I haul a haulmark toy trailer and have installed 2-93 amp/hr deep cycle batteries in it. I upgraded the wire from the junction box of the trailer to the batteries with an 8 gauge cable, no fuse. I believe the 7 pin connector to the junction box is also 8 guage but it could be 10. I believe my truck has 30 amps or maybe 40 out to the battery charge circuit in the 7 pin trailer wiring harness. I do have the trailer towing package. I have looked at the manual and am not sure about the size of the fuse and/or relay? When the truck is not running no power is provided from the truck. Even on accs mode with the key, no power to the trailer. That is why I was not worried about a fuse.

What determines how much my batteries will draw from the truck, if there close to dead. I am pretty sure my alternator can handle it no sweat, being a Lariat, but what about the wiring in the vehicle to the the 7 pin plug at the rear. If I installed 2 more batteries would that be too much for the trucks wiring and blow the fuse/relay?

I can wire a house 120V no problem, but 12V DC is still confusing to me. I also have a 1500 watt inverter installed in the trailer. It is connected to the batteries with a 4 guage cable. Could I run that steady at 1500 watts while the truck is running?

According to some charts this is the formula: Convert Watts to Actual DC amps: (Watts / 12) x 1.1 = actual DC amps which is at 137.5 amps for 1500 watts so to my calculations I could run 1500 watts for 1.35 hrs on my 2-93 amp/hr batteries alone, I think....

Any help at all that someone could give me would be great even if it's just on the truck output. I assume my alternator is pretty big but I don't know how much. :o :o

 

The wiring I believe is 10 ga and the fuse is 30A. To get the charging circuit to work, you have to plug in a relay into the fuse box, and it came in a plastic bag in the glove box that also includes a wiring harness for a trailer brake controller. The 12V circuit is active only when the ignition is in the ON position.

 

Not sure on the amp output on the 12v charge, but like jljue said, you need to make sure you have the relay in order for it to work with what's already wired up.

There are some 30a fuses related to trailer towing in the fuse box under the hood as well.

The only time I know my 12v charge is going to the trailer is when the engine is running. Then, it's only bringing the battery volts up to around 13.5v. At rest, the trailer battery is usually at 12.5 or so. While the truck is running, trailer hooked up and getting 13.5 the truck battery is getting 14.5. The truck gets 14.5 when the trailer is not hooked up too.

So, you won't get much of a charge rate on the trailer battery, unless you drive for several hours. I charged up my Dad's dual battery setup on his trailer while we were 'dry camping' when his batteries went down to 11.5v..... I hooked up my truck because we didn't want to listen to his diesel running while sitting in camp! Anyway, it took about an hour to bring it back up to around 12.5v at rest. The whole time, it was charging at 13.5v.

Good luck!
Mitch

 

The relay was installed with the brake controller.

If it has a 30 amp fuse does that mean it can send 30 amps back there? The voltage it charges at I don't think is very relevant 12-15 or so. I am still not sure if I have 4 dead batteries back in the trailer would it demand more amps from the truck. Someone told me a battery will only draw 10 amps while charging from a vehicle, does that mean 4 will try and draw 40? I sure wish this was as easy as AC.

 

If you send 30 amps back there the fuse will blow. It is designed to blow at 30 amps. Ideally you wouldn't want to stress the electrical system and wouldn't want to pull more than 60% of the capacity of the circuit...meaning...18 amps of steady pull.

 

'99 wiring dia says Trailer battery feed is #10AWG and Maximum allowable current is 11.5 amp. Fuse is 30 amp. We all know more than 11.5 amp will flow through that wire but it may not be happy doing it all day (HEAT). Every connector can add resistence.