Do ryobi batteries wear out? Do we replace the battery only or the charger too?

SOURCE: 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.

SOURCE: can i charge a 14v battery using a 18v charger (both are ryobi)

Absolutely not. The risk of damage to the battery is only a minor concern compared to the fire hazard you would be creating.

SOURCE: light on ryobi 18v charger

More than likely a defective battery sinse the other ones charge fine..
Bring it back for exchange of another one..

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SOURCE: Does it harm a Ryobi 18v lithium battery to leave on Charger?

Lithium-Ion batteries require careful charging and temperature monitoring while in the charger. Ryobi's Charging station monitors many functions and shuts off when the battery has gone through a complete charge. This is indicated by a steady Green light. Once the Charge indicator is a steady green the charger stops charging. With any battery It will slowly discharge and I don’t think the Ryobi will maintain a battery that is never removed. For example, if after a complete charge cycle if the battery was left there for 2 years and then you went to use it, it would likely be completely discharged. However if the battery charger is unplugged then plugged back in a new charge sequence would start. I lose power at my house about every 6 months or so. Once power is restored a new cycle would start and the charging cycle begins again.
Hope this helps and have a great day.

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SOURCE: can you charge ryobi lithium batteries in a

NO! Never charge lithium batteries in a charger designed for nickle based batteries like the NiHi your old charger was meant for. Ryobi did design the new Lithium charger to be backwards compatible so you can charge your older 18V NiHi batteries in the Lithium 18V charger.
Also note, NEVER miss-match the voltage, for example, make sure you are not trying to charge 12V batteries in an 18V charger unless the manufacturer has specifically stated that the charger is capable of multiple voltages.