Does it matter if there is a voltage difference between series connections?

[Kadenmac]

Hello,
I am building a 20s 12p battery pack and I have made the parallel packs already and when making them they were all the same voltage. Does that also apply when making a series connection. If so what is a safe voltage difference? I am using a bms by the way if that matters.
Thanks in advance.

 

[harryS]

You want all the parallel groups to be at the same voltage before you do the series connections. This allows you to start with a balanced battery,

 

[Kadenmac]

You want all the parallel groups to be at the same voltage before you do the series connections. This allows you to start with a balanced battery,

Ok, even if I’m using a bms?

 

[harryS]

Does the BMS have a balance function? It still takes a while if it does, of the cells are off by more than a quarter volt,