I'm going to get an extra battery and leave it in the truck. These lithiums don't give you much notice when the charge is gone. I read that they can hold their charge for a year. As long as I charge them on the weekend I'm usually good for the week, but sometimes I use the flashlight a lot like yesterday when I was in an old dark basement working on a dryer. They had one 40 watt bulb in the ceiling. You've heard of a 3 dog night? That was a 2 flashlight job!
Not cheap, but the DEWALT DCB119 12-Volt MAX and 20-Volt MAX Li-Ion Vehicle Battery Charger
would let you charge them back up from your service vehicle.