8s/24v battery balancing

[jjaj823]

Greetings all!

I have 4 x 8s/24v batteries that I put together, with each set of 8 cells attached to a BMS. I accidentally tripped one of the battery breakers manually (i.e. it did not trip due to an electrical fault) when it was depleted.

Now, it appears that at least one of the batteries is unbalanced with the rest, which is causing my inverters to shut down as they sense low voltage. I spoke with the inverter rep and he recommended ordering 5 48v/4 cell balancers and running daisy chaining them together to balance all the cells.

But, in addition to being expensive, it seems like the BMS already balances each cell it is connected too, so shouldn't I just be able to balance the batteries as a unit? If so, does anyone have a link or recommendation on what to use for that? I can only seem to find balancers designed for 12v batteries.

Any other suggestions or recommendations are appreciated!

Jeremy

[rffffffff]

I could be wrong for sure but my approach would be to take the low bank out of the situation and charge up the other 3, then connect only the 4th bank and charge that up until similarly full to the others and reconnect the whole mess at that point.

There may be some issue with having 4 separate batteries as opposed to a single 4p8s battery. From what I understand (again, could be wrong) these batteries like to be connected in parallel first then series so that you don’t end up in this kinda circumstance.

[john61ct]

Use balancing gear that accurately displays the individual cell voltages.

Then get one set of cells balanced at a time.

When all are good then reassemble the bank as a whole.

[Booyah45828]

I'd separate the system and charge each battery individually. Then reconnect them into your system. What BMS do you have?

[Bert06840]

Originally Posted by jjaj823

I have 4 x 8s/24v batteries that I put together, with each set of 8 cells attached to a BMS. I accidentally tripped one of the battery breakers manually (i.e. it did not trip due to an electrical fault) when it was depleted.

I hope you don't have a circuit breaker between your cells and your BMSes. You shouldn't, and if you do, you may have deep discharged one cell.

Also, how long did you store your packs in a discharged state?

Have you measured the cell voltages of each of the cells in each of the packs? That will give you much of the information you need to make further determinations.

Originally Posted by jjaj823

Now, it appears that at least one of the batteries is unbalanced with the rest, which is causing my inverters to shut down as they sense low voltage.

So you have four battery packs, each made out of 8 series connected cells, each with their own BMS. And these four packs are wired up in parallel onto a 24V bus. Is that correct?

If your inverters shut down in that configuration, you have FOUR bad packs, or something else is going on. I'm assuming here that you don't draw that much current that you need four parallel connected packs to supply the current.

Originally Posted by jjaj823

I spoke with the inverter rep and he recommended ordering 5 48v/4 cell balancers and running daisy chaining them together to balance all the cells.

That is terrible advice. You can have vastly different cells in each of the packs. Only the cells INSIDE one pack, when connected in series, need to be similar. He's trying to sell you stuff you don't need.

Originally Posted by jjaj823

But, in addition to being expensive, it seems like the BMS already balances each cell it is connected

Yes, but the balancing current of BMSes is often in the sub 100mA, which is adequate for a healthy pack, but not necessarily for one that is out of whack.

, [QUOTE=jjaj823;450557]You don't need any of that. You just need a 3.5V power supply with current limiting. Attach the supply to each cell individually, until it stops pulling current.

Something like this if you don't have it already: https://www.amazon.com/dp/B08HQ92R8C...NsaWNrPXRydWU=

But before you do that, measure first. Always measure first.

Originally Posted by jjaj823

If so, does anyone have a link or recommendation on what to use for that? I can only seem to find balancers designed for 12v batteries.

There are plenty of 8S balancers out there. Like this one https://www.amazon.com/gp/product/B0...73ZEKLGH&psc=1 but I have never had the need for one.

But yeah, make sure you're not being BSed, because you do not need identical packs to parallel. I have a 130Ah and a 200Ah pack in parallel and they play nice just fine.

[jjaj823]

Thanks everyone for the responses!

I think I'll go ahead and remove each pack individually, test/balance if needed and then reassemble & reinstall.

Anyone have any ideas on how to check if the BMS is OK?

