self-discharge current than the one with highest self-discharge. A redistributive balancer is able to return some % of that energy to other cells.It is good to understand the existence and principles of redistributive balancing, because a future battery technology could emerge, one which has appealing energy density, price per kWh, or some other parameter, but which suffers from high/unstable self-discharge or poor, unstable coulombic efficiency, necessitating large amount of balancing "all the time". But with currently available li-ion, this is not a concern.Another case for redistributive balancing is, if cells cannot be matched in capacity, so you transfer charge during runtime to dynamically run all cells from 100% to 0%, to avoid having excess charge left in larger cells at end-of-discharge. But this requires large redistribution power capability, so is really expensive.Example case: we built a 20kWh demonstration system with second-life LFP in an EV conversion, some of the cells had potential damage due to overdischarge (as destroyed by a fancy, expensive, active balancing BMS). Using dissipative balancing with mere 40mA balancing current never caused issues in that system, despite some 240Ah or so cell capacity.But that one did use a clever algorithm allowing the balancer to stay on for longer than just minutes during the CV phase. Competitors had balancing currents in range of 200 - 500 mA, which already creates a possible thermal problem in case something goes wrong with the control.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2023, 08:03:12 am
content: Quote from: tszaboo on February 28, 2023, 03:30:46 pmBalancing at top maximizes available energy anyway, but as you mention, the voltage curve of LFP is so flat in the middle that if you try to do any balancing there, you are just creating errors.But, at the end of charge, the voltage of LFP rises quite steeply. At this point, you can easily detect if some cells are fully charged or not. What I did with the "clever" balancing algorithm was simply this: when the BMS detects "pack full" condition - which isanysingle cell hitting 3.65V - the voltages of all cells are recorded, to calculate required amount of charge removal. Then charge is removed from the cells so that most is removed from highest voltage cells. For example,V1 = 3.50V -> lowest, do not remove chargeV2 = 3.55V -> remove 1 units of chargeV3 = 3.60V -> remove 2 units of chargeV4 = 3.65V -> remove 3 units of chargeRemoval of charge is simply keeping a resistor on for X minutes.This worked very well. Voltages were sensed during low-current charging. Actual shunting could go on regardless of use (i.e., the vehicle can be driven and the balancing goes on, based on values acquired at the end of charge.)

author: paulca
author_rank: Super Contributor
timestamp: March 09, 2023, 08:15:09 am
content: Yea. On the question above "How do you get power for each cell?"I used a floating power supply set to 3.65V and manually moved it between cells. Individual cell charging/monitoring is only tricky when you are also referenced to the pack ground and VBat. For that you need some interesting circuitry and the real challenge is measuring the top VBat voltage as it's your max + rail and thus measuring it has challenges. I considered writing/making a cell monitor to publish those stats to my data system. However...I am going to buy a BMS. Not least for the protection. Reviews are pointing me to a JDB BMS on AliExpress. Probably just a 40A one, although given it's source, maybe the 100A one as I have a 40A system. It has RS485 comms and enough settings to basicly turn it all the way down to minimal/essential.I find the balancing is most important near the top and near the bottom. Without cell monitoring a 12V pack pulled down to, say 10V (0%) might on inspection find 3 cells above their minimum and 1 cell well below it. The opposite happened during charnge.For LiFEPO4 this isn't such a bad deal, certainly on the charge side as the minimum and maximum voltages are not contained with in the packs normal charge/discharge region but well outside it. Unlike a Li-ion which is 100% charged at 4.2V a LiFePo4 cell is pretty much fully charged at 3.65V per cell. However, you can, continue to charge them all the way up to 4.2V per cell. It's just there is no point. The sell discharge at 4.2V will drop it back to 3.8V pretty quickly. Similar thing happens at the other end. Below about 3.3V there is very little capacitor there to get. It means your charging regime can cut off long before MAX and your BMS cut it off long before MIN. Li-Ions when trying to maximise your pack life, usually results in a full absorption charge at 4.2V and a cut out around 3.0V so you can get the full capacity out of them. Or if you want them to last, you do the 20%-80% thing. It's just that the LiFEPO4 cells the 20% 80% based on voltage is about right, though gives you 99% of your capacity. So easier to manage.I need to readjust my solar charge controller. I was sure I set it correctly with an absolute maximum VBatt of 14.40V, but I have seen peak voltages of 15.1V. Only for a few seconds when the sun pops out from behind some cloud the controller doesn't appear to cap the voltage that fast and produces a slight overshoot. Again the pack is technically good all the way up to 4*4.2V or 16.8V, though you would never take it there delibrately.When the pack is nearly fully charged and the panel is doing absorption (which is also set to long at 4 hours), I measured the cells and found an imbalance of 200mV. It's hear the risk lies. Without a cell monitor one cell could end up quite high. At the same time however the self discharge rate rises for all the cells, so, in a normal solar day, assuming the pack full charges, the cells will all look imbalanced at the top. However within a few hours they will have settled down and put under a little light load they appear with in 10mV of each other.

