Nickel cadmium (NiCad or NiCd) rechargeable batteries can last longer than the more recent type of lithium batteries, provided they are looked after properly. It's possible you think your NiCd battery needs replacing, because it doesn't power your device for long, but this may not be the case; it simply needs revitalising. This means giving your NiCd battery a full discharge to break down crystal growth and return it to a similar condition to when you first purchased it.
Put your NiCd battery into the electrical item it usually powers. Turn on the device.
Leave your device to run until you find it turns off, due to lack of power. Leave the NiCd battery in your device for at least 30 minutes.
Turn on the device and let it power up. It does, but probably lets you know it's low on power. However, leave it on, until it turns off again. Leave it about 10 to 15 minutes then try it again. Chances are it powers up. Each time it powers up, the battery crystals release a little energy and this makes them break up. The smaller the crystals the more energy your NiCd battery can subsequently retain.
Repeat the procedure until your device doesn't power up. It doesn't power up once the battery is fully discharged meaning the crystals have broken down and the revitalisation has worked.
Put your battery on charge. It needs a long charge to complete the revitalisation. The broken down crystals can now take a deep charge so the process takes longer than you got used to.
Turn off the charger and remove the NiCd battery, once charged. Let it rest for a few minutes then put it back into your device. You find your device powers up immediately and operates for much longer, possibly as good as the day you first purchased it.