[Bert06840]

Measure voltages, it’ll become immediately clear if one is out of line.

And does your bms not have a user interface like so:

[jjaj823]

My BMS definitely does NOT have an interface like that!

What are you using?

[jjaj823]

Follow up question also:

Would I be better* off reconfiguring into something like 2 x 2p/s or 1 4p/8s battery instead?

*= with the understanding that "better" can be subjective...

[Rucker]

Originally Posted by jjaj823

My BMS definitely does NOT have an interface like that!

What are you using?

I'm using a Daly 250A unit. Yes, it does have cell balancing but I leave that function to the cheap capacitor-type active cell balancing PCBs. The BMS theoretically balances but as mentioned, only at very low current, and even then only when cells are almost fully charged. Will Prowse has a video on the performance of the Daly. I consider the Daly nothing more than a glorified circuit breaker. It has temp monitoring, but apparently that doesn't work correctly, again, per Prowse.

I've wired up the cheap balancer, which is actually really effective, but I unplugged it after initial balancing and so far the cells have been maintained by the Daly.

[rffffffff]

Originally Posted by jjaj823

Follow up question also:

Would I be better* off reconfiguring into something like 2 x 2p/s or 1 4p/8s battery instead?

*= with the understanding that "better" can be subjective...

This is my understanding. Cells like this prefer to be paralleled at the cell level instead of pack level like you have currently. Do a little more digging but it’ll solve some of your balancing issues cause it’s only one battery at that point.

[Bert06840]

Originally Posted by jjaj823

Would I be better* off reconfiguring into something like 2 x 2p/s or 1 4p/8s battery instead?

The short answer is a tiny, reserved maybe, with asterisks and disclaimers.
A more sensible answer is: almost likely not.

If you parallel multi-kilowatt-hour packs on cell level, you get the potential for large currents. That comes with a series of issues, most notably risk for high contact resistance (in this context: milli-ohms), which causes fires. The plating on the nut, washer and lug, the torque, the material of bus bars start to become very significant. For currents up to, say, 100A you get away with "nah feels about tight". In the 300-500A range you do not any more.

Also, there isn't much to be gained. I can think of only one - really terrible - scenario. Say you bought 16 cells to make a 24V pack, but they test all over the place. Say five come in 80Ah, four at 100Ah, and seven at 30Ah. Normally you would make only strings of similar capacities. But if you don't have enough cells from each capacity bucket, you may take for example four 30Ah cells and four 80Ah cells, and parallel them up in 80Ah-30Ah pairs, so you end up with four de facto 110Ah cells. Then, you could make a string of four 110Ah duo-cells, and four regular 100Ah cells, and you you could access 100Ah of the total pack. You'd have a 80Ah cell and three 30Ah cells left over, but at least there is that.

Now, that is a terrible idea for many reasons, and cells that are so far gone have no business being anywhere near humans. So don't do this.

It is much wiser to have multiple smaller current pack paralleled after the BMS, all fused individually. When things go haywire, you have the BMSes and the fuses to CYA in at least some horrible scenarios.

Even if you had a series of identical cells, bus bars the size of your arm, copper nuts, and a calibrated torque wrench, you still want to avoid paralleling at that level. If nothing else you are going to need a very large BMS, and there are very few of them for the reasons mentioned above. Up to say, 100A they are a dime a dozen, figuratively speaking.

[jjaj823]

Whelp,

I decided to take the packs apart, balance everything and rebuild using the Overkill Solar BMSs. I chose them because they are at least sent from the US and are not much more than the Daly shipped directly from China.

Hopefully, performing a top balance and getting new BMSs that allow me to monitor at the cell level will solve my issues.

Thanks everyone for your input on this...it is very much appreciated!

Jeremy

[Bert06840]

I also got one from overkill Solar. It is a regular Xiaoxiang, but it doesn’t matter. He supports his stuff, that’s worth the difference in price.

Ps those BMS work just fine.

[jjaj823]

Originally Posted by Bert06840

I also got one from overkill Solar. It is a regular Xiaoxiang, but it doesn’t matter. He supports his stuff, that’s worth the difference in price.

