Yeah, I'm pretty sure you have to have a battery box to charge batteries.you have a coal generator/battery/any power source powering a battery box and then put the empty battery into the top slot. You'll know if it works if with no battery the top number on the right side of of battery box gui is going up. hope it help's
yeah forgot about that.Just make sure you wait a minute for the Generator to charge up enough before it starts charging your Battery.