18V Lithium-ion battery doesn't charge. That is, when inserted into charger, the charger indicator lights alternate red-green. This is one of the original batteries supplied with a hammer-drill purchased several months ago. It has never before been charged by me.

The Ryobi charger will flash green as it charges and then remain steady or slowly flashing green when fully charged. Flashing red means testing and if it remains steady or slowly flashing red it is defective.

check output on transformer, 2 check pcb ,could be a regulator

I use the Ryobi Li-Ion tools the L-I batteries will cut off when they get to a low charge level this is to protect the batteries Li-Ion batteries life span is shortened when fullt discharged so when the tool suddenly stops put the battery on charge,also if the battery has a charge indicator light for a more accurite idea of the charge (green is charged,red discharged) hold the test button on then run the tool the light may go from green to red if it does that shows the the charge is about depleted if the light stays green you have a little run time left.

This is not right. They should last much longer then that. I have one of the more expensive 18V lithium batteries (the one with the button on it that tells you if it needs charging or not) and I can use mine almost all day before it needs to be recharged. I do construction work and I use my Ryobi products almost all day. Either both batteries are bad (I don't believe that is the case) or the charger is not charging them all the way to a full charge. With this charger, you do need to make sure that you remove the batteries as soon as the green light comes on. It is not good on these batteries to stay charging after they are fully charged. You also should not leave the charger plugged in without any batteries charging.Please tell me more about the charger. When you plug in the charger, does the red light come on? When you plug in a battery, does the red light stay bright and the green light blink? Does the red light go off and the green light stay on bright when you remove it from the charger?
If the charger is not working correctly, then return it for a new one.

The battery will power your equipment for 30 to 45 minutes, that is normal. What you're thinking of is the lifespan of a lithium battery. It with your battery will last 10 times longer than a regular NiCad battery. So you will be able to charge the battery and use it Hundreds of times before the Battery Life is depleted. Sounds like it's working as designed

This isn't true because I have a second charger and it works fine

on my charger (DC18SC) alternating red and green dual flashes mean a defective battery.
http://ubermanuals.com/product/makita-dc18sc-8866 also same for the DC24SA

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.

I think your tryiong to charge NIMH batteries oon a charger designed for Lithium Ion batteries. With all the new style batteries such as NICAD, NIMH and LI-ION, charging becomes difficult. Most chargers pre NIMH and LI-ion, were designed for NICAD's. The best advise, match your batteries with the correct charger