Ps those BMS work just fine.

Good to hear! Support was another reason I went with them. Glad to hear another good experience!

Jeremy

[BusT]

If you have 16 280ah cells could you make a pack of 8 using overkill solar 100amp bms and parallel that with another 8s pack that has a bms for 24v 560ah ?
That’s what I’m trying to do have my cells but alittle lost on the bms but did order two on Monday. Also purchased dc bench top charger and hoping to top balance pack in a 8s pack altogether .

[jjaj823]

Originally Posted by BusT

If you have 16 280ah cells could you make a pack of 8 using overkill solar 100amp bms and parallel that with another 8s pack that has a bms for 24v 560ah ?
That’s what I’m trying to do have my cells but alittle lost on the bms but did order two on Monday. Also purchased dc bench top charger and hoping to top balance pack in a 8s pack altogether .

Yes, you *could* technically do that.

However, you would be limited to 100a discharge current.

Just to give you an example, my 120AH cells have a max sustained discharge of 120A, and a surge higher than that for about 30 seconds or so.

If your electrical doesn't have a ton of DC draw, that might work for you. For me, I have serval high draw items where 100-150A is common for relatively short duration. I also have dual inverters in split phase providing 240v AC. For me, it made more sense to create 4x 8s packs, giving me a theoretical max current draw of 480A. I approximate that I will never exceed 225 though, so it gives me some room.

Basically, all that is to say you may be limiting yourself if you go with a 2p/8s pack instead of 2 x 8s packs.

Hope that helps a bit!

Jeremy

[rffffffff]

When you build a 2p8s pack you’re doubling the current capacity of the battery too so you can draw the same current as 2 x 8S packs. You have to make sure the bms and all the rest can handle that but the way you build it shouldn’t create any specific limitation one way or the other as long as all the bus bars etc are up to handling it.

[Bert06840]

Originally Posted by jjaj823

Yes, you *could* technically do that.

However, you would be limited to 100a discharge current.

Just to give you an example, my 120AH cells have a max sustained discharge of 120A, and a surge higher than that for about 30 seconds or so.

If your electrical doesn't have a ton of DC draw, that might work for you. For me, I have serval high draw items where 100-150A is common for relatively short duration. I also have dual inverters in split phase providing 240v AC. For me, it made more sense to create 4x 8s packs, giving me a theoretical max current draw of 480A. I approximate that I will never exceed 225 though, so it gives me some room.

Basically, all that is to say you may be limiting yourself if you go with a 2p/8s pack instead of 2 x 8s packs.

Hope that helps a bit!

Jeremy

Agree!

And it is safer, and you have quad redundancy. Right now I have three 8S packs in the same configuration.

[Rock-N-Ruth]

Originally Posted by jjaj823

Greetings all!

I have 4 x 8s/24v batteries that I put together, with each set of 8 cells attached to a BMS. I accidentally tripped one of the battery breakers manually (i.e. it did not trip due to an electrical fault) when it was depleted.

Now, it appears that at least one of the batteries is unbalanced with the rest, which is causing my inverters to shut down as they sense low voltage. I spoke with the inverter rep and he recommended ordering 5 48v/4 cell balancers and running daisy chaining them together to balance all the cells.

But, in addition to being expensive, it seems like the BMS already balances each cell it is connected too, so shouldn't I just be able to balance the batteries as a unit? If so, does anyone have a link or recommendation on what to use for that? I can only seem to find balancers designed for 12v batteries.

Any other suggestions or recommendations are appreciated!

Jeremy

So if I understand you correctly, you accidentally removed one of your 8s batteries from the charging circuit and cased it to be drawn down but not recharged along with the other three? If this is the case the solution is simple. Top balance your batteries manually.
Remove all the batteries from the load circuit.
Remove the three from the charging circuit. Allow the low battery to charge to the place where it has the same voltage as the other three.
Then place all four batteries in parallel, not under load or charging, for 24 hours.
Measure the voltage on each battery. They should now be the same and you can configure them back into your system.