Motor Vehicle Maintenance & Repair Stack Exchange is a question and answer site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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

share|improve this question

migrated from electronics.stackexchange.com Jan 20 at 23:42

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

    
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 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 at 11:01

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

share|improve this answer
    
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 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 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.

share|improve this answer
    
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 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

share|improve this answer
    
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 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 at 9:12
    
I think the intent was for short term tows. – Dee Jan 21 at 20:17

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

enter image description here

share|improve this answer

maybe you can fit a a battery with more reserve power

share|improve this answer
    
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 at 21:33
    
@Bob this comment is on the wrong answer – Jasen Jan 23 at 6:39
    
Guess it was on the wrong answer but I would imagine anyone that read could figure it out. – Bob Jan 24 at 12:54

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.