I have a Nu tool cordless electric drill, Which I bought second hand and both the batteries were charged up and it worked really well but there was no instuction book and I dont know how long to charge the batteries for , they are 20.4 volts. Can anybody help ?.
NiCad cells should be charged to 1.35 volts per cell. A 20 volt battery would therefore contain 15 cells. The cells should not be allowed to discharge to below 1 volt per cell, ie 15 volts overall. Charging should ideally be carried out at a constant current if you want to get the maximum charge into the battery. This means that if you have the charger supplied with the drill you just plug the battery in and leave it there until it starts to warm up. Ignore slight warming as that's normal but rapid warming towards being hot is too much. The cells will tolerate some degree of overheating but best not to overdo it. If you want to calculate how long it will take to reach full chatge from a fully flat condition you need to know the capacity in AmpereHours of the cells. It's usually marked on them but to find it you'll have to dismantle the battery. Do it carefully so as not to short anything out. AmpereHours is just Amps times hours so if the charge current is, say C/10, it will take 10 hours. Otherwise fing out experimentally. Charge from flat until the battery overheats.