I'll plug in my 400watt inverter into my truck's 12V lighter jack and leave it all day. I'll charge receiver batteriers, transmitter batteries, etc, some times more than 1 at a time throughout the day.

For example, today at the dd race, I plugged in my inverter at 8am and didn't turn it off and unplug it until around 4:15pm when I was packing up. During this time I charged both vehicles at the same time first thing in the morning, then again after each heat. I also charged up the 3000 packs in my starterbox and my transmitter batteries (all 8, 4 at a time) too. Never once did I think I would drain the battery.