Like if I wanted to make a 12 volt generator with a 100 amp capability but wanted to be able to turn it down to limit it's output to 30 amps for charging two deep cycle batteries (15 amps each). Two deep cycles will draw as much as you give them (theoretically). Ideal rate of charge is typically 10-15% of it's amp hour rating. 100 amp hours, give it 10-15 amps. Two 80 amp hour batteries in a parallel bank, ... give 'em 16-24 amps. So if I want to build a generator for charging those batteries, I'll want to be able to reduce the amount of amps they receive so I'm not feeding them 70-100 amps or so. How is that done?