Battery experts - question for you, please.

[alcatraz]

Hey fellow riders

I'll get right to the point. Is it possible to take a gotway pack out and piggyback a DIY pack off of the other pack's BMS? I know it will balance slower but is it "safe"?

I'd use balance wires that are safe up to 3A continuously. My fear is obviously that if the cells have different characteristic they might behave differently under high current charge (emergency braking) or discharge (fast acceleration) to a point where there are currents traveling between the packs that exceed the limit for the cables. They then overheat and then some form of catastrophic failure could occur I guess.

My common sense tells me that there's no way two healthy packs could behave so differently that currents even close to 3A would pass through the wires. 

Then there's the tesla v2 1480wh where they use one BMS. They didn't decide to do it "my way" but the other way by making two 42v packs and hooking them up in series. Why? Is it because my idea is too dangerous? 

Thanks for reading and commenting.

Edited September 27, 2021 by alcatraz

[Tawpie]

Caution is wise here. Anytime you mix two topologies there are potential risks... different cell characteristics (cell internal resistance, cell capacity) may mean one pack may deliver more power than the other especially under load. Likewise, they may want to charge at different rates and I don't know how the BMS would handle that. It's key that battery packs that are connected in parallel behave pretty much identically unless you have additional circuitry to manage the differences that develop.

A lot more detailed information about the DIY pack is needed, and I personally wouldn't go this route without a very good understanding of both packs and the BMS.