I have a motorhome and tow my car behind. Since I have to leave the car in accessory mode so the steering wheel is unlocked, the battery doesn't last a long time.

I want to run a DS line from the motorhome hookup (fused at motorhome) to the battery in a car to utilize the motorhomes charging. I plan to fuse at the car battery. Really would like to add a diode to prevent the car battery from backfeeding into the motorhome.

All batteries are flooded lead acid, 12V. Since there is a good possibility of backfeed when starting (diesel draws a lot of power to start), I'm not sure what size of a diode I should use or even where I could purchase one.

I live about 40 miles North of Toronto Canada.

Any suggestions on where to get or improve the circuit would be greatly appreciated.

Thank you

  • What is the model of the car you are towing? AFAIK all cars should have lock, unlock(for towing), acc, run, start. Does the manual state anything about towing?
    – rpmerf
    Jan 21, 2016 at 0:26
  • @rpmerf: Actually cars vary in what key positions they have - mine just has lock, run and start (the accessories are controlled by the car's computer - so the radio can always be turned on, even in "lock", but turns off after 20 minutes, whereas the windows only operate in "lock" until the first time you open a door after stopping...).
    – psmears
    Jan 21, 2016 at 11:01

5 Answers 5


The fastest, cheapest and easiest solution is to add a battery disconnect switch to the car. After placing the ignition switch into acc mode the open the hood and disconnect the switch. When you arrive at your destination, open the hood and reconnect the switch. This costs under $20. It also eliminates the need to run anything or calculate anything. Many auto parts stores carry this in stock.

PS put this on the negative battery cable not the positive.

enter image description here

  • yeah, if you're traveling in a motor home you don't need the presets in the card radio, and presumably can live without the clock too.
    – Jasen
    Jan 21, 2016 at 9:07
  • This was my exact idea. I've also seen under-hood mounted switches that are similar to this but quicker to operate.
    – JPhi1618
    Jan 21, 2016 at 13:14

adding a diode would probably be counterproductive as it will cause an extra voltage drop, I'd look into fitting a relay such that the socket was not connected during engine start.

  • This would be very similar to a split charge relay circuit, commonly used to disconnect the leisure battery of a motorhome during starting.
    – Chris H
    Jan 21, 2016 at 10:25

Disconnecting the battery messes up your radio, etc. Instead of a diode which causes a voltage drop you could put a 55 W lamp in series. It will limit the current to the battery but as the battery charges up the voltage drop across the bulb will reduce and the battery gets full alternator voltage.

When cranking the motor home its battery voltage may drop to 6 V or so. The charging lamp will glow at half-voltage (about 1/4 power) and won't load the car battery.

enter image description here

  • The car is a 2013 Ford Edge AWD with push button ignition. The manufacturer states to place in accessory, put vehicle in neutral and run engine for 7 minutes every 6 hours. There are many parasidic loads on these cars that drain the battery quickly. I can presently tow for 5 hours, run for required time. Go back on road and in 3 hours I have to boost to get started. I know the fusing will provide protection to a certain extent on heavy backfeed loads (while starting motorhome but I figured a diode would eliminate any possibility in the event I forgot to disconnect the cable at night.
    – Bob
    Jan 21, 2016 at 2:27
  • maybe you can fit a a battery with more reserve power, or fit a second battery to the car.
    – Jasen
    Jan 21, 2016 at 9:12
  • I think the intent was for short term tows.
    – Dee
    Jan 21, 2016 at 20:17

I found this online.... Should solve your issue:

enter image description here


maybe you can fit a a battery with more reserve power

  • Dee, thanks for the lead. Ive checked into that one and its just what I want to do. Diode and fuse but costs $75. or more. I figure I can do for about 15-20 and that leaves extra for diesel 👍. Thank you very much for the info.
    – Bob
    Jan 21, 2016 at 21:33
  • @Bob this comment is on the wrong answer
    – Jasen
    Jan 23, 2016 at 6:39
  • Guess it was on the wrong answer but I would imagine anyone that read could figure it out.
    – Bob
    Jan 24, 2016 at 12:54

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