Lexie's response is a disclaimer and no help at all- checking with the manufacturer of the tool is no help when you are buying aftermarket parts that you hope will be superior.
If you buy a Ryobi 18V tool set that uses the yellow NICAD battery, the charger is designed for NICADS and is so marked. NICAD chargers used to supply a continuous charging current while NIMH chargers supplied a PULSE current. Some aftermarket NIMH battery sellers claim you can use the Ryobi charger. If that is the case, there must be additional circuitry within the battery itself to compensate for the uninterrupted current the charger is putting out. So the question is, are the NIMH batteries compatible with the stock Ryobi charger? The answer should be YES (and here is why) or NO. Not, "check with the manufacturer".
I have contacted some of these vendoer with this question, and they simply do not respond.