Don't use a "car battery charger". They don't have enough regulation to keep the Swallow functioning properly.
If you want to plug into AC you need a 12 to 13.8 volt regulated power supply with enough current capacity for
the batteries you are trying to charge. If you don't understand what I'm talking about seek help from someone
that understands. Doing the wrong thing when it comes to batteries can be dangerous. Have fun, but do it safely.

Bruce