if I were to connect two 6s4p batteries with a bms inside both of them, would I need a bms for the newly connected 12s4p battery?
and on youtube, people only connect lipo batteries together, could I connect two lithium ion battery packs together?
Each half would use its own BMS, no third BMS needed. But you have to note YOU CAN ONLY CHARGE ONE AT A TIME, if you charge both from a similar power source, they will short out across the charger input and kill everything. If you have 2 independent power sources (different ac networks then you can)
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No. You don’t need a bms if the liion packs has bms inside. Lipo and li ion both should have a bms
It might be better to disconnect the 6s bms and merge the batteries to a 12s and connect it to a 12s bms. But we run into the problem of new cells and old cells and are the cells all the same kind problem.
could I use a charge balancer to charge both at one time
Not while they are in series. If you separate the packs to charge then its totally fine, nothing unusual will happen since it would be 2 completely separate batteries. If you want to charge both while they are connected, then you need 2 isolated power sources or the negative lead will be connected at 2 points in the battery and short out.
I think you can take a 12s power source and wire 2 ‘6s’ charge ports in series for the output instead of a single 12s charge port, but any imbalance would cause one pack to have more resistance and the other pack would have to much voltage.
Best bet is to use a 12s BMS or separate the packs completely. For separate batteries you can use a balance charger (2 individual or 1 dual) or 2 BMSs with either a shared (parallel) 6s voltage source or just separate chargers