how to make an automatic balancer for 2 25.6v pack in serie

[Jeff23spl]

Hello
I friend of mine have a autonome home system with inverter, solar and stuff, the system works on 2x 8S 25.2v 200Ah battery in serie for 51.2v nominal
It is 2 battery with integrated 8S BMS on each with display.

The system ran fine since fall 2023 but last week-end, he came back to me telling that 1 battery was at 5% and the other at 50% or something around that.
I was aware that each pack would only remain balanced internally and nothing is taking care of 1 pack against the other. But since both battery was new and the same, that it would remain not too bad....but apparently it is not.

I know i could open each pack, mod them to arrange a common 16S BMS to take care of this but he paid the price for brand named expensive RV pack with long warranty, if i do that the warranty will be gone and if something happen it will be on me...

I could arrange a manual switch to change configuration from serie to parallel with some good length of wire to make a resistance high enough to discharge the voltage difference. This would need to be done to balance both pack and return to normal after. If it would be me, i would do that but this is for someone without any clue about how electricity works...It get risky and sometimes the inverter/automatic transfer dont like to not be connected to batteries so it add complexity that could create anxiety to the user...The other issue the main connection to inverter can go up to 10kw so about 200A, a DPDT DC switch isn't common at this amperage....i found a 125A transfert switch that could probably make it but $$$ and it remain a manual action...

I did search 48v balancer that would take 2 packs like a 2S of 25v each but almost all of them are for 12v packs. Some allow lithium but most are designed for flooded batteries. It is strange that no one sell a product for this problem, that would not be much different to set for 25v lithium batteries, it is the same logic, just different voltages...this bring me here to ask the community if there could be an easy way to make one or solve this problem.

I'm open to suggestions ?

 

[docw009]

I think your buddy needs to address the main problem. I would expect two new batteries of similar size to discharge evenly when put in series. Isn't that a warranty bug?


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[leffex]

Something is wrong. At least you found it out.

You are not responsible for someone else and from my knowledge they won't understand and do exactly what you don't want. My electrician brother had a fire after charging 4 lipos with a 5 bulk charger. I told him more than 3 times not do to exactly that. But he did do it anyway like so.

Doesn't your friend have warranty? If you have the brand name you can find out if there are common errors on the web that someone has written about?

 

[Jeff23spl]

I think your buddy needs to address the main problem. I would expect two new batteries of similar size to discharge evenly when put in series. Isn't that a warranty bug?

That could be it. Both are identical, size, brand and model, both was supposed to be new. As said, i did balance them to connect them toghether last year. I wasn't sure if it would still unbalance naturally with identical pack...

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[Jeff23spl]

Something is wrong. At least you found it out.

You are not responsible for someone else and from my knowledge they won't understand and do exactly what you don't want. My electrician brother had a fire after charging 4 lipos with a 5 bulk charger. I told him more than 3 times not do to exactly that. But he did do it anyway like so.

Doesn't your friend have warranty? If you have the brand name you can find out if there are common errors on the web that someone has written about?

Well you are right, im not responsible for this part... Yes there is warranty, i will check for common errors, it is a good idea.