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Charging BMS issues with 2 12v 100ah batts in series to 24v?

jbird526

Solar Enthusiast
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Feb 16, 2021
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Maryland
Maybe I am overthinking or not thinking at all regarding this BMS question. :unsure:

If I build a 12v 100ah 4s lithium battery with a 12v 100a BMS, and later decide to build an identical unit but hook them up in series to run 24v components, is there any issue charging with 24v to the BMS's?

If I were hooking up 2 12v Battleborns in series they are still using 12v BMS in them, so maybe I just don't have my head right today.

Ultimately breaking my custom batts down and setting up as 8s and a 24v BMS would be more efficient but that is a different thread.
 
4S w/BMS in series with 4S w/BMS is no issue - just like BB.

The issue arises from batteries of different ages, wear and capacities placed in series. It's very important for all cells in series to have the same capacity to ensure you capture that capacity. If two batteries are of different capacity, you will be limited by the capacity of the weaker battery.
 
If the older set of 4s cells has reduced capacity due to age or use, they could require more time to charge than the new set of 4s cells. In that case the new cells would get to 100% faster and the older cells would never see 100% - whatever 100% is considered to be in your system. This is basically what snoobler said above.

If I knew I was going to go 24v someday, I would buy 8 cells now and build two 4s 12v batteries, each with their own 4s BMS. That way they are approximately matched for age and use.
 
Thanks snoobler and HRTKD for the responses.The age of the cells is a good point and one to definitely consider. Have one set of 3.2 100ah on a slow boat from China right now. Another set after a pay check or two may be a good idea. In the future the 280ah look promising also but that would be for a separate system entirely.
 
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