I am upgrading my camper battery packs -- if I put in a 300AH Docan Power battery pack, can i safely add in series within a year a DIY 300ah battery pack to make a 24v system?? or would it be better to add it in parralel to keep it 12 v??
Two individual 12V batteries in Parallel will naturally stay balanced even if they are not exactly the same capacity. If they are placed in Series but have 2 different BMS modules there is no way to keep all 8 cells balanced. If you top balance both batteries then place them in series it will probably work OK for a period of time since they are the same capacity but eventually will drift and you may need to take them apart and top balance every few months.
Of course a 24V system is preferable as it reduces Amperage draw from the batteries so cables can be smaller. If you do go with a 24V system in the future how hard would it be to get an 8S BMS and wire the cells up as one 24V battery?
I would go parallel. Or get a balancer to keep the pair matched if 24v is needed. Also verify with the seller that series is permitted as the BMS must be compatible.
