it all depends on how much time you have between heats really. most of the time i have 45minutes to an hour to play with between heats. when its like this, ill start charging the pack for the 1st heat 40 min. beforehand. after that is over, i start charging the next pack for the 2nd. same throughout the rest of the day.
if you immediately put another batt. on the charger after one finishes, you shouldnt have a problem. i'll repeak right before the race if the batt has sat with a full charge for more than 10 minutes. thats my method anyway.