Steve
The battery is firmly mounted in the controller. It's not normally user accessible, though there are a few YouTube "how tos" on changing the battery.

There is a charging port that normally charges the controller with the same charger that recharges the batteries. I know that when you plug the charger in, DJI says it's something like 90 minutes to charge a battery that will play out in 45-50 minutes. So apparently the controller can eat up the battery faster than the charger can supply power. I'd be happy if I could just get an extra 30 minutes however. I'm not sure of what voltage to put into the charging port. The charging connector looks like what we used to call a mini-phono pin, hollow in the inside, I assume the outer case of the plug is the ground.