Hi,
NiCad batteries last longest when they are worked hard everyday. They hate sitting around not being used - their ideal use is to be fully discharged and then recharged everyday. That is what makes them still the best battery for use in tradesmen's drills and other tools.
I had a 7.2v Hitachi drill where the battery died after 6+ years - I then took it to a battery recelling place who put in good quality Sanyo NiCads which had higher capacity. I recommend this path. It is more expensive but the way they rewire things up means that the wiring can cope with the high electrical currents.
Paul