First thing is to find out how much current the tow vehicles lights draw. If you go to the service manual it should give the normal current draw with the parking lights on. Then check the size of the fuse for this circuit. You should not exceed much more than 75% of the fuse ratting. You can pull the fuse, turn on the parking lights and use an amp meter across the fuse contacts, to measure the current draw. But if you short out one of the test leads to ground you may loose you car. I do not recommend doing this!
The cheep and dirty way to charge the battery is to use a rectifier diode as a blocking diode, in series us a light bulb with a 3 amp current ratting at 12v. This goes between the tail light circuit and the batteries + lead. This light bulb works like a current limiter, holding the max charging current to 3 amps or less as the battery charges. The diode keeps the taillights of the trailer from discharging the aux. battery. It lets the current pass to charge the battery but keeps the battery from lighting the taillights.
Electronically replace the light bulb with a current regulator power supply circuit.
If you go on to Goggle and enter LM723 you will find circuits for current limited power supplies. The LM723 will not handle very much current so you will need to use a pass transistor like a 2N3055 or a P-channel power FET. This circuit will take a hand full of parts and you will have to test it out on a bench before installing it. Just remember not to over load your vehicles tail light circuit. If you are using a small battery just lower the maximum charging current.
Chuck
