Hello,
I don't know if you still have the problem, but I had the same trouble.
It would appear that if the batteries are too low, or one is defective, the charger will not work properly. I lifted the battery cover lid and used an ordinary car battery charger connected to each battery directly and charged them one at a time.
Once the batteries had taken some charge for a few hours. I tried the sterling charger again and they continued to charge ok.
I hope this helps.
Remember the batteries are only 12v each. So the car battery charger will only charge them one at a time.
Regards,
Paul