Nikon, the generic charger is always on green?

I have bought a second hand nikon d70s, i'm not sure if the battery is fully charged, when i plug it in it stays on green which i thought means that the battery is fully charged, when i put it into the camera, it sometimes shows me that the battery is low, sometimes shows me the battery is full.i'm confused now.

Just use the camera with flash, shooting any old cr*p until the battery runs down, then recharge it.