Check to see if it will charge Nickle-Metal Hydride batteries. You may need to check with the Rayovac website for details on your specific charger. My guess is probably not. Your charger is probably a trickle charger meant for Nickle-Cadnium type batteries. You can get a NiMH charger that will do 4 at a time for about $14 from these folks. At the cost of batteries these days, it doesn't take long to recoup the cost of the charger, especially if you use a lot of battery powered stuff.
FWIW, I've ordered dozens of batteries from these guys and I've always been very impressed with them. They stock high capacity, industrial quality batteries. Although store bought rechargables are the same can size, the ones these guys offer give as much as 8 times as much capacity than the off the shelf rechargables available at Home Depot, Wally World, etc.
I read the real small print on the back of the charger and it says it will charge Nickle-Metal Hydride.