If too slow a rate is used, then the charger won't get as strong an indication from the battery that it is full and that can lead to overcharging. That would be less than 500mA for an AA battery. I think the sweet spot is 2-4 hours charging. The Sony charger I have uses a low charge rate (360mA), but it is actually pulsing a higher rate (1400mA). The higher rate gives the good signal from the battery.