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Open Roads Forum  >  Travel Trailers  >  General Q&A

 > How to: Battery Disconnect

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opnspaces

San Diego Ca

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Posted: 06/07/13 03:37pm Link  |  Quote  |  Print  |  Notify Moderator

Jerem0621, thanks for the writeup sometimes we take what is perceived as the simple things for granted and assume "everybody knows" I think it's great that you took the time to post this for everybody. But I agree with many of the posters who have concerns about the breakaway switch. Below are my suggestions and reasons why it might be better to move the switch to the positive terminal.

As far as your emergency breakaway switch, I would think you could just take the negative wire from the switch, put on a ring terminal, and attach it to the threaded negative post on the battery. At least I believe mine has a separate negative wire. BUT, if your breakaway is grounded through the mounting bolts to the frame this won't be an option. At that point the positive terminal is your only real option as stated in the next paragraph.

If your emergency breakaway has only one wire, the only thing you can effectively do is to move the disconnect to the positive terminal on the battery. Then using a separate ring terminal connect the positive on the breakaway to the threaded battery post. This way you can disconnect the loads on the battery and the breakaway will work fine whether or not you install the green knob.

As far as which side to put the disconnect on in reality I don't think it matters as long as you can keep the emergency breakaway available regardless of the disconnect position. I know of two different reasons I've heard for disconnecting the negative side of a battery and they both make sense. But they probably don't really apply in this application.

Reason 1. When disconnecting the battery on your tow vehicle you always disconnect negative first and reconnect it last. This is a safety issue as if you happen to slip with your wrench and connect a circuit between the negative terminal and the vehicle body nothing bad happens. But if you try to disconnect the positive first and slip with the wrench to the body you will get a direct short causing lots of sparks and heat and possibly welding the wrench to the body which would be really dangerous.

This is not the case on the green knob as there is nothing long enough to cause a short to, regardless of the terminal you attach it to.

Reason 2. I once worked as a technician for a major automobile manufacturer. During one of the many training classes I had to attend, I was told that in the computer circuits that they always switch on the negative side of the circuit. This is because the current has already been used and therefore there is less arcing and therefore less wear on the circuits.

I can see this as a concern on a microprocessor where there is very little material on a conductor that can erode before it stops working. In the case of the disconnect switch you have a large conductor and almost no sparking. And if by chance you manage to erode away the switch through sparking, a new switch is cheap and easy to install.

All that being said, if I did buy and wire a disconnect I would wire it on the positive cable with a separate wire for the breakaway.


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jerem0621

Tennessee

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Posted: 06/07/13 03:55pm Link  |  Quote  |  Print  |  Notify Moderator

beemerphile1 wrote:


jerem0621, if you try to direct wire the breakaway system to the battery you will quickly discover why the disconnect on the negative won't work.

Someday, someone, somewhere will turn the disconnect after the trailer is hitched and tow with a non-functioning breakaway system. Current owner, next owner, your brother-in-law, somebody will do it. Why not be safe from the get-go? It is a simple change that could save lives.


Thanks, I will address the break away system. Even though I have to reinstall the green knob before I can use my tongue jack..... Mr Murphy can remove it during travel.

Question, why can't I take the positive and negative leads from the break away switch, add some loop attachments, attach them to the posts of the battery and still use the disconnector I have for the rest of the trailer?

I want this to be as safe as possible, we all know a power tongue jack can fail. I want to save the battery but at the same time be safe doing it.

Thanks!

Jeremiah


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beemerphile1

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Posted: 06/07/13 04:42pm Link  |  Quote  |  Print  |  Notify Moderator

Look at the breakaway switch. Typically they have one wire in and one wire out. It opens/closes the positive (power) to the brakes. The brakes are grounded to the frame. If you open the ground by switching the negative the brakes will not work in a breakaway situation. The pin will be pulled, the breakaway switch will close, power will be available to the brakes, but the circuit will be incomplete due to the open ground.

Some breakaway switches do have a negative/ground wire. The purpose of the negative/ground is for an indicator light on the breakaway switch and does not ground the brakes.


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jerem0621

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Posted: 06/07/13 05:10pm Link  |  Quote  |  Print  |  Notify Moderator

beemerphile1 wrote:

Look at the breakaway switch. Typically they have one wire in and one wire out. It opens/closes the positive (power) to the brakes. The brakes are grounded to the frame. If you open the ground by switching the negative the brakes will not work in a breakaway situation. The pin will be pulled, the breakaway switch will close, power will be available to the brakes, but the circuit will be incomplete due to the open ground.

Some breakaway switches do have a negative/ground wire. The purpose of the negative/ground is for an indicator light on the breakaway switch and does not ground the brakes.


Makes perfect sense. Gotta ground the brakes and the switch. Got a little project for the weekend to make this work right.

Note to everyone I did not take the grounding of the breaks themselves into consideration. I will correct this and repost for evaluation.

Thanks

Jeremiah

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