KatTruck

A friend and I recently picked up a 21' snowmobile trailer.

He drives a 2017 F350, and I drive a 2018 Raptor. We'll be towing the trailer with both of our trucks.

Ford cuts the 12V power to the trailer when the ignition is not on, so we're looking to add a battery to the trailer in order to run the interior dome lights when the trucks are not on, or when the trailer is unhooked from the trucks.

Surprisingly there is not too much info on the subject online. That is, there is a lot of info, but a lot of it contradicts itself, some makes sense, and some is clearly wrong, so trying to get an idea from others if we have some legitimate concerns here, or if they are unfounded concerns.

Originally we were just going to add the battery and wire it to the trailer dome lights 12V+ and ground it to the frame. We've read that the battery wont charge from the truck (which is OK, we can top it off with a battery tender every once in a while since the trailer gets very little use), and...we've also read that the battery will charge from the truck, and that if we don't have the same spec battery in both the truck and trailer it could over charge. We've also read that there is a concern of the trailer battery pulling juice and draining the truck battery when not in use.

We've seen many DIY setups online and on YouTube that don't address these concerns, so we're wondering what really is happening between the two batteries (trailer battery and truck battery), and if we need to worry about one of them overcharging, not charging at all, or running down the juice from the other.

Any help from those who have done this and understand the "science" behind it is much appreciated.

Steve83

That should be interesting to see - like love bugs! It's just like the house battery in an RV trailer. The difference is that you don't have nearly as much "house" for the battery to power - just some lights.That's the wrong way to look at it, which is probably what's confusing you. Think of it as "connecting the battery to the trailer battery charge circuit & the frame, and connecting the light SWITCH(es) to the battery (through a fuse, of course).It doesn't have to, if you ONLY connect it to the lights & trailer frame. But there's no reason for it not to, since the truck is already built to charge a trailer battery.Yes, if you connect it to the correct terminal of a 7-blade (or 7-pin) trailer connector. Click this, study the diagrams, and read the caption:


(phone app link)That's voodoo. Think of all the RV trailers that have deep-cycle batteries, connected to all the pickups with cranking batteries - some diesel, with 2 cranking batteries.Obviously, that can't happen since, as you said: the truck disconnects the trailer charging feed when the key is off.

raisin

You don't have to worry about trailer battery spec vs truck battery, other than 12v and enough capacity to do the job. People towing rv trailers don't try to match truck to rv battery or vise versa.
Trailer battery can't drain truck battery because truck +12v line goes dead when truck is turned off as you said. Than no connection at all.
The reason you can't depend on the truck to charge trailer battery is voltage loss over small wire size thru truck to trailer battery. Think of small hose to fill big tank.


The time to worry about trailer battery size is when 2 batteries are hooked up in parallel to give greater cap.


You could go 2 ways, get cheap 12v car battery or I would get the smaller size deep cycle battery, couple pieces angle iron or premade battery frame, add it to a-frame, Get black plastic battery box. Wire from proper pin in trailer 7-in male connector to battery. If you already have proper 7 conductor cable you may be able to dig out proper wire. I'll stop now to keep this short, asked if you need more info...


steve83 is a faster thinker than I am...lol

KatTruck

Thanks folks.

Our original idea was to mount a smaller/mid-size deep cycle battery in the V nose of the trailer (inside as there's not enough room on the tongue to mount on the outside cleanly).

We were going to connect the + of the battery to the trailer dome lights 12V+ wire. We were going to connect the - of the battery to the frame of the trailer (steel frame, the trailer wiring is all grounded to the frame).

Thoughts on this setup?
Issues with this setup?
Would this setup "charge" the battery from the truck?
Or would this setup basically make it a standalone system so we would just hook it up to a trickle charger every once in a while?

Really appreciate the help.

Steve83

Connect the (+) to a small fuse block, or inline fuse holders. Then the switch, then the wire to the lights.


(phone app link)

You CAN connect the (-) to the frame to get power to the trailer lights, but then you're dependent on that screw & the trailer ball/receiver to ground back to the truck frame for truck charging. It would be MUCH more-reliable to ALSO connect the B(-) to a real Copper wire going to the 7-blade ground wire. Both the ground & charge wires should be ~6~10ga between the 7-blade & battery.

Either way, you can still connect a trickle charger (a true float charge is better, like the BatteryTender & BT Jr.) to the trailer battery for land-charging. As before, when the truck key is off, the truck battery won't be charged by the trailer charger. And when the truck key is on, you probably won't have the trailer charger plugged in to land power.

KatTruck

Thanks for all of the help folks, much appreciated.

Any other tips/tricks/recommendations, or does the below look good?

Looks like our best bet is to:

- Connect the + of the battery through an inline fuse to switch of the 12V+ on the trailer dome lights.
- Connect the - of the battery to the ground on the 7 pin connector (for best ground).
- Top off the battery with a battery tender when needed. (We did purchase a Battery Tender Jr. for this purpose as that was our original thought for "charging" the battery).
- We also purchased an LCD readout of the battery life/condition for a better/quick reference as to the status of the battery.

raisin

You could use led inside and outside (RV) lights to get more light and use less battery.

Steve83

Yes, but there's no reason NOT to connect the B+ to the 7-blade "trailer battery charge" terminal.

PerryB

Agreed, that's how I'd do it. To the OP - Do a little wiring mod to the trailer and make life easy. Get a 5 or 6 wire (extra for later if 6) junction block and mount it to the front of the trailer in an opportune spot. Cut the original plug off the trailer and terminate the wires (ring terminals) to said junction block. Now get a piece of RV cord (has a 10 ga. ground and a 12 ga. charge wire) and make up a new lead cable with an RV-7 plug. Assuming a single midsize battery on the trailer, it'll charge just fine. Sanitary and functional.

KatTruck

Thanks everyone for all of the input and tips.

We are using LED lights (btw). The trailer came stock with 2 dome lights that were both LED. We replaced those and added a few more LED lights. The lighting inside the trailer is perfect now. Full coverage, and bright...but not too bright.