There's maybe 6 of us that fly consistently at our field. We generally charge our batteries at home but we have an automotive sized 12v battery at the field in a weather sheltered stand. On that stand is a 12v solar panel that feeds an automatic charging circuit that keeps the battery pretty well topped off. We generally don't charge more than 3 batteries at a time and usually the biggest pack is a 3s 2200. It works great for us. Another thing is most of us are of the "old fart" variety so we fly a bit, talk a lot, fly a bit, and talk some more.
On cloudy days at least one of us will bring along an additional extra 12v battery (lawn tractor size) just in case the field battery is weak. (the charging circuit pulls a little bit on it's own). The lawn tractor batteries are pretty reasonable at places like Lowe's Building Supply or Tractor or Farm Supply stores--like $30-$40. They'll run a couple of iMax B6 chargers near all day depending on the size of packs you're charging.