I would like to put a diode in the line to the small brake battery on a trailer. I want to be able to charge the battery from my AUX power line from the truck, but not allow power to run out of the battery should an overhead light be left on in the trailer when the trailer is unhooked. I know the breakaway switch will be wired directly to the battery to allow that to work when needed.
I realize that a diode works like a check valve in a plumbing system. I dont know how to size them.
If there was 480volts in this system I would be able to deal with it. This 12v stuff hasnt got enough kick to get my attention I guess

Thanks for the help

 

Go to the local auto parts store and buy a automotive 12v diode, used for charging systems just like your application.

MikeyB

 

That sounds like a loaded question to present to a Autozone or Advance auto. How do they plug that year make and model into their computer?

Too bad there is not a real auto parts in these parts to make that work.

 

I do not understand just what you are trying to do?

You can install a diode to isolate your trailer battery from your tow vehicle battery, this is done to prevent you from draining your starting battery if you leave your trailer lights on and do not disconnect the plug, if your tow vehicle is set up properly it will either have a relay or an isolator in the charge wire to your trailer that will open the circuit when the engine is off or this can be easly added.

But if you install a diode inline with the load from the brake battery to prevent the trailer lights from draining the battery either the lights would not work at all or the lights would work and the diode would be useless.

How is this brake battery set up? Is it a drycell "Lantern” type battery specifically for the breakaway or is it a rechargeable "wet-cell" house battery that runs the lights, accessories including the trailer breakaway?

Do you want to use the battery only for the brakes and run the lights only when the trailer is connected into the truck?

For isolating small loads you can get diodes from Radio Shack, they are rated in amps, you will need to get one rated high enough to handle all of the current that will have to pass through it, always get the largest you can find and then let me know and I will find a heaver one, it will need to be mounted on a heat sink to dissipate the heat because they can get HOT.

You can use alternator diodes but you will have to mount them in a heat sink, one possibility is to use the diode array from a heavy duty alternator, they will have at least 3 negative and 3 positive diodes already mounted on a finned heatsink.

For a lot of my projects I use some 70-amp stud mount diodes mounted on a finned heat sink, I get these from Fry’s Electronics, I think they are only about $5.00 each.

You can also use a device called a Low Voltage Disconnect that will disconnect all of your loads from the battery when the battery voltage drops to a predetermined voltage.

Another thing to remember about most diodes is there will be about a 1.0-volt drop across the diode meaning that unless your charging system is in good shape and charging better than 13.8 volts at the diode there is a possibility you could never fully recharge your battery, but remember I said MOST, there is a special diode called a Schottky that can be used here.

I know this was a lot of information to drop on you for such a simple question but it is really not too complicated, I work with this all of the time, and just last month I had to rewire and elevator motor control, whoever installed it originally connected the LINE and the LOAD backwards on the disconnect and when the elevator service man went to pull the 100-amp cartridge fuse he almost got burned by 480-volts 3-Phase.

Maybe I did not understand what you were trying to do.

Jim

 

Jim
I use the AUX wire in the 7 way plug to charge the small (gel cell I think) brake battery on my trailers.
The last enclosed trailer I bought has a decent set of overhead interior lights in it. They are currently tied to the parking circut. I would like to hook them to the AUX circut. I was looking for a way (diode) to be able to allow the small battery to charge from that circut but not be able to let the lights run off the small battery.
Basicly I want to have a one way path for current to the battery from the AUX line but no way back.

I am going to GUESS a 2 to 5 amp 12volt diode would work to maintain the charge in that small battery. I do not feel that a 1volt drain will adversely affect the breakaway system in a trailer.

Thanks Jim

 

you need a larger diode. get a 30 amp 50 volt minimum. 30 amp 400v are easy and cheap to get. the diode will do exactly what you want to do. you can use one of the diodes for a alternator but is a mechanical pain on most because the case is mechanically pressed into the heat sink and the connection made to the heat sink. if you do not find one pm me and i will look up a nts part # for you

 

First of all, the trailer brake battery has nothing to do with and in not connected to the trailer lights in any manner. You can add a 7 way plug on both the trailer and vehicle, and google a diagram to hook it all up. Then your vehicle will charge the battery. The battery will ONLY be drained if the lanyard is pulled on the breakaway switch. To prevent anything in the vehicle from draining that battery, which can really happen, you can simply unplug the trailer or like you said, add a diode. Instead of Autozone, which all you'll get from them is a stare like a horse looking at an open gate, go to a good car audio place. They will know what a diode is and will have some good ones in stock.