Why does ryobi 18V cordless not charge

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: The Yellow and Green light both come on and stay

http://oneworld1.inetu.net/manuals/ryobi/hp1442m_1802m_874_eng.pdf
It sounds like your battery is bad. If yellow and green are on for more than 15 minutes it probably a defective battery pack. See page 8 in the manual.

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: how do you take apart a ryobi lithium 18v one

This applies to Li-ion. NiCd is probably somewhat different.

Unscrew four security screws on the bottom of the battery. If you don't have the right bit you can grind down the center pins in the screw heads and use a T15 bit.

Unscrew the hex-head screw in the center of the protruding post at the top of the battery. It may be non-standard in size but a small metric hex key wrench works if you are gentle. The four corners of the battery cover should be loose now.

There are still two plastic clips holding the top cover to the main unit near the centers of the long dimensions. Gently push a knife blade into the crack between bottom and top to release the clip on each side.

As you lift the top cover off you should push down on the contacts near the top of the post so you don't pull on the attaching wires. Eventually the contacts should pop free and the top cover detaches.

Detach the wiring harness for the status LED from the circuit board. You can now remove the combined circuit board and battery pack from the lower case.

Six screws attach the circuit board to the battery assembly, along with several solder connections, but you probably don't need to go this far. You can now test and possibly replace individual cells.