trailer break away system Q

Hi All,
Any reason why I cant use a full size car battery to power the breakaway switch? Right now I have a full size battery on boar that is independent of the tow trucks electrical system. The battery just powers a radio and some lights on the trailer . The little breakaway system battery crapped out and I would like to eliminate it .
Thanks for looking

12 volts is 12 volts….. hook it up!

Not a problem at all.

Only issue I see, is a small partly charged breakaway sized battery if seeing a system short for some reason, will likely drain itself rather quick, vs an unfused fully charged full size car battery might burn the trailer and its cargo down promptly. I don't know the current needed for brakes in a breakaway situation, but a large fuse, 30-50A? would give you some peace of mind I would think.

If I’m in a situation where that system has to be used….. a burning trailer is a few items down the list for sure.

Fwiw I’ve had trucks towed into work with stone dead batteries only to figure out faulty brake controllers or bad brake switches energized the trailer brakes all night…. no fires to report on my end.

I’m betting in a lot of cases those tiny batteries are in a state where they wouldn’t apply the brakes for too long anyways….. great, going to check mine.

You know I was addressing an unfused short, right?

The only time the brake system could be energized is if the dead man switch plug is pulled out. The seperator plug attached to the pull cable is plastic. When its pulled the contacts inside the switch touch each other completeing the positive side of the circuit. If there was a short from the battery to ground it wouldn't be any different than if you were using a car battery in the trailer for a winch, lights, etc. Simple enough to place a knife switch at the battery itself and open it while the trailer is parked.
Doug

Yes…. and this conversation peaked my curiosity and it appears you’ll find on most rv’s, toy haulers that utilize their larger battery as a source for the “break away” switch that circuit is not fused. I read some discussion on a 40amp self resetting breaker being installed on that circuit.

…..now thinking how most RV, toy hauler, race trailers are wired that’s kind of scary.

Yes, I agree, but one aspect here stands out to me, The OP has a dead backup battery and has not yet IMO confirmed why it's dead, and there are a number of benign reasons it is, but if a short is involved, installing a battery with 10? times the capacity, even if the same voltage with no current limiting could be a recipe for disaster.

On my 7k axles, they draw 6 amps per axle, So stick a 30 amp fuse in it and send it.

Joe

This is actually what I was wondering. Do they use the small battery so that the available current is very small as opposed to a larger battery. In the case of a short in the wiring between the battery and plunger switch the wiring is so short and small I sure it would simply burn off and it all outdoors.

When you apply your trailer brakes do they see a full 12 volts? Or would a full 12 volts damage the system? I think I should check that .

Thanks for all the replies.

The brakes are connected to the tow vehicles battery as well. So where ould there be a difference?
Doug

Do you know if they see a full 12v or does the brake switch limit voltage?

Small battery is just old .

My brake controller shows it ramping up proportional to the force required. For emergency braking, it is directly connected to the battery, the full 12 (13.2) volts.

My trailer has living quarters and a generator, and i wanted to hook to the large battery, so I ask the inspector about the battery, he said "as long as the battery is independent of the towing vehicle" and able to fully apply the brakes if disconnected from the tow vehicle.

Joe

The controller varies the voltage based on need, more voltage, more stopping power. I think most controllers have a variable setting that adjusts trailer brake application based on pedal application.

I'm going to say no, at least most of the time, if you set the gain to max it may. My understanding is that electronic controllers are essentially a rheostat, more gain = more voltage to the brakes, which equals more braking. Years ago, my brother had a hydraulic/electric controller, the more you pressed the brake pedal the more brake the controller applied to the trailer, it worked very well.

Just like sr4440 said, my trailer is a 48' goose neck w/ living quarters and it doesn't even have a "small" battery, it has 2 large deep cell 12v batteries. EVEY\RYTHING runs off of them.

Brian

Ok good enough. Tossing the small battery . Will put a 30 amp fuse in line after the switch.
Thanks again everyone

A 25 amp relay may be a better choice than the fuse

I'd suggest a maybe 30a breaker if wire size is larger than 14g, being this is only potentially for intermittent use during breakaway. and 14g+ will handle short periods of 25A as breaker resets.

One more trick. Get a second dead man switch. Mount it under the trailer close to the axle. Run the pull cable through the wheel. If the wheel ever rotates it'll pull the pin. The brakes will be on. Good theft device.
Doug

Was thinking the same

Very smart

Until you forget it's hooked up and drive away, not that I would ever do that.

That's how I know it works. You don't move very far.
Doug