Originally Posted by SeaSon
If the OP is using the battery
charger as a 12v power supply with either no batteries or dead batteries, than likely to ruin the charger. Chargers are not power supplies.
Have to agree with SailRedemtion on this. Plenty of chargers are also suitable for power supplies.
Even if not designed as a power supply I don't understand why using a charger to power 12V DC loads directly would damage it. Unless you're drawing more than the constant rated capacity of the charger I don't see why a charger cares what it powers.