How to: Battery Disconnect

A common problem for RV'ers (not just TT owners) is a low or a dead battery. The reason for this is parasitic power drain. A single light left on can obviously drain the battery quickly but there are things in our RV's that pull some power all the time. A few examples could be your radio, power inverter, gas detector, smoke detector, etc, etc, etc..

One method to remedy this issue is to leave your RV plugged in to a power source (if you have an onboard battery charger) or leave your battery on a trickle charger. However, for many of us we have to leave our RV at a storage facility, or parked away from the house without a plausible way to leave the rig plugged up.

One simple method is to install a battery disconnector, as the topic of this post, I will show you how I installed a battery disconnector on my deep cycle battery. I had previously fully charged it after I found it dead (again) after fully charging it and reinstalling it on the trailer. :M

Kudos go to this forum for educating me about parasitic power drain and turning me on to a "battery disconnector".

Ok, first things first, Start with a fully charged deep cycle battery. (these things take a LONG time to charge from dead. )

Then pick up a battery disconnector. I got mine from Wal-Mart in the automotive section for less than $5.00.

I also picked these Coated Marine Battery Terminals up for about $4.00 at Wal-Mart. More on them in a few moments.

Install the battery terminal protectors that come with the Marine Battery Terminals

Now, Install the battery disconnector on the NEGATIVE terminal. Then I installed the black Marine Battery Terminal on the battery disconnector. I have it pointing down to fit in my battery box, You can position this any way you need to to fit your situation.

Fit your battery in your battery box. Notice that I also installed the Red Marine battery terminal? I did this because I wanted to use the wing nut for my wires, yeah, I know.. but I had it so what the heck right. In this picture the red connector is pointing to the left. After I got through with pics I moved it to point forward so the battery box lid would fit.

Now, remove your wing nuts and install your negative cables to the black Marine Battery Terminal. Attach your positive wires to the red Marine battery terminal, reinstall the Wing Nuts.

Now, test that you have a complete circuit. I tested the light on my tongue jack. Everything works!!!!

Congrats, you have successfully installed your battery disconnector. Your Deep Cycle battery thanks you. Now, Remove the green knob from the battery disconnector and you have disconnected your battery from the system. When you need to use your TT again, simply reinstall the green knob and your battery will be connected again and you will have full power. ( don't lose the green knob!!)

Oh, yeah, I think I used a 1/2 crescent wrench for the install. Super simple. You may have to use different parts depending on how your wires connect to your battery. You can get the parts for pretty much any need at Wal-Mart or any automotive section etc.

I know that this is a simple install but for those who don't know about these neat little disconnectors I hope you got a little from it.

Thanks!

Jeremiah

13 Replies

jerem0621
Explorer II
Jun 07, 2013
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
beemerphile1
Explorer
Jun 07, 2013
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.
jerem0621
Explorer II
Jun 07, 2013
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
opnspaces
Navigator II
Jun 07, 2013
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.
beemerphile1
Explorer
Jun 07, 2013
JN_B, I am talking about the emergency breakaway system which functions if the trailer is disconnected from the tow vehicle. It has nothing to do with a brake controller.

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.
coolbreeze01
Explorer
Jun 07, 2013
The disconnect is a good thing, use it often. Came OE on my TT.
JN_B
Explorer
Jun 07, 2013
beemerphile1 wrote:
Really bad idea putting the disconnect on the negative. It is inevitable that sometime, somewhere, someone will tow the trailer with a nonfunctional emergency braking system.

Maybe I'm wrong, but if my brake controller senses an incomplete loop it doesn't work. The first thing I do when I pull out of the site is push the brake to make sure they're working (or at least getting power).

Back OT, I'm planning on doing this soon, as my battery box is not easy to open, and I'm getting sick of doing that, I just need to find a good place to locate the cutoff switch (out of sight)..
jerem0621
Explorer II
Jun 07, 2013
Thanks for the comments. The instructions on the disconnect said to install it on the negative terminal.

I know there is a more elegant way to do this... But this method is effective. I may direct wire the break away system directly to the battery.

I found some nice waterproof disconnectors with a knob last night. 30-60$ depending on what you want. I think that would be a cleaner install. I probably will do that in the near future and make it look cleaner. Plus I won't have to open the battery box to activate the kill switch.

Thanks for the tips, keep them coming. When I go for the cleaner install ill document it here if you all are interested.

Thanks!

Jeremiah
CincyGus
Explorer II
Jun 07, 2013
beemerphile1 wrote:
Really bad idea putting the disconnect on the negative. It is inevitable that sometime, somewhere, someone will tow the trailer with a nonfunctional emergency braking system.

Anyone reading this should do it the safe way and put the disconnect in the positive line and connect the breakaway directly to the battery.

I see nothing wrong with wiring the breakaway directly if that makes anyone feel better but since he has a power jack and can not hook to his trailer without power, I don't see how it can be inevitable that it WILL be towed without EBS. Seems a bit overstated to me.

Unless he's going to install the green knob, hook to the trailer then remove the green knob, there is NO chance his implementation will cause a problem with the EBS.

There is also no reason that I have ever found that it makes any difference if the disconnect is installed on the negative or positive. If there is one, please enlighten us. How does doing one or the other improve or hurt the situation?
beemerphile1
Explorer
Jun 07, 2013
Really bad idea putting the disconnect on the negative. It is inevitable that sometime, somewhere, someone will tow the trailer with a nonfunctional emergency braking system.

Anyone reading this should do it the safe way and put the disconnect in the positive line and connect the breakaway directly to the battery.