I assume you have NiMh batteries and a good charger (one that uses a power supply, not A/C or a car battery) We use Novak Millenium, but others, like the Pit Bull, are equally good. Chargers that work with timers are junk.
Charge at 5.0 amps with a .04 V cut-off. Use a fan to cool the batteries while charging and after until they are cool. Heat kills batteries.
A good pack will charge up to 9.2 volts and beyond. They heat up, but this is OK because you want the volts. That's why the fan.
Re-peak about 20 minutes before the race.