Add 4 x 280ah cells to existing 280ah Pack

moonlitsouls

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Hope all is well ,

I have a 12v 280ah pack with a 120a bms

i want to add another 280 pack to double my capacity

i understand that i can wire the cells in parallel but im having a bit of confusion about the bms

i assume i will need another 120a bms , whats the best way to wire this up?

am i just making an exact copy of the first battery and then wiring positive to positive and negative to negative ?

or should i break apart the first pack and rewire the cells in series and make one big pack?

thanks a lot
 
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Ampster

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Opinions vary. If by increased capacity you mean more overall current draw, I would go with two packs and two BMSs. If you only want more Amphours and redundancy is not a factor then a single pack may be preferable. I have done both and the tradeoffs are based on use case and one's own desire for simplicity versus redundancy.
The best way is the way that fits your needs.
 

moonlitsouls

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Opinions vary. If by increased capacity you mean more overall current draw, I would go with two packs and two BMSs. If you only want more Amphours and redundancy is not a factor then a single pack may be preferable. I have done both and the tradeoffs are based on use case and one's own desire for simplicity versus redundancy.
The best way is the way that fits your need

with the single pack , i still need two bms?
 

Ampster

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with the single pack , i still need two bms?
No, assuming you are talking about 2P4S. What that means is two cells are paralleled into what I call buddy pairs and each pair is then in turn put in series with other buddy pairs to make a single 12 volt battery. Only one BMS is needed because the pairs in parallel appear to the BMS as one cell. I am sure there are other opinions about the alternate configuration and you should understand the pros and cons of each and how each fits your use case.
 

moonlitsouls

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No, assuming you are talking about 2P4S. What that means is two cells are paralleled into what I call buddy pairs and each pair is then in turn put in series with other buddy pairs to make a single 12 volt battery. Only one BMS is needed because the pairs in parallel appear to the BMS as one cell. I am sure there are other opinions about the alternate configuration and you should understand the pros and cons of each and how each fits your use case.

Thanks for the help , i prefer the 1 BMS , but what about the balance cables? am i only attaching the 4 cables to the series connection points?
 

DLTooley

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I would make sure to construct my parallel cells from the old and new cells respectively so as to insure a good match.

There is a benefit to having a bms on every cell, but haven’t seen it explained in detail.

My thinking is that it’s certainly ok short to medium term but as cells age it might be more important. That’s certainly testable.
 

time2roll

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What method are you inclined to do?

If it all runs great and there’s no issue removing the battery for the rebuild then I would combine cells into the existing battery.
 
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