how to balance battery banks?

[erhard]

i have 3 12V 50Ah LiFePO4 batteries in series to get 36V. i did the initial balancing by connecting the batteries in parallel after they were all charged up to 13.4V.
i read that this process needs to be repeated like every 6 month.
so yes at very low voltage <=12V or larger voltage >=14.4V they can be differ by 0.1V-0.4V ( which is my resolution of the small DVM attached to each battery.

question:
i cannot find a BMS that allows to connect the 3 batteries and it manages this series setup continuously as usually is done with individual cells.

 

[sunshine_eggo]

Individually charge to 14.4V and charge in parallel to 14.4V. Parallel at 13.4V doesn't do much.

Not aware of any specific for 36V, but you can use 3X of these:

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Once you've properly top balanced them, one each of the above can be connected to each battery and then connected together. Can go a long way to maintaining balance.

I used these to maintain 2x 4S strings of trojan T-1275 FLA.

 

[erhard]

thanks will try next time. i just followed the LiTime battery manufacturer suggestion saying >=13.3V

want to clarify. i do have 3S setup (see initial email). so why is their no BMS for 3s 12V each. should be doable and enough demand.

 

[sunshine_eggo]

thanks will try next time. i just followed the LiTime battery manufacturer suggestion saying >=13.3V

Most low end battery manufacturers are stupid. They really can't be trusted.

want to clarify. i do have 3S setup (see initial email). so why is their no BMS for 3s 12V each. should be doable and enough demand.

36V is the redheaded stepchild of the battery world. You may think there's enough demand, but when you compare 12V, 24V and 48V to 36V, 36V is an infinitesimal share of the market.

 

[erhard]

Most low end battery manufacturers are stupid. They really can't be trusted.



36V is the redheaded stepchild of the battery world. You may think there's enough demand, but when you compare 12V, 24V and 48V to 36V, 36V is an infinitesimal share of the market.

sadly you are right and i did not know that when i setup my system. suddenly i realized that there are not too many controllers for 36V. i have now 6 x 100W with Vcc=15V each and use 3s2p config. my inverter accepts 25V - 65V but 4s panel Voc would be 80V so no good. i could use a DC-DC step up and would be able to charge the 48V battery bank.
anyway, not sure why 36V is such a rare thing?
also DC/DC is another loss in efficiency.
currently i measure for example 2.9kWh produced/sunny_day and 2.4kWh put in the house grid --> 82%, not too bad i think.