Some Ryobi 18v batteries charge, others don't.

Had the same problem with some makita batteries, Was going to throw them away so decided to take apart and see what was happening. The are made up of 1.5 volt cells soldered together, found that many had .9 volts or less and others had 1.5 or more. This means that some of the cells had gone bad (probably due to lack of chemical reaction). Found that if I collected all of the 1.5 volt together could charge just fine. Problem is it is very hard to get them all soldered back together and even harder to enclose them. If a mfg. could come up with some way to change cells individually they could make a lot of money and save us a lot. So the answer to your question is buy you at least one more battery and recycle your old one.