I have a Black and Decker LST136 Cordless String Trimmer. I have two lithium ion batteries. One is 36 volts and one is 40 volts. When I place them in the drop in charger, I will sometimes get the flashing red light and the battery stops charging. Lately, I have been placing a battery in the refrigerator before charging it. It seems to reduce the charging issues. Is there goingany issues with doing this?