This article is from the Radio Control (R/C) Flying FAQ, by Shamim Mohamed firstname.lastname@example.org with numerous contributions by others.
You need a charger of some sort. If you are using six or seven cells, any
RC car charger will do the job. You don't need peak detecting or any of
that fancy stuff to start with. For larger packs, there are good
high-voltage chargers around. Check out Astro Flight and TRC, among
others. Remember, the biggest enemy of NiCads is heat, so try and keep those
batteries cool when charging. Expect to pay about $40.