Mechanos

Just wondering about charging accessory batteries through the stock trailer tow harness. If you run accessories in your trailer and draw the trailer battery down significantly, what keeps it from drawing more power through the harness wiring than it can handle when you go to charge the battery when you plug in the trailer? In other words, will a depleted battery try to pull more juice through the trailer harness to charge itself than the harness can handle (resulting in blow fuses)/

ratsun

I've charged some pretty low batterys in my trailer but I know its real hard on the alt I generally unplug once at my site and just run of the trailer battery. On my truck the power to the plug is hot all the time so if your somewhere without hook ups it will draw from all three batterys and I dont want to chance killing the starting batterys. I will also disconnect the trailer battery before the hook up to go home and charge the battery at home with the on board charger. As far as blowing fusses I have not had any problem.

Maccandy

ratsun wrote:
"I will also disconnect the trailer battery before the hook up to go home and charge the battery at home with the on board charger."


If you disconnect your trailer batteries and you have a brake away, your trailer brakes will not work! That is what the brake-away switch does. It sends power from your trailer battery to your brakes so it will stop. Not good to disconnect your trailer batteries when you are towing.

Casey Balvert

There is so much resistance in the charge circuit going to the trailer you are lucky to get ten amps throught it with even a fully depleted battery. I have tried it. I don't think you need to worry.

Casey

Hemi Cat

The charge circut is "supposed" to have a circut breaker in line to protect it from overloads providing it was wired correctly. I wouldn't worry about it too much. I've run down my RV batt quite a few times while traveling and stopping for the nite and unplugged the cord, then plug it back in after starting the truck. So far I haven't had a problem.

Rattletrap1

I'm with Casey.

Scotty

But Casey is in Ontario Canada not Ontario CA.

How can you be with him and why would you want to be with him...its too dang cold up here.

Ok, back to my nap.

Casey Balvert

Yeah, go back to sleep Scotty.

The circuit is protected with a 40 amp fuse which also supplies the brake circuit. The most I ever monitored going into the trailer battery is under 10 amps and that is with a battery that is almost fully discharged. The wire is way too long and small to provide any real current for charging. Why would DC spend any extra money on heavier gauge wire??? Even the brake wire is too small for the potential current it is supposed to carry. Just check the wire charts to see for yourself. No way will charging your trailer battery pop the fuse.

Casey

FiverBob

If you stop by an RV supply, you can pick up a relay to put in series in the charge line on the truck. You can either use a diode (rather expensive) or a simple relay that is activated when the key is turned on. They cost about $20. It will save you pulling the truck batteries down when stopped if you don't unplug the truck for a period of time from the trailer. I have never had a problem with an overnight, but I still try and remember to pull the plug - - the hardest thing is remembering to plug it back in the next morning. Wow, these lapses in the cranium are terrible.

Another good option is replacing the 40a fuse with a 40a circuit breaker. Not fun if you pop a fuse and have no trailer brakes - - the breaker will reset itself (assumming the source of the trip was momentary or intermittent). These can be found at most auto parts suppliers.

ForgetfulBob

ratsun

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Good point I never thought about that.

Rattletrap1

I use the solenoid like Bob speaks of to feed my camper batteries with a dedicated 8 gauge wire. That's about minimum size for approx 25' run if I intend to pull up to 30 - 40 amps with <10% voltage drop.

The small wire from the factory basically protects itself. If there's enogh current demand the voltage drops and hence the current demand drops because hrere isn't enough differential between the charging line and the weak batterie's voltage. The same principle applies as to why the ammeter on your home charger goes down as the battery is charged up.

Mechanos

Just out of curiousity, where did you draw your "ignition hot" power from to trigger the solenoid when the key is on? I have a 10 gauge power wire I ran for my trailer plug.

Rattletrap1

I can't really explain it. Blue wire I believe at the harness near the underhood fuse box. I just found it with a test lamp. The solenoid only draws <1 amp so it doesn't need to be substantial size wire to trigger.

I can take a look under the hood and try to be more specific or take a pic. I used a dreaded scotchlok to tap. Mounted the solenoid above the brake hydraulics on the firwall. I'll take a pic..it came out really clean.

Supposedly there's an orange wire at the firewall that is ign. hot, but I didn't find it..and found this one.

The only negative about this setup is if say the camper battery goes south, the truck batteries can drain to it when the key is on. I put two golf cart batteries in series for my camper battery..so I'm not too worried for several years. I also have the stock position camper battery, a disconnect plug and a 1, 2, both, off switch at the camper for the camper batteries.

The stock truck batteries have no isolation between each other anyway. Isolators are overrated IMHO.

FiverBob

I went in the fuse box and found a switched hot - - can't remember which one, then put in a jumper available at auto suppliers. It is more of a pain to do this than the diode, but diodes that pass that kind of current are expensive. Mounted the relay on the side fender wall.

Mechanos

Hmmmm..... I just remembered that I happen to have a 140 Amp isolator diode thingy tucked away in my "20 year box". It does gobble up a fair amount of real estate though..... I think it's about 4" x 8" x 2" if I remember right.