author: paulca
author_rank: Super Contributor
timestamp: March 09, 2023, 08:26:21 am
content: On this particular BMS you get to set quite a lot. The preferred balancing method is to do it during or close to absorption. So when the cell voltages are below say 3.50V no balancing occurs. When cells are above that they become balance candidates. I am not sure, but similar at the bottom end might be nice too to prolong the "single low cell" cuts pack.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2023, 10:04:04 am
content: Quote from: paulca on March 09, 2023, 08:15:09 amYou can't balance at both top and bottom. You choose one (usually top), and then the pack is out of balance at the other end, because capacities differ. That's life.As I explained above, you could design a high-power, redistributive, high-efficiency balancer which rebalances the pack during each and every cycle, so that it drifts from top balance to bottom balance. But this would be colossally expensive, for a small gain in added capacity.Remember that balancing is only for maximizing energy. Cell-level LVC and cell-level HVC is needed anyway. It's hard toguaranteeperfect balance (for safety). Hence, safety is always provided by the HVC and LVC; when any cell hits any limit, charging/discharging the pack is stopped. In a perfectly top-balanced pack, HVC happens in every cell nearly at the same time (so it's basically random which cell triggers first).

author: paulca
author_rank: Super Contributor
timestamp: March 09, 2023, 10:47:22 am
content: Dumb idea, but...A single "auxilary" cell. It is not part of the main pack.Under discharge when a cell gets within 10mV of LVC the aux cell is used to support it, using whatever concuction of relays/mosfets to float that aux cell in paralell with the low cell and rotate across them supporting the low cells while we drain the last bit out of the high cells.Under charge, once the main pack has cells exceeding "Nominal charge" voltage when a passive resistive balancer would cap it, instead the aux cell is placed in parallel with the high cell to pull it down. Completing the charge when all cells including the aux cell are at nominal charge voltage.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2023, 12:00:26 pm
content: Good idea, but WITHOUT disconnection of the "aux cell". I did exactly that with the LFP EV pack mentioned above; out of the 26 (IIRC) cells, I think two had significantly less capacity than others, bringing the usable capacity of the whole pack down. I simply added some tiny 2Ah cylindrical cells (of the same chemistry, of course - voltage curve needs to be similar enough) in parallel to get their capacity up. Worked fine.All you need to do is to run the pack to empty once and see which cell(s) caused the LVC, and increase their capacity by adding the tiny aux cell (permanently in parallel).Dynamic disconnection/connection is a lot of work and expensive components, and if something goes wrong and your logic connects the cells together at wrong time, it's catastrophic, as large currents will flow unless the voltages are equal to maybe +/- tens of mV.But make sure you have a problem with capacity differences first. If you buy decent cells, they are very close to each other in capacity, so there is not much to gain.This is a more general comment: make sure you understand the problems you are solving (or if they exist at all) before thinking about how to solve them.

author: paulca
author_rank: Super Contributor
timestamp: March 09, 2023, 12:37:55 pm
content: Quote from: Siwastaja on March 09, 2023, 12:00:26 pmYes. My thought experiments around cross linking between cells etc. Tended to include, "I'm a software engineer and no, I don't trust even my own code during development with real cells.", not without safety interlocks.I was considering using a chain of MCU->74H logical gate array-> Mosfet drivers -> Mosfets. The logic array hard coded to prevent any invalid mosfet configuration with basic NAND gates.It is far, far too much effort for an "hobby" class nano gen off grid system. Might be a different story if you are developing the next bleeding edge multi-cell EV pack and have 1000 cells to balance and a product with a £10-15k market value.

