pengyou said:
I am assuming that you mean one battery is 24v and one is 48v, or each of two is 36v, for a total of 72v? If I understand you correctly, it is a good idea to use serial strings but they should be charged in serial.
Basically. If you series packs with the same type of cell type it's fine. Just, of course, use one BMS and not one per pack.
My problem is, the more information I give you the safer you feel about doing something really dangerous if you misunderstand me. I want to avoid that. Lithium Ion batters are scary dangerous if used wrong.
Let me try to package it.
Parallel connection of cells is safe and good. But you MUST use cells with the same voltage. You can mix different capacities.
Series connection of cells is good. But here it matters that the cells are mostly equal in capacity and power. (BMS balance those cells and react to dangerous over or undervoltages due to imbalance)
Packs are usually a hybrid of the two.
You probably came about ?S?P nomenclature before?
A 10S4P pack consists of parallel bundles of 4 cells which are connected in series. Resulting in say 4.1Volt * 10 == 41V.
Those are first principles.
What is absolutely forbidden is to build the same pack by connecting 4 series strings of 10 cells
When you are trying to combine packs, you almost inevitably run afoul of those first principle rules unless the packs are identical.
Say you have two of those 10S4P packs. Similar age and cell capacity.
It is totally fine to connect those packs in series to create effectively a 20S4P topology.
What you can't do is to combine wildly different types of packs with different capacities. The cells will get unbalanced during charging and discharging. The BMS might prevent a fire. But they usually can't balance them strong enough to avoid the misbalance. And balancing happens by burning off excess energy. Not very useful to do unless you really can't avoid it.
If you parallel packs you have to follow a ton of safety rules to even have a chance of doing it safely. Definitely stay away from that.
And in the context of charging the packs. If you parallel the chargers, they combine power and it's totally safe.
You might be tempted to attach one charger to one pack of say a string of two packs in series. It looks like a good idea. But you are fighting the BMS in terms of balance and you deny the BMS the capacity to protect the battery by disconnecting the charger.
But let's get back to actually helping you particular.
What types of packs to you have? What do you want to do and what do you want to achieve.
I'm still not sure about the motivation behind your question.