There is a "Quiet Power Mode" function on the right engine. Two minutes after starting this engine you can put it in QPM mode, which is a slower than normal idle so it acts as a kind of APU. In QPM mode you can use up to 250A of electrical power.
You can put it in that mode before passengers board to heat / cool the cabin and operate some other electrical appliances.
I'm actually not sure if it charges the battery when in this mode (I've never used it other than to observe it working), but I don't see why that would matter - you're not draining the battery while sitting in there in QPM.