author: shapirus
author_rank: Super Contributor
timestamp: March 09, 2023, 01:17:00 pm
content: I don't know. It sounds to me like you are trying to reinvent the wheel for an unclear purpose.Here is my personal example. Three LiFePO4 batteries. All three use Heltec active capacitor-based balancers which balance the cell voltages continuously regardless of the SoC level, unless disabled via the RUN jumper.1. 4s, 600 Ah.2. 4s, 18 Ah.3. 8s, 18 Ah.No direct access for multimeter, so in this case cell voltages are taken from the BMS reporting over a serial port.Code:Command: cellVoltages - Cell Voltages Information--------------------------------------------------------------------------------Parameter Value Unitcell_01_voltage 3.409 Vcell_02_voltage 3.411 Vcell_03_voltage 3.411 Vcell_04_voltage 3.411 Vcell_05_voltage 3.411 Vcell_06_voltage 3.411 Vcell_07_voltage 3.411 Vcell_08_voltage 3.409 VThe only situation where this wouldn't be sufficient that I can think of is very high-capacity batteries made of poorly matched cells where the balancing current achievable by these balancers won't be high enough to catch up. But they can be parallelled, if necessary.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2023, 03:18:42 pm
content: Quote from: shapirus on March 09, 2023, 01:17:00 pmFun to see how the world goes on, and it seems Chinese are now producing cheap redistributive balancers and people are apparently buying them. Didn't know that. And I'm sure they work better than the overly expensive and complicated products I have seen years ago.... but then again:QuoteSo an energy saving device the only purpose of which is to possibly maybe save 1mA is wasting 12mA by just doing the saving.If it works, then it could be usable with semi-faulty packs which have high-leakage cells. Although such cells could be a fire risk anyway.

author: shapirus
author_rank: Super Contributor
timestamp: March 09, 2023, 03:54:45 pm
content: Quote from: Siwastaja on March 09, 2023, 03:18:42 pm...a significant part of which is the indicator LED. Can be removed if necessary, and then again, there's the on/off jumper which can be shorted, for example, only when the charger is connected.Quote from: Siwastaja on March 09, 2023, 03:18:42 pmIt's not an energy-saving device, at least, it's not its primary purpose. Having a balancer is essential to prevent an uneven degradation of the cells and, in case there is no per-cell voltage monitoring, to prevent cell damage.When there is no balancer, even with well-matched cells, disbalance, even if tiny in the beginning, increases over time, cycle after cycle. When the battery is used in buffer mode (e.g., a typical UPS), battery without a balancer will slowly drift out of balance even if no switching to battery occurs, because the self-dicharge rate of the cells is inevitably unequal while the charger will keep the total voltage of the pack constant.Different types of balancers exist. Some require the user to make sure that the cell voltage at which they start balancing is actually reached at the target charging voltage of the entire pack (so as not to end up with one cell being at 3.65V and others at 3.35V or so).For LiFePO4 packs, especially in buffer mode (float charging) and/or with final charge voltage well below the typical 3.60V/cell (say 3.35-3.45V/cell) these capacitive active balancers are what I find to work best for me. It's as plug-and-play as it gets, no worries about the acceptable voltage range or anything else. I think they also go into a soft power-off state when the cell voltages go below a certain threshold to prevent overdischarge (such as when the battery is kept unattended for a long time), but I'm not 100% sure about this.For LiPo packs used in the cyclic mode, those typical for the RC hobby use, I'm satisfied with the built-in resistor-based balancer in my Turnigy Accucell 6.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2023, 04:26:58 pm
content: Quote from: shapirus on March 09, 2023, 03:54:45 pmCompared to a classic dissipative balancer, it is. Both perform the job of keeping cells balanced just as well.There is, of course, a corollary to saving energy: larger balancing currents also become possible because power dissipation is not a problem. Although I highly doubt the need for such high balancing current. My experiments on aged, damaged, second-life 240Ah LFP pack showed that 40mA balancing current is sufficient, with a proper algorithm. Other products of the time used dumber algorithm and worked well with 200mA-500mA. I find it extremely unlikely that 5A balancing current is truly needed.Voltage ranges, user experience, plug/play or not, is totally independent of if the underlying type is dissipative or redistributive.OP is considering designing their own BMS, for whatever reason. "Why do that when you can buy an off-the-shelf product" is useless advice, although one can and should look at off-the-shelf products for motivation.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2023, 04:54:39 pm
content: If a redistributive balancer with high current capability continuously balances the pack (at any voltage), there will be two sides to this coin:1) if the cell capacities differ a lot, usable pack capacity will increase. Classic top or bottom balanced pack gets the capacity of smallest cell. Continously balanced pack gets the average capacity of the cells (assuming 100% efficiency for redistribution)2) if the cell capacities do not differ a lot, but ESRs do differ, usable pack capacity will decrease, as the balancer misidentifies small voltage differences as the cells being at different SoC even if they aren't, and redistributes charge incorrectly, having to then later correct itself. Energy is wasted in this back-and-forth redistribution.This uncertainty is why I'm not a big fan of continuous redistribution. It looks nice when the voltages are seemingly equal, but equal voltage at every moment is not exactly the right metric to look at.The big case for this continuous strategy would be packs where cell capacities are unstable and large differences emerge. With current li-ion not having such a problem, this future chemistry with said problem should be competitive in some other way.

