HOWTO:  Install an RV Battery ON/OFF Switch


     Whenever I store my RV trailer somewhere that I can't keep it plugged in my battery will drain down to nothing in about 3 days.  Some of the trailer electronics put a constant drain on the battery.  It is unwise to ever drain your deep cycle battery below around 50%, draining it all the way reduces the lifespan of the battery considerably.  So, I've had to pop open my battery cover and unscrew the battery terminal to disconnect it.  Then repeat that process when I am ready to use the trailer again.  My deep cycle battery only drains about 10% of it's charge in a month so this works great for storing the trailer for up to a couple months.

     So I decided to install a cut-off switch on top of the battery cover to make it easy to disconnect and reconnect the battery.  Boat/Marine batteries have this type of a switch so I bought one for about $12 and a short 19" battery terminal 4AWG cable for $5.  So this project cost me under $20 and took less than 30 minutes to install.  Now I can just simply turn my switch ON/OFF without opening the battery compartment.

  • Drill
  • Drill bit small diameter
  • Drill bit large diameter
  • Allen wrench
  • Socket wrench w/socket
  • The boat/marine on/off switch
  • Short battery terminal cable

     Drill holes in the battery compartment cover.  I put blue painters tape on the cover and placed the switch over it and marked the 4 small holes with a felt Sharpie pen.  Then drilled those holes right through the tape.  Removed the tape.  Then centered the larger bit in between the smaller holes and drilled out a larger hole big enough to fit two 4AWG battery cables and their connector ends through (about 3/4 of an inch in diameter).

     Connected the "positive" black wire from the RV trailer in through the hole cut into the battery lid to one of the two switch terminals.  Then connected the short terminal lead wire to the other switch terminal and ran it in through the hole cut in the battery compartment lid.  Connected that lead up to the "positive" RED post of the battery.  NOTE:  The RV Trailer follows home/residential electrical color code and NOT automotive color code:

                      AUTOMOTIVE .    RESIDENTIAL.
POSITIVE .      RED                    BLACK
NEGATIVE .     BLACK                 WHITE


     Screw the switch to the top of the lid using the provided screws.  Pop on the battery compartment lid.  All done.

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I have a question . I thought you could disable the battery by removing the 40 amp fuse in the panel (main power fuse). I have 2 batteries and thought about a disconnect switch but my is always on shore power when no on th road. When we camp I run the gen everyday for 1/2 hour.GBA!!SVJ
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Edit: since Dwight's method is at the battery, one must ensure that it is NEVER turned off when towing, or the emergency trailer brakes ("breakaway") won't work. They need battery power to work.

I simply replaced the cheap auto-reset breaker they had mounted on the front of the frame with a Bussmann CB185 (High Amp Switchable, Waterproof, Flush-Mount Manual Reset Circuit Breaker). A CB187F would be a step up, as they're rated for switching under load. I would normally only switch when everything is already turned off (or possibly in an emergency), so the extra cost wasn't worth it for me.:
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looks more hd than the one supplied with the Retro. I guess you got it online? GBA!!SVJ
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It's more heavy duty only in that I put a larger rated breaker in (along with thicker wiring). The stock breaker was similar to a Bussmann 121. The CB185 may look larger in the picture that it actually is - about 2"x3", which mounts just fine on the frame. It's a pretty common form factor, Littlefuse makes similar. There are also similar, cheaper Chinese ones, but I wouldn't trust those to either trip properly or be waterproof.
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Dwight, I am interested in doing this but have read that the NEG terminal lead should ALWAYS be used and not the POSITIVE lead. I am no electrician and wonder why you chose the POS cable?
Please buy Made in USA
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mkevenson wrote:
Dwight, I am interested in doing this but have read that the NEG terminal lead should ALWAYS be used and not the POSITIVE lead. I am no electrician and wonder why you chose the POS cable?

As far as kill switches on your battery go it really doesn't matter if you have it on the positive or the negative side because either terminal will kill power from the battery.  The current flows from the negative terminal to the positive, so it follows that you should disconnect the outflowing terminal first to deenergize the system.

I believe what you "heard" about taking off the negative first applies to manually by a human being with a wrench, for safety -- If just disconnecting the battery for maintenance or repairs etc, the negative comes off first. Prevents you from causing a direct short with a wrench while removing the positive.
  • Remove The Negative Cable First.
  • Re-connect The Negative Cable Last.
Why? Because the wrench or socket is touching the live part of the electrical connector. There is a good chance that the wrench or socket handle will accidentally touch something. The entire trailer is connected to the negative terminal.

Every new OEM boat that I have seen has the battery cutoff switch installed on the positive side near the battery. One reason for doing it that way is that the AC ground and DC ground are usually tied together. If you disconnect battery ground, you could still draw current through the AC ground and it might be through a small wire size wiring harness that would not handle the current.

  • You do need to wire the breakaway switch directly to the battery so it will work even with the battery switch turned off.  I did not cover doing this in my original tutorial.  Or alternatively at least make absolutely certain that you have the switch "ON" whenever towing, or the breakaway brake won't work (probably best to wire it in permanently, easy to do).
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Mike, The Bussman CB185, what amperage rating did you get?
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