Low temperatures will obviously reduce Ni-Cd and Ni-MH battery discharge rate. electrolyte is at its freezing point, charge speed will greatly slow down. At low temperature (under -15?) charge will raise the internal gas pressure and probably unseal the safety vent. Ambient temperature of 5?to 30? is the best range to get effective charge. Generally with temperature rising charge efficiency will become higher. But when the temperature rises to 45? or higher, the materials performance in the battery will be degenerated and battery service life will be shorten greatly.