If you fly larger helis, you may want to use a portable generator instead of a field battery and a field battery charger.
Honda manufactures a line of very quiet generators which are ideal for use at flying fields. The Honda EU1000i uses a four-stroke engine, is rated for 1000 watts of AC output, has a noise level of only 53-59 dB and will run from 4-8 hours on a single tank. The older discontinued Honda EX350 and EX650 are also good choices for a portable generator if you can find one. These use two-stroke engines so oil will need to be mixed with the gasoline. These generators output a square wave instead of a sine wave, so some equipment may not work well with it. Be sure to test before you buy one.
Yamaha manufactures the EF1000iS which is rated for 1000 watts of output, and is also a very quiet (47-57db) generator.
The 12 volt output of most portable generators is very weak, so the AC output should always be used for charging batteries. Portable generators should be run at least 30 minutes a month to keep the generator properly lubricated internally.