author: shapirus
author_rank: Super Contributor
timestamp: March 09, 2023, 05:04:14 pm
content: Quote from: Siwastaja on March 09, 2023, 04:26:58 pmIt depends on the goal. If it's only to get a working product in the end at a reasonable price, then an off-the-shelf component is often the best solution. If it's also for educational purpose, then yes, DIY may be the way to go, however, it's necessary to understand that in this particular case implementing a DIY device may be too complicated (if it is to be done properly) to prefer it over a ready-made one. It very much depends on personal preference, so I'm giving food for thought rather than an advice.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2023, 06:23:59 pm
content: Quote from: shapirus on March 09, 2023, 05:04:14 pmLooking at paulca's threads, it seems to me he wants to explore all kind of ideas, some totally impractical. So DIY designing a complicated active balancer is not a surprising idea from him. Of course, if one just wants to install a battery pack, they use off-the-shelf components. And if one just wants to develop a reliable and usable BMS in least amount of time, while still learning the relevant concepts, they should use the dissipative balancer design because it achieves the same, but is simplest to develop. On the other hand, if one wants to explore as much design space as possible, prototyping active balancers is great fun. My design over a decade ago, designed for 300-400V packs, used a flyback converter per cell, bidirectionally delivering to/from a common 30-40V "balancing DC bus". I hand-wound the transformers, and got finally to some 70% charge transfer efficiency, with synchronous rectification, before losing interest and just develop a simple distributed BMS with dissipative balancing, which ended up working great and being used in a few projects.

author: paulca
author_rank: Super Contributor
timestamp: March 10, 2023, 04:04:58 pm
content: The reality in my case... today, battery 80-90% full, taking 20 amps at 14.xV and the min-max balance was 0.009V.Now the panel has fully absorbed them up to 14.40+V I expect a different story.

author: rteodor
author_rank: Frequent Contributor
timestamp: March 11, 2023, 06:12:33 am
content: I am considering building a battery and while enumerating the available options of BMS-es and balancers I ran out of paper.My current understanding is that active balancing is not really needed because its more expensive and then is excess energy anyway. Still there might be cases for active balancing:1. bottom balancing (I did not see anybody doing that). Probably done during a polar night when conserving every bit of energy is important.2. large and aged capacities might need active top balancing. Even more so if charging is done at lower voltage levels (like 3.45 bulk / 3.35 float for LFP). That is because the charging current is bigger at lower voltages and aged cells will have larger differences in capacity.Quote from: SiwastajaAndy from Off Grid Garage (yt channel) noticed the following about Heltech capacitive active balancer: it is terrible if its used continuously (unbalancing the cells actually) but it does a wonderful job if its only used at top in float. This balancer seems to work great with a BMS capable of activating an external balancer (some JBD variant in his setup). The balancer is also unable to balance advertised [5 or 10] Amps even in a test setup with fully charged and fully discharged cells in the same pack. The voltages are just not enough to drive such currents.And there are also inductive balancers (Heltech has one too).... And there is JK BMS with integrated active balancer....

