well the only other scenario i can think of that might make the charger think there are low power or no batteries hooked up would be the charger itself is bad.
These chargers did have issues with the wiring where it comes out of the charger at that strain relief plug. I remember one of the other tech's i work with having this same issue and he found the wires burned on the inside of the charger, he couldn't get that harness and fix it. They made him replace the whole charger.
If you have another charger maybe you can rig it up to that battery pack and see if it will charge them?