I think the easiest thing you could do is rent/buy a portable generator and use that to power your charger(s) while at the field. If noise is an issue, there are solar powered generators that could also work. I know, generators aren't cheap(I have a pair of Honda 2000s, so I speak from experience on this). That's why I suggested renting one for a day. That said, I see people running generators to charge their batteries at boat races all the time. Some even run multiple chargers since the boats run 8S packs with multiple heats being run in a day