author: shapirus
author_rank: Super Contributor
timestamp: March 11, 2023, 10:58:13 am
content: Quote from: rteodor on March 11, 2023, 06:12:33 amI wonder what the application conditions were. I've never noticed anything like this with mine (which I have three of). Whatever happens, it always tries its best to equalize the voltages. Yes, it may fail to balance fast enough, when the charge/discharge current is too high and the cells are too unequal (and the advertised balancing current is not reached, yes). But it doesn't mean that it makes things worse.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 11, 2023, 12:44:41 pm
content: Quote from: shapirus on March 11, 2023, 10:58:13 amWhich is a wrong thing to do. You are assuming this is the right metric, but it's not, because voltage is not indicator of SoC under charging/discharging current,especiallyon LFP mid-curve. The balancer is doing needless work and wasting energy as a result. When on-grid, maybe the wasted energy doesn't matter, but if the "balancing" is going on while discharging, the wasted energy then reduces the usable pack capacity. This might be more serious than just the power consumed by the indicator LED.It is impossible to say if the net effect is positive, neutral or negative by following the voltages alone. Much more sophisticated monitoring and analysis would be needed, involving accurate current measurements (for charge integration).A proper algorithm would monitor a full cycle without any balancing to figure out the amount of active balancing needed during cycle, and then repeat just that pattern. All the small details need to go right.This is clearly a cargo cult engineering product. As an expert, I would not recommend it.

author: paulca
author_rank: Super Contributor
timestamp: March 11, 2023, 01:14:30 pm
content: Some of the balancing boards I am looking at provide a whole set of parameters allowing things like, only enabling the balancer when certain conditions are reached. Such as maybe only enabling the balancer while in "absorption" phase for an hour before cutting charge or maybe bring the balancer back in at the bottom when your pack starts on the descent again. Many options.The Victron "Smart shunt" thing sounds interesting, but it also sounds expensive and proprietary. It's basically a learning fuel gauge type affair, with a coulombs counter maybe, you can get it to watch the pack for a full cycle, then pick what you consider 100% SoC and it goes from there.

author: shapirus
author_rank: Super Contributor
timestamp: March 11, 2023, 01:38:24 pm
content: Quote from: Siwastaja on March 11, 2023, 12:44:41 pmDepends on multiple factors. In my case, which is to keep well-matched cells equalized and not allow them to drift apart over time, and equalize them initially after they are assembled into a multi-cell battery, it's exactly the right thing to do.I didn't care to add the circuitry to turn off the balancer during the discharge stage or only turn it on when the charger is on. Doesn't make any difference in my case: a few mA of the balancer's quiescent current or tens or even a few hundreds mA equivalent of energy wasted at charging and discharging the capacitors don't make any difference compared to the typical 10-30 A discharge current.It may be a good idea to do it in the general case though.Yes, these balancers may not work well (and, the rest of the variables being equal, neither will the resistive ones) with poor quality batteries made of cells that are unmatched in terms of capacity or internal resistance or both.Those will obviously require more sophisticated logic to get the best performance of them and extend their life as much as possible, but it's a special case: normally it's preferred to pick matched cells of decent quality to build a battery.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 11, 2023, 01:52:25 pm
content: Quote from: shapirus on March 11, 2023, 01:38:24 pmIt's totally wrong thing to do, although as a side effect, if you equalize the voltages all the time, indeed the cells can't drift apart in SoC very far. However, this is very inefficient way to keep a pack balanced and reduces usable capacity, and the primary reason to balance a pack is to increase the usable capacity.Especially if your cells are well-matched, then all you need is a small classic dissipative balancer.You have absolutely no idea what you are talking about. You are a consumer buying some Chinese shit and based on that "experience" you come to teach experts who actually have researched and developed such systems. This is just getting ridiculous. I'm out of this bullshit discussion.

author: shapirus
author_rank: Super Contributor
timestamp: March 11, 2023, 02:24:08 pm
content: Yes it's difficult to admit that a dirt-cheap Chinese board can now in most situations solve the problem which previously required a skilled engineer and costed a fortune to solve. I understand.However, there are still many cases where complex solutions are required, and skilled engineers are still in demand there. So there's no reason to get frustrated, not before AI replaces them in a few years, anyway.(but balancing a damn battery pack is not one of them.)

