Why You MUST Balance Your Lifepo4 Cells and Why They Won't Stay Balanced 

I just used my ipad (with the apple pen) and ziteboard. Hope this helps!

Ok never mind just checked out one of your older videos... Again great explanation.. thanks for the info

Good question. I think the answer is complicated and I think I will do a follow up video to explore the topic. Basically it depends.

@@TheWilltoBuild I'm looking forward to it. The ones I've seen are fairly low amperage if you are pulling a lot of amps so it can't keep up, but yeah there is a lot more to it then that. I've been playing with one for the past week and still don't know how I feel about it. :) Curious to see where your thoughts and test lead!

This is a good question. It is possible in theory - but are on large cells, given the extremely flat Lifepo4 voltage curve, two cells could have extremely similar or even identical voltage but be at different levels of charge. Given that top or bottom balance will work, it just makes more sense to top or bottom balance if you can.

@@TheWilltoBuild The penny dropped with that explanation - thank you! At the bottom and top, the voltage changes quickly with capacity, so paralleling cells up when they are at either end means when the voltages equalise they will also equalise to roughly the same capacity..in the middle, all you can say is they end up having the same voltage but capacity could be almost anything! So equalising at 3.55V is not as "capacity equalising" as paralleling them at 3.65V.

If they are sitting there at that state of charge they are fine. After a few months it is worth using them a little but for the time being I wouldn't worry about it.

3.4 volts when? resting, charging or discharging?

You CAN see the SOC through the VOLTAGE; between 3.000 and 3.400 there are 400 elements of display; especially AFTER YOU GET USED TO ALL YOUR BATTERIES, ONE BY ONE; like you knoe your family, yoyr pets etcetc

It's a little complicated. Going totally full to totally empty is hard on the cells. But storing your cells at close to full all the time is also not ideal. Generally cycling less rather than more is good, but I would always advise never discharging below 10% of capacity and above 90% (except for top balancing).

I just recording on my ipad on ziteboard, hope this helps!

@@TheWilltoBuild Ah,ha! I hadn't thought of using an Online Whiteboard. I'll go and experiment. Thanks very much.

This is a really interesting question. On the one hand, what you describe will increase capacity, but at the cost of longevity. This is because all the higher capacity cells will be constantly discharging into the lower capacity cells they are paired with. So this will help to ensure that you get "average" capacity from each set of cells in your series, but it will also mean that some of your cells will be working extra hard and may not last as long as others. Matching cells that go in parallel with similar capacity will mean a longer life for your cells, but it will also mean limiting the capacity of the battery to the smallest set of cells that are pair together.

@@TheWilltoBuild Thank you for your helpful and informative comment. It isn't easy to be sure of the relative merits of capacity versus battery life. As I have recently installed the cells in the high low pairing format rather than close matching pairs I am inclined to take my chances and see what happens. The differences between best and worst cells are only about 3% so I am not sure that this will be significant enough to show up a difference in total battery life. Having spent the last 4 weeks capacity testing 32 cells I am disinclined to do two further tests of both layouts. It would take me the best part of 2 days to uninstall and install the cells between formats. Two identical discharge/charge tests on the whole assembled pack would be time consuming needing close monitoring. If it then proved that the original format was the better one it would take a further 2 days to return to the original format. My planned application is a mobile one where the installation is in a very small trailer transporting power from an off grid location to my home (only 1 mile away) where solar power is not practical. The power will be mainly used to top up an EV on occasions when it isn't practical to use the off grid power. Any spare electricity might be used by the house to reduce that supplied by the grid. Even without this mobile supply so far we have managed to avoid using grid electricity for one EV for 5 months except on two occasions providing enough power for a driving distance of 4000 miles. These two occasions cost us the small sum of £2.21 (about $3). It is true that I will never get payback on my investment versus grid electricity but the difference compared with an ICE car will result in a payback over about 4 years and avoids the use of any fossil fuels.

Parallel will help, but that can have its own problems too though. And the same problems can exist, they are just less likely to occur.

@@TheWilltoBuild what kind of problems? I've been running a 10kwh set for 5 years without a bms. I just keep the voltage within a certain range.

@@agtlewis small remark: if you only measure the voltage of the pack and switch off when it hits the max, if one cell goes bad, the other cell(s) will overcharge and die. A bms will prevent that.

64 cells?

@@josepeixoto3384 yeah 64 250Ah cells makes a decent battery

It is scary. I was super worried about it as well - it's a lot of money. I would go to Will Prowse's forum (diysolar) and go the section on batteries. People post there a lot about their orders and you'll be able to see where people are ordering from and what their experiences are right now.

@@TheWilltoBuild thank you very much

You are suggesting simultaneously wiring them all up in parallel with 100 ohm 5 watt resistors? I see how that could work, but it will make your batteries look ugly and seems like alot of trouble no? At least relative to just doing a balancing. Interesting idea though.

The difference in bleed current between the highest and lowest cells would be tiny, the balancing effect would be miniscule. The balancing effect might even be offset by any difference in the resistor values or the resistance in the connection between the resistor and the cell. Didn't seem like it would be an effective method to me.

No. The battery's are the exact same size and shape. Your analogy makes it way harder to understand

@@JM-yx1lm Thank you but I disagree completely....What makes some cells lower in capacity is their real world or actual Amp Hour rating...which if we are talking about analogies ....just like litres of water in container....the level in that container is a very good if not perfect analogy for voltage....

@@mekuranda The different heights represents different Ah capacity and as such show the battery has more depth of discharge available than another.

