Just charge it at 1C (1600mAh for 1 hour).
If you're really worried about it then charge a .5C (800 mAh for 2 hours). No need to charge it any lower. In fact it might be better to charge at 2C for 30 minutes if you need to grab the battery and leave the house to go to the game.
Now as for the batteries, are they high quality? Are they working? What charger are you using? Is it a "smart" charger or a "dumb" charger (typically the wall chargers you plug in directly into the socket that you get stock with your gun)? If it's charging for only 15 minutes before shutting off then there's a problem, is your battery warm/hot after this? If not then you're not charging at 4C.
Secondly, are you sure it's not a problem with your gun itself? ie. Motor maybe dying, gears shimmed badly, or motor height set wrong? It shouldn't be struggling to turn the gears. If it's fine for 70-150 shots then it's most likely a battery charging problem.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE