Do I need to disconnect each of the batteries when I charge them individually or is it ok to just turn off the power?
Turn off your main switch.
ARe you talking about charging the string (72v) using the onboard charger OR charging them sequentially one at a time using a 12v charger?
If the former, turn on the main power switch to charge them. If using a 12v charger to charge them one at a time, I’d turn off the main switch. I do the latter occasionally just to “balance” the charge in each battery rather than “overcharging” the stronger batteries to bring the string to fully charged.
Al