Tapping into rear 12v accessory to charge the trailer

[boxfullofyertoys]

Hey everyone,

I've been doing a lot of browsing on this site in the last few days trying to get everything compiled for my build. One of the things I've been trying to figure out is how to charge the battery in the trailer from the car when I'm under way. I've seen most of the posts on the subject, and I have no problem with running a lead from the front of my car ('03 Subaru Forester) to the rear if need be. During my thought process it occurred to me that I've got a 12v accessory plug in my trunk area that could also possibly be used (I'd tap into it from behind and run a lead from there). Plus, it only works when the car's ignition is on, thus stopping any battery drain.

I guess my question is: can it be done (safely)? Or am I barking up the wrong tree?

-Dave

[SteveH]

Dave, what you are proposing will work, electronically. Don't know if it's physically practical, or not. The worst that would happen would be a blown fuse in the circuit that powers the accessory outlet, and then only if the trailer battery is pretty low when you connected them together as most of the accessory outlets are fused 30 amps.

[Dee Bee]

My concern for you is that the wires of that circuit in the car have to be able to handle the max amps that will be used to charge the 12 battery system.

I doubt that it was wired for that.

Every circuit has to be able to handle the max. expected amperage and more to be safe in my opinion. Otherwise the wires will heat up, fail and cause an electrical fire. Compare the size of the wires directly at the auto battery terminals with the size of wire that energizes a tail light or a turn signal. Even though they are both handling 12 volts, the big difference is in the amperage that goes through the wire. Higher amp must have large wire.

Others are able to give you more precise electrical info..

Read this as a start in your 12 volt education.
http://www.ccis.com/home/mnemeth/12volt/12volt.htm
Here is a quote:
Battery Charging. Many of the better inverters designed for RV use are also high quality battery chargers. This is an excellent option, as it allows you to get rid of that old, inefficient converter and enjoy faster, safer 3 stage charging. Most of the major inverter manufacturers offer this either as standard equipment or as an add-on accessory on their inverters. These chargers typically can deliver from 25 to 150 amps of charge current and run very well with generator power, allowing you to quickly recharge your batteries while out in the boonies. These 3 stage chargers will also not boil the water out of your batteries. Instead, they will bring them to full charge and then taper back to a true float charge... these units can safely be left plugged in continuously, unlike the standard converter found in most RVs.

Dee Bee

[Dale M.]

My concern for you is that the wires of that circuit in the car have to be able to handle the max amps that will be used to charge the 12 battery system.

I doubt that it was wired for that.

Every circuit has to be able to handle the max. expected amperage and more to be safe in my opinion. Otherwise the wires will heat up, fail and cause an electrical fire. Compare the size of the wires directly at the auto battery terminals with the size of wire that energizes a tail light or a turn signal. Even though they are both handling 12 volts, the big difference is in the amperage that goes through the wire. Higher amp must have large wire.

Others are able to give you more precise electrical info..



Read this as a start in your 12 volt education.
http://www.ccis.com/home/mnemeth/12volt/12volt.htm
Here is a quote:
Battery Charging. Many of the better inverters designed for RV use are also high quality battery chargers. This is an excellent option, as it allows you to get rid of that old, inefficient converter and enjoy faster, safer 3 stage charging. Most of the major inverter manufacturers offer this either as standard equipment or as an add-on accessory on their inverters. These chargers typically can deliver from 25 to 150 amps of charge current and run very well with generator power, allowing you to quickly recharge your batteries while out in the boonies. These 3 stage chargers will also not boil the water out of your batteries. Instead, they will bring them to full charge and then taper back to a true float charge... these units can safely be left plugged in continuously, unlike the standard converter found in most RVs.

Dee Bee

That is why most cars have fuses inline with accessory outlets....

One could actually try it and if one finds that fuse consistently fails, then one may consider dedicated circuit wired to rear of vesicle....

And I believe if you have no AC source to power inverter, you are left with alternate (12v dc ) charging sources....

Dale

[G-force]

I doubt that accesory plug in your vehicle it rated to pull as much current as a dead tear battery can draw. Typicaly this wire needs to be 10 ga the whole run. But there is no reason that could not be added to your vehicle directaly off the battery, using a continuous duty relay so that the wire is only hot when the car ignition is on. Do a search on the internet about installing 7 wire trailer wiring into a vechicle, thats basically what you need but without running the wiring for the electric brake system. Most trailers have a 4 wire system, the 7 gives you 3 additional wires: Charge wire for the trailer battery, wire for the electric brakes, and an accessory wire (sometimes used for backup lights or lockout for hydraulic/eclectric brake systems.)