author: paulca
author_rank: Super Contributor
timestamp: March 11, 2023, 05:22:21 pm
content: Assuming we are talking about the same active capacitve balancers... aka these:https://www.aliexpress.com/item/1005005050192601.htmlI gather it's a series of logic counters and mosfets. The tests I seen, A*V in > A*V out. By a fair margin of double figure percentage.That is what you would expect with a PWM on a mosfet gate every rise and fall it goes through the variable resistive region. Every time you turn the mosfet on you have to charge the gate. Everytime you turn it off you dump that charge to ground. It's all wasted.If you have a LiFePO4 cell at 3.30V and it's neighbour is at 3.20V and you attempt to balance them, you could end up transferring 25% of the capacity of the cell via a 80% efficient balancer. I think the flat portion on most cells is about 0.5V from 0% to 100% resting OC. 10mV difference can amount to quite a chunk of the capacity. Its that reason balancing is not recommended specifically for LiFePO4 within that voltage curve region. Li-Ion is more linear and affords balancing during charge. I would always balance charge my high current LiPos, unless I'm in a real rush, when "fast charge" canes them at 2C until one cell hits 4.20V and takes half the time.

author: paulca
author_rank: Super Contributor
timestamp: March 22, 2023, 03:56:29 pm
content: Back on topic.I have been using the lead-acid boost charge profile at 14.40V to "effectively" top balance the 4S pack through "absorption".It's been a week or so now and ... it's not exactly working out as planned.I have one cell stuck in it's flat charge region while the other 3 are climbing up over the 3.65V target. The highest got to 3.900V while the lowest was still dragging in the flat region at 3.460VWhile I was expecting them to appear unbalanced at the top undercharge the self discharge back to the 3.6/3.7 region would effectively equal them out.That is assuming however that all cells reach their upper voltage curve on every charge. Once that stops being try the cell imbalance will probably continue to get worse until there is a cell taking far too much voltage on it's own.While 3.9V is not terminal for a LiFePO4 cell, it's not optimal or recommended.The only balancer I have had a total "capping" current of 100mA per cell. Thankfully, the sun is now out of scope on the panel and that 3.9V cell is rapidly falling back under 3.7V.I suppose it bumps the BMS board up a notch in priorities. I was hoping to get away with it until the other 4 cells arrive mid april, then I don't need a 4S BMS, but can go to the main range which start at 8S.I have a 2 Amp balancer/BMS but it's 3S.When I finish work I suppose I will have to go out and manually top balance them all again... and probably repeat that at least weekly until the other 4 cells arrive.We don't have much sunny weather planned anyway

author: shapirus
author_rank: Super Contributor
timestamp: March 22, 2023, 04:51:21 pm
content: I'm not sure there is such thing as absorption when it comes to lithium cells. You can't "overcharge" lead-acid cells, because once they reach a certain voltage, they won't be able to get any higher, because this "extra voltage" is "spent" for dissociating the electrolyte, which is known as so-called "boiling". Overcharging them simply means slowly (how slowly? I don't know) destroying their electrolyte, which, in non-sealed (or manually unsealed) batteries, can be restored by bringing the acid-water solution back to the required density.This is why it is possible to balance lead-acid batteries by charging them at a higher voltage: when some cells begin producing hydrogen and remain at a certain voltage level, the others will eventually catch up and become equalized. This is my understanding of this process, at least.It apparently doesn't happen like this with lithium batteries, which is why it is important to have their cells balanced, especially as you're nearing 100% SoC, or, rather, as some of the cells are reaching their maximum allowed voltage.

author: paulca
author_rank: Super Contributor
timestamp: March 22, 2023, 05:04:54 pm
content: There definitely is absorption during charge.Take an average small LFP cell.Resting voltage, lets say 3.4V, close to 90%+ SoC.Apply 1 Amp of current to it and the cell might rise to 3.8VRemove that current and it will settle back to 3.4V.Apply 5 Amp of current to it and the cell might rise to 4.0VRemove that current and it will settle back to 3.4V.It's that "boost" differential I was hoping would scale in the same way as the upper voltage curve scales. When cells have 14.40V across them a lot of those volts are accounted for by the "boost" differential across the cells. Because there is so little actual capacity up there I was assuming it is caused by the resistence to charge rising as fast as the voltage. Thus a reasonable amount of time spent unbalanced in that region, assuming of course that no cell exceeds 4.2V seemed likely to balance the pack.I think ultimately it would/will, but with such high charge currents over 0.1C it does not appear to be working out.

