My CMM1000 cordless mower says its charged but its not. Any thoughts?

I have a great B&D CMM1000 cordless mower, but lately the charger cuts out before charging it. Have you heard of other situations where the light turns green and the charger stops charging BEFORE the batteries are actually charged? Right now it goes for a few minutes and at first the charger appears to work properly by putting bursts of 26 to 29 volts in into the battery. The battery voltage then starts to climb steadily from 13.5 to up around 15 volts (should be up over 24v when chraged) but then the charging light goes from red to green and the charger stops doing anything even though the batteries are still very weak. I'd like to upgrade the batteries if that's what is needed, but don't want to do that if the charger or something else is the problem. Have you run into this before or do you have any ideas? Perhaps it would be safe to hook the charger up directly to the batteries to see if the circuit and charging indication light are what's causing the trouble... any thoughts? Thanks!