I'm sorry you didn't follow my analogy. The heights are the capacity of the battery cells in my visual (nothing to do with how they actually look).

@@TheWilltoBuild I understood your analogy..but took a little while to understand that your line graphic must have meant AH capacity...even though the word voltage was used often... having been in the position to educate non EE's on the idea of cell balance..I always struggled in getting them to understand until I used the universal analogy of water volume being current and voltage equals pressure...and in the same vein water capacity is easier to link with AH capacity. As I am not a content creator, I would like to provide your explanation YT links to all my clients...the few who I tested this video on were still confused (admittedly these were non engineering... farmer, worker type people)

Definitely not that case. Cells will continue taking power long past the point that it is good for them. You can overcharge cells, which will destroy them and is potentially dangerous. That is why you should always use a BMS.

Yeah BMS balance is way, way too small to do much.

@@TheWilltoBuild my JK bms does 2 Amps; manages 200 Ah mismatched junk cells up to 20 Amps,charge or discharge; after 20 amps,the delta voltage goes haywire.

I’m not a professional and I don’t have a lot of experience But it seems the same on AH . Maybe you won’t hurt all cells going to 3.65 V if you do bottom balance. 🤷🏻

Most people do a top balance. There is a lot of back and forth on this though and either will work fine.

Internal resistance has minimal impact on the most common lithium chemistries, but will cause drift in balance over time. So why choose top vs. bottom balance? * Bottom balancing is slightly better if you are typically running on the bottom part (mostly discharged part) of the battery, like in an EV. * Bottom balancing must be done while the battery is basically empty which means the battery is not available during balancing - maintenance takes the battery out of service. * Top balancing is better if you are typically running on the top part (mostly charged part) of the battery and frequently charging (like in a solar power or UPS system). * Top balancing is also easy for a simple balancer to maintain while the battery is in use every time the charge is nearly full. TL;DR: Top balancing is nearly always the best approach.

@sylvan dB - I think that is very good explanation. Can you say more though on the reasoning behind why bottom is better when you run them low, top when you run them high, etc? Also @jason the whole point of the video was to explain why they don't stay balanced during a cycle, so I'm sorry that didn't come through. Also you are right about the 10/90% charging. But that won't solve the problem of getting less than optimal capacity. You'll still hit your floor and ceiling too early relative to what is possible. But I do talk about that type of charging in my other videos.

@@TheWilltoBuild Bottom vs top has to do with the characteristics of batteries, including the internal resistance changing depending on state of charge. The better matched the batteries at the point where charging or discharging is halted, the more accurate the halt and the less chance of damaging a single cell. Obviously a pack of real world cells cannot be both top and bottom balanced at the same time, so you choose whichever is your more common use case. For example, your BMS may cut off discharge at 2.5v which is quite low. If the entire pack is matched while discharging then the pack voltage of a 4s pack would be 10v and most inverters would have shut off the pack at 10.5v or even 11v or 11.5v - higher is better for the battery. But if 3 cells are still high the pack voltage will be high and we rely on the BMS to shut off when one cell is down. Then that one cell is stressed more than the others. If that kind of full discharge is normal operation for your pack a bottom balance to better match at termination is likely to more evenly wear the cells. Similarly when charging. If your charge controller stops at 14v that would be 3.5v/cell if they are top balanced. There is very little energy beyond that point, and less stress for the cells to stop. But if the cells are not top balanced and one cell reaches a 3.65v or 3.7v BMS cut out that one cell will be more stressed. If that is your "normal" then best to top balance so the regular charger can stop before the BMS safety kicks in. In addition, it can be hard on the rest of the system when the BMS disconnects the battery. Solar charge controllers, for example, often have warnings to connect the battery first, and disconnect the solar before disconnecting the battery. What happens if the BMS disconnects the battery while the charge controller is charging? Having the cells top balanced means your charge controller can stop charging before the BMS is triggered to protect a cell.

@@Sylvan_dB - Excellent information. Cells should be matched for internal resistance and capacity before building a battery from them. If that advice is followed, the white board work shown in this video becomes a gross exaggeration of the "problem", and completely obviates the need for an active cell-balancer. Well-matched cells are not going to gradually become worse, in terms of internal resistance and capacity, and the net capacity of the battery is not going to be radically lower than expected. If this were an actual problem, active cell-balancing would be something included in batteries built by reputable companies. Instead, they simply build their batteries from well-matched cells and use a good BMS to protect them. Active cell-balancing is not exactly a parlor trick...but it's certainly not essential to use, if you started out with a good build in the first place. And again, if you stick to a 90/10 charge model (programmable into a quality BMS) then you never lose any of the capacity described in the video AND you get the best possible life out of your battery. If you need more capacity, build a bigger battery, don't rely on cell-balancing to squeeze every mAh from your cells.

@@jasonbroom7147 I agree active balancing is unnecessary and I don't have active balancing on any battery I've built. But as a hobbiest I do not have the highest quality matched cells and every time I assemble a battery I top balance the cells first. That is a critical step and that is what I took away from this video. The video may have exaggerated the difference between cells, but that is a difference in magnitude not kind. After all if cells were perfectly matched you would not need a BMS, but I assure you no two cells are ever that perfectly matched. You need a BMS and you need to ensure the cells are at the same state of charge before putting them in series.

Thank you for pointing that out. I feel like the suppliers on aliexpress change the listing, or even the supplier themselves, so this definitely got outdated. I've updated the links with more reputable suppliers - thank you for pointing this out. You did me a favor.