I assume your 18V battery is Ni-Cad? In that case you may want to stump up for a new brushless tool and Li-ion battteries. They are labeled 20V as a marketing gimmick in North America - elsewhere in the world, they are still 18V.
The Li-ion batteries are a darn sight better, both in longevity, duration of use on a single charge (esp when coupled with a brushless-motor tool) and weight.
I triied what Tom did - I am a Dewalt Drill bigot, as I find the 3-speed gearbox matches my needs best, so after repacking and replacing Ni-cads several times in a 10-year period, I went Li-ion, but also purchased the adaptor - I could not bear to throw out my trusty 18V drill, which had performed “beyond the call of duty”. Same problems as Tom mentioned, plus the drill was slowly discharging the battery while not in use (I suppose some leakage in the speed control, or across a worn commutator, even with new brushes). Should not have been sentimental (I still have the old drill) - the adaptor was surprisingly expensive, and I almost never use it.
The two technologies are not directly interchangeable, and all the newer Dewalt stuff (I also have the circular saw and jigsaw) is Li-ion). I suppose the the smaller batteries have their uses, but I agree with tooler2 - you can use a larger battery for smaller jobs, not the other way round.
‘I am intrigued by the the Ridgid warranty - does that mean that they have a swop-out programme for batteries for the next 20 years? What does lifetime mean? (the HD site is off-limits outside of N. America).