A nicad cell is a nicad cell. Size does matter when it comes to charging
them but the voltage requirements are the same.
The problem you are probably having is that the new charger is putting too
high of a voltage on the batteries and thus they are overcharged.
If you are using the vacuum regularly, leave it off of the charger until it
runs down and then recharge and pick off the charger the next morning.
There are nicad FAQs out there in webland that can provide good info on the
care and feeding of a nicad battery pack.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!