1. The batteries have to be the correct tier (too low, and they won't work; too high, and they probably risk blowing up the machine)
2. The slot can also recharge batteries and electric tools, so a full battery in a machine connected to a power supply won't have an obvious effect. However, an empty battery in that situation will recharge (though this might not be immediately obvious). Alternatively, a full battery in a machine that isn't hooked up to an external power source will allow it to operate from the battery (although if the operation requires more than the battery holds, or more than one amp, such as arc furnace or thermal centrifuge recipes, it won't complete the operation). If you stick the full battery into the machine without attempting to operate the machine, the effect might not be obvious.
Ahhh alright, it was just the tier of the battery that I had wrong. Thanks a lot!