If you have 13V at the converter, you should have the same voltage on your battery. If you don't, one of the wires from the converter to the battery is not tight.
In my case, it was the negative wire on the converter negative bus that was loose.
If you can connect pieces of wire to the positive and the negative at the converter output, use your jumpers to take first the negative to the battery. If that helps, it's the negative that is loose. If when you connect the positive it helps, look for a loose positive connection.