author: Zucca
author_rank: Unknown
timestamp: March 22, 2023, 05:54:24 pm
content: This is why I decided to use my LiFePO4 from 20% to 80% SoC and to avoid any possible chinese or cheap or professional BMS.BMS does cause more harm than good on the long run.Yes I had already some fun with thermal events and white smokes events. All because of (poor designed) BMS.BTW designing a good BMS is a nightmare.I mean instead to built clever protection in the battery we should use clever chargers and clever loads.That said, I think it is crucial to have a stupid passive monitoring in the battery pack that reports the voltage of each cell to the outside.Don't know but I am a fan of stupid battery packs that are not a pain to replace (look at here what a horror movie), and have all the protection outside.And yes, for portable application where 100% of the SOC needs to be squeeze out, then and only then, I will bent over and install a BMS.PS: Very interesting discussion, thanks for all the good info here. Especially thanks toSiwastajafor sharing his knowledge!

author: paulca
author_rank: Super Contributor
timestamp: March 22, 2023, 08:58:02 pm
content: I have seen people white smoke BMS's by testing the overcurrent protection. I believe the device had 20 mosfets (some odd back to back "bridge" config) 10 x 170A brutes in parallel and claiming something like 300A breaking current.Turns out it did break the current path, but unfortunately for one poor little mosfet at the end, it was the slowest. Puff! Cracked through the case, skid marked the board.Chinese electronics, halve the stated ratings. If you "need" 300A breaking current, buy 600A.I personally plan on only really using 40A, but technically the battery is 100A capable and should be cabled and fused accordingly. Thus a 100A BMS. The only time the over current protection which I believe is 300A rated is going to be a question is in a short circuit or serious overload scenario and the BMS popping a few mosfets trying to halt that, I'm ok with that. £50 BMS is cheaper than the potential damage a short will ultimately lead to. To be honest, I might consider setting the overcurrent protection on the BMS above the battery DC breaker rating, which is probably cheaper still to replace if it dies breaking 300A short circuit.I don't need to worry about overcurrent under charge, my solar panels have known limits.On the "balancing" aspects, I am picking a BMS that has fine grained settings over the balancer. When it comes on, when it goes off, what imbalance it will tolerate etc. Having it come on as soon as one cell hits 3.65V and remaining on while there is a >20mV imbalance, using it's full 2A active balancing path... ONLY for top balancing. Then disconnecting 99% of the time.The BMS would also have solved todays 3.9V mishap. The "charge mosfets" would have shut down the incoming current from the solar panel as soon as a single broke a 3.75V limit (by example).

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 22, 2023, 09:09:32 pm
content: Quote from: Zucca on March 22, 2023, 05:54:24 pmUh, no. I certainly would not second that.While some BMS in cheap (read: crap) batteries may not work well, adding a reasonable level of protection inside the battery shell itself is a must IMO.LiFePO4 are not as dangerous as standard LiIon, so they can take a good amount of beating, but still.That said, for sure if you buy "random" chinese batteries from shaky sources, the odds of getting something reliable is pretty low, and I'm not surprised about your experience with the built-in BMS.Generally speaking, you protect what needs to be protected (here the battery) as close to it as possible, rather than rely on "clever" stuff connected to it.Besides, your clever stuff is never going to be as clever as you thought.Maybe you can also rely on clever users rather than add any protection to a product, in the same vein.Just my 2 cents.

author: paulca
author_rank: Super Contributor
timestamp: March 22, 2023, 09:11:39 pm
content: Quote from: SiliconWizard on March 22, 2023, 09:09:32 pmI tried this recently. I put zero protection on a PCB I had printed. Willingly. Believing, "It's a dev board, only I will be using it. I'm not stupid."Well. Guess what? 2 voltage regulators and an Optical->I2S module later, I'm regretting that decision.On my other battery builds, most appropriately the one in context is the "growing, budding, house battery style" with LiFePO4 cells. It's pay day in a few days.The shopping cart contains* a BMS (the best reviewed one from AliExpress) JK-BMS. Which includes RS-485 monitoring and control.* a small dual way din enclosure* a 100A DC compatible breaker (not from aliexpress!)* 2 large 100A capable 'female' bulkhead style screw terminals, ideally with safety caps.The DC Breaker will be overcurrent protection and (assuming I can find a dual pole breaker) the isolator switch. I know using a breaker RCBO as an isulator switch is frowned upon, but much like your air-cond or heating system the battery will very, very rarely get turned off.As I have the mechanical skills of a lettuce that has been microwaved for 30 seconds, I am placing the battery inside a "Really Useful Box" with the only exposed bits being the female + and - terminals and... the BMS display/rs-485 cables.

author: Zucca
author_rank: Unknown
timestamp: March 23, 2023, 12:48:59 am
content: Quote from: SiliconWizard on March 22, 2023, 09:09:32 pmfor "clever" I meant KISS or appropriate solution.Example to limit the current and prevent fire, a simple fuse is a very clever solution.To me better than any µP or high tech BMS.Why the closer to the cells the better?I mean a good protection circuit will work either if inside the battery pack as well as integrated in the load or charger.Of course a temp sensors on the cell can`t be avoided in some application, but why this general concept to put a protection or BMS or whatever in the cells pack?A battery pack is a consumable device, we should be able to replace it throwing away the less components as possible....

author: Zucca
author_rank: Unknown
timestamp: March 23, 2023, 12:50:39 am
content: Quote from: paulca on March 22, 2023, 09:11:39 pmIMHO the problem here is the zero protection of course.

author: paulca
author_rank: Super Contributor
timestamp: March 23, 2023, 10:03:27 am
content: As I am also spec'ing a new battery for "Audio electronics", because I gave up on understanding power supply filters, I have been looking at some smaller, lesser, BMS's.That puts me in the market for some of those "cheap and possibly nasty" BMS boards that seem to get put into every multi-series cell battery pack. They even sell kits sans the cells.The issue I see with them, well, some of the issues I have seen with them.Breaking current. Most use bridge mosfets and the smaller board only have 4 in parallel. The BMS may be advertised as, say, 30Amp. But also claim a breaking current of 100A. However under test you will find the over-current protection has a delay, sometimes that delay is quite long, several seconds. Even a large lunch box full of 21700 or 18650s in 3 or 4P could exceed 100A in a short circuit ,especially if the battery has been wired "beefy" as it should. The issue is that those 4 mosfets trying to break that fault current may fail or more likely will succeed releasing white smoke from several of the mosfets that never switch off at exactly the same time.Balancing current versus charge current. None of these cheap ass BMS's are actually "charge" aware. They are completely passive to the charge current. The only thing they can do about it is disconnect the whole battery if they don't like it. So when a solar panel is pushing 10-15 Amps into the pack and it only have 0.5A balance current, one cell will hit 3.65V and the whole pack will disconnect. Then a few minutes later when the cells rest down it will turn on again. On, Off, On, Off, On, Off until those poor little mosfets die or overheat. That battery disconnect is absolutely NOT recommended for anything else on either side of the battery, or for the battery itself. Solar charge controllers, for example, detest having their battery removed, with such large inductors and buck converters removing that huge balast from them, I'm guessing, causing huge transient inductive spikes and removes any "system voltage" reference the charge controller had.This is something that only caught up on me recently. I was too familiar with using combined balance chargers. These ARE charge aware and in the scenario a cell gets to 3.65V they will engage the balancer, but they will also lower the charge current such that the balancer can keep that cell under 3.65V while giving the others a chance. This can prolong charging, but gives you a full absorption cycle at 3.65V.Large battery BMS's are not like this. The only thing they can do with charge current is (a) ship it between cells with capacitors or (b) resistively cap high cells (c) disconnect the whole pack.

author: paulca
author_rank: Super Contributor
timestamp: March 23, 2023, 10:19:19 am
content: BMS's are essential to maintain the "DO NOT EXCEED" values of the Lithium cells.Mostly because almost all consumer batteries sold today are lithium. However, correct adherence to those "DO NOT EXCEED" values, cannot and should not be placed into the hands of the layman.People out there will watch a video on TikTok on how you can charge an 18650 with nothing more than a 5V USB charger! Seriously. Go search YouTube and TikTok and you will find hundreds and hundreds of similar videos.Each and everyone of them a fire harazard.When "lads" buy one of the larger "replaces a lead acid" 100Ah lithium pack and it turns out to be LCM not LiFEPO4 and they... without reading a single line of the packaging least any manual and without the slightest understanding of what they are doing, go ahead and charge it with the 20Amp lead acid charger.... leaving it completely unattended for days.That is why lithium batteries come with BMSs inside. I would not be surprised is that becomes a law/regulation in places like the UK. That all lithium battery packs sold much include a BMS. Of course that BMS must pass a certification programme which will cost the manufacturer thousands, so UK sold BMS's will tripple in cost and everyone will go to china anyway.