Wont charge - Ryobi Two 18V Lithium-Ion Batteries + Lithium-Ion Charger Upgrade Combo Pack

One thing I have found with the ryobi 18V batteries in particular, and cordless tool batteries in general, is that if the battery is completely dead, the charger simply will not recognize it. This is referred to as a "threshold level" by some manufacturers. One solution that has worked well for me in the past is to take a battery that is known to be good and has a charge and use small jumpers to hook it to the problem battery, terminal for terminal, for about 1 to 1 1/2 minutes, much like jumpstarting a car. Then place the problem battery back in the charger and more often then not, it will take a full charge. Wear safety glasses any time you're messing around with batteries, because although the risk of explosion is minimal, it is still a risk.