by7antine

Had this same problem with a cordless tool once.  I ran the battery so low in voltage that the charger wanted to call the battery defective.  I force fed the battery a bit then the charger was fine with it.
 
I think the same thing is going on here.  The controller will let you discharge the batter so low that when you plug into power the charge circuit wants to call it a defective battery.  Use TH3DOCTOR's method to force feed the battery up to its minimum 'acceptable' voltage level and then the circuitry will call it OK and keep on going.
 
TobyD81 is right, an old dead battery is an old dead battery.  But my battery will run for 3 days of 8 hour sessions before needing recharge.  In this case I would call it a design failure of the controller circuitry that allows the battery to be discharged bellow the critical threshold required for recharging in the same controler.