If you are charging the batteries on the truck, will the trailer battery also charge if you're hooked up to the truck?

 

only if wired up to do it. need wiring and relay.

 

The trailer has lights inside that work when trailer is hooked up to truck but the trailer has a small battery under the trailer that's supposed to be for the electric brakes to work. My question is how to charge the trailer battery? How do you do this relay?

 

If your small battery on your trailer was factory installed, you should not have to do a thing. If all your trailer wireing is in good shape and working as intended, you do not need to add any relay. The batt. stays charged as long as the trialer plug is connected to the truck, and the truck is running. An easy way to check is to put a volt meter on the batt., it should read around 12 volts. Then start the truck, it should then read around 13-14 volts. good luck.