You buy some cells whether they are grade A, B or they are used LiFePO4 cells, whether they are the same voltage or not, you still need to do the initial balance.
Why do you need to do the initial balance before you use them to build a battery pack ?
The initial balance means to bring all cells to the same state of charge. But using the voltage to determine if they are the same state of charge, it’s unreliable for the chemistry. This is because there will be relatively flat charge and discharge curve. That means it could be the same voltage with your meters but they can be slightly off the state of charge wise.
It is important to have a solid initial balance so that when you do your first capacity test, you have accurate results. If one of your cells is too high or too low, it triggers your BMS high or low voltage disconnect and your capacity results will be off. You’ll think you’re getting ripped off. Also if you have balancing problems with the used or new pack, and you are slapping active cell balancers on there to compensate for that. You need to do the initial balance and see if this fixes your problem.
You should do the initial balance, then capacity test and watch the cells under large loads, to see if there’s a variation with internal cell resistance.
Also keeping in mind if you connect the cells in parallel, let them naturally balance over time, they will balance.But it takes weeks or months depending on what size battery bank you have.
The very initial voltage between them is very small, when it’s in the high state of charge, this will pass current from each other in balancing very quickly.
But if you have a very large battery bank, or if you have very ancient in-cell internal resistance, if you have large loads, or if you want to balance the capacity, you should do a manual top balance.
If you don’t know how to do it, and just depend BMS to balance. The text is what you need:
http://www.deligreenpower.com/deligreen-news/73-en.html What is the difference between “active” balance and “passive”balance?
Do you need active balancer except BMS ? You can find the anwer here.