If you have a short somewhere outside of the ignition, the battery won't charge properly connected. I would recommend disconnecting the vehicle from the battery and then charging it (and if it is not a maintenance free battery check the levels first). Trickle chargers take an easy 12-24hrs to charge a battery, depending on their amperage. It should end up 12.75V+ charged.
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick