For NiMH battery packs, you need to do simple math. For instance, if you have a small 50mah charger and you want to charge a dead 1200mah RX pack, then it will take you 24 hours to charge that pack (1200/50). If you have an adjustable rate charger, RX packs charge rates need to be kept low, 1 amp or less. If you had a fully discharged 1000mah RX pack, and you charged it at 1 amp, then it would take one hour to charge. If you charge that RX pack at 5 amps, you will ruin it. Just ask me, as I forgot to turn the adjustment knob on my charger and it ruined the RX pack in short order.
Information below taken from:
http://www.servocity.com/html/nicad..._batteries.html
Many modelers use high rate, peak detection or time-based chargers to charge NiCAD batteries. Such chargers are NOT recommended for NiMH batteries (unless otherwise specified in the charger or battery literature) as they can cause permanent damage to the NiMH cells. Also, NiMH batteries will not perform well in high rate discharge applications, typically providing only a small fraction of the rated capacity in these instances.
NiMH batteries also have approximately twice the self-discharge rate of NiCAD batteries when in an used state. For example, when your radio is off, a 1650mah NiMH battery can discharge itself nearly twice as quickly as a NiCAD battery, typically within one week. Therefore, you must charge your NiMH batteries the night before each use.