[boxfullofyertoys]

Thanks for everyone's input. I think I'll hold off for now and maybe add that feature if I need it. I'm wiring the trailer for both 12v and 110v, and I don't think I'll be out in it for more than a weekend at a time. Worst case scenario, I'll be using it for a car stereo, lights, and a fan...so the battery should hold me for a weekend if I'm just strictly DC. I just wanted to set up the charger in order to recharge the battery on the way home after a weekend camping trip instead of plugging it in at home

[mexican tear]

Why not just wire the car and trailer with a standard 7 prong plug for RV's? This plug has provisions for charging and brakes, the backup lights connections will probably not be used ( a little humor).

If you ever decide to add brakes, then you are good to go on that. By the way, as cars get lighter and lighter trailer brakes make much better sense.

kai

[kargoki]

Thanks for everyone's input. I think I'll hold off for now and maybe add that feature if I need it. I'm wiring the trailer for both 12v and 110v, and I don't think I'll be out in it for more than a weekend at a time. Worst case scenario, I'll be using it for a car stereo, lights, and a fan...so the battery should hold me for a weekend if I'm just strictly DC. I just wanted to set up the charger in order to recharge the battery on the way home after a weekend camping trip instead of plugging it in at home

How about a solar panel on the roof of the trailer to charge while sitting/driving/weekend adventures, you can pick up the portable power packs and a solar panel to charge it back up..there are several solar chargers on the market now

[Cutterpup]

Check the fuse panel for the amp rateing of the fuse, for the rear accessory power port. On my 98 Subaru we ran a small 12v cooler from it with no problem.

Dan

[wlooper89]

My after market hitch installer ran a charge wire to a 7-pin connector. It is a # 10 wire connected directly to the battery positive terminal. A 25 amp inline fuse is connected close to the car battery. I have not done so yet, but a solenoid powered by an accessory wire can turn the charge wire on or off with the car ignition. The solenoid does not require a heavy wire or highly rated fuse. Just the wire from the car battery and fuse to the trailer battery passing through the solenoid switch needs to be a heavy one.

Right now my charge wire is hot all the time without the solenoid. So I have to remember to unplug the cord between the car and trailer when camping to avoid depleting the car battery along with the trailer battery. Normally I unhitch the car anyway so it is not usually a problem.

A solenoid is planned for a future upgrade when I get around to it.

Bill

[len19070]

Charging while driving is a great way to charge. Its a no brainier. If you have to drive to where your going...then you have a charged battery when you get there. Simple.

A fused 10ga wire directly from the battery to the plug is the best way. Its the way the RV industry has been doing it for decades.

Yes, it is possible if you leave some high amp draw accessory on in the trailer you could run the battery's down while parked, But who leaves fans, lights TV's on in there trailer when towing? Not me, I turn everything off before I tow. If nothings on theres no drain.

If you are still afraid of a drain, pull the plug.

Happy Trails

Len

[wlooper89]

Many thanks! It is nice to know my hitch and wiring installer did it the standard way. Normally I turn everything off for towing, including the D/C master switch in the trailer. Mostly I was thinking about camping with the tow vehicle connected. Not something I normally do but then as you say one can unplug the cord to the tow vehicle.

Bill

[brian_bp]

Yes, it is possible if you leave some high amp draw accessory on in the trailer you could run the battery's down while parked, But who leaves fans, lights TV's on in there trailer when towing? Not me, I turn everything off before I tow. If nothings on theres no drain.

I run the refrigerator on 12V while towing. It takes signficant current, and could run down the tug's battery if left during a long non-camping stop. It has run down the trailer battery this way.

I also leave a thermostatically controlled roof vent on when parked, to keep the trailer interior from getting too hot.

[Lucky489]

Well the refrigerator I would be willing to bet uses more power from your battery than your charger is charging the batteries. At least that was the way it was on my pop up.

Yes, it is possible if you leave some high amp draw accessory on in the trailer you could run the battery's down while parked, But who leaves fans, lights TV's on in there trailer when towing? Not me, I turn everything off before I tow. If nothings on theres no drain.

I run the refrigerator on 12V while towing. It takes signficant current, and could run down the tug's battery if left during a long non-camping stop. It has run down the trailer battery this way.

I also leave a thermostatically controlled roof vent on when parked, to keep the trailer interior from getting too hot.

[Gaston]

I built a charge cord that has 2 male connectors on it . put a acc. 12v outlet on the tongue of the trailer and plug the cord into the 12v acc plug in the cargo area of my cruiser then into the acc plug on the tongue. the male plug has a15amp fuse in it. works just fine.
As a side note, I find it is better to fully charge the trailer battery before we leave home, recharge by connecting to RV park power and letting the charger in the converter charge the battery. I only use the car connection as a last resort as it won't charge as well as the converter