I have a 4S 25c 5000mah zippy and it takes forever for me to use up all the charge. After 3 race days I still had around 60% left. I think a 2200mah to 3000mah would be the most practical. You can get a lipo voltage checker on eBay for a few bucks. When the volt per cell gets to around 3.7V/cell @ no load (resting) go ahead and charge it.