I tried a trick I learned on a 18V drill battery that would not charge. I thru it away and then I got it out of garbage. Somehow the circuit board inside Ryobi battery has a failure and when battery gets too low, it won't allow charging. If you have a cordless battery that will no longer take a charge, this may work. It did for me. Remedy: Take battery apart, 4+1 screws, pull out the whole assembly from the base. Observe rechargeable batteries in series.(5 of them in my case). Take a baseline voltage reading of each battery (probably 1V each). Take a baseline reading across the entire pack (maybe 5V) Take a baseline reading at the master contacts on nose of pack. (might be zero) Weird right? Use your voltmeter to determine pos and neg of top battery to bottom battery. (not the main nose/tower contacts) Wear safety shield, use your automotive 12V battery charger. Touch pos to pos and neg to neg for 15-20 seconds. You are touching so that the entire battery pack is being charged. (both ends of the series). After 15-20 seconds, NOT MINUTES, you will get a reading, maybe 10-11 volts. If you get that you are in fat city. Put back together and use the dedicated charger. It's like after the internal batteries get a certain charge level, the PC board will allow charging.
Warning: My 18V battery had Torx security screws. Bastards! That tells me they know a person can fix it!
If the heads were bigger I could have drilled a hole in the end of my Torx bit. They make Torx security drivers. But guess what? I had to use a die grinder, CAREFULLY, to cut thru plastic and cut off 4 screw heads. Left the one in the middle of the power nose. Came within <1/8 inch of cutting circuit board! Had to pry out the pack past the lock tabs. My battery looks like it exploded.
It was worth the effort (30 minutes to save a bunch of money on these batteries.)
I will post if battery "locks up" again after use. I don't think you can do this by touching automotive charger to the power tower contacts which go thru the PC board to the cell batteries. You must bypass the circuit board. DON'T get the polarization wrong when charging!!!
I'll let you know if anything bad happens. I have to electrical tape it back together.
It seems like the actual smaller batteries inside are not failing, but the circuit board is. This is said to be much easier to do on other brands.
