I have a battery switch between my batteries so I can easily isolate them before charging. I always thought that would be a good idea. I hadn't considered the BMS angle though.make sure you call Dakota and ask them if you meed to equalize the charge before making the series connection. with seperate control modules most manufacturers want you to charge them seperately and be within 50mv(.05v) before making the series connection. that keeps them equal so the BMS doesnt get confused and shuts one battery down before it is charged.