Simple electrical hack.

[MGfromBC]

I'm currently installing a deep cycle battery and isolator in my car for road trips.  For years I've used a 7Ah gel cell in a small ammo box with lights, 12v and USB outlets.  At home it plugs into a small solar panel in a window in case of power outages.

In the car I built a cord to charge it from one of the lighter outlets.  One is hot all the time and one only with the ignition. Originally, since my dash cam was in the ignition only outlet, I plugged it into the other one and unplugged it when it was charged.  Over the years I decided the set the dash cam on motion sensor and put it into the always hot outlet.

On the next trip I plugged my spare battery into the ignition only outlet.  When I turned off the car and removed the key the music continued to play.  After checking this out I found out that I have a circuit/fuse with the radio, the clock and a 12 v outlet.  

Its not that the stereo and clock use a lot of power but, I can always use them knowing that the power is not coming from the ignition battery and there is no need to carry an additional entertainment system.

 

[bullfrog]

You will also most likely be able to remove the vehicle starting battery without losing your radio presets and won't have to reset the clock as well. I used to do that with the always hot plug to keep from loosing those as well as any computer information and trouble codes when changing a vehicle's battery, you just have to keep in mind not to short the battery cables as they are still "hot".

 

[XERTYX]

They used to make a 12v ciggy plug to 9v alkaline battery for that very purpose. You wouldn't lose the presets.

 

[highdesertranger]

I would not make a habit of leaving it plugged in. More than likely you are backing power up through more circuits than you have identified. For short periods of time to keep the memory of stuff intact like bullfrog said shouldn't be a problem but long term I wouldn't do it. Remember you are backing power up in the opposite direction it was meant. Highdesertranger

 

[bullfrog]

^^^ In other words there is a chance you do not have fuse protection as you are powering the circuit after the fuses.

 

[highdesertranger]

Bingo

Also you might be energizing the ignition circuit, The computer, and who knows what else.

Highdesertranger

 

[VanFan]

Yep.  Back feeding.  I did something like this when I first got the van.  It involved an old-style power pack and a fused cord with male 12volt connections on both ends.  I thought it was so cool to have a "house battery" to run the van's goodies (electric sofa and twinkle lights).  Looking back on it, it was probably not a good idea, for the reasons cited above by HDR, et al.  I plugged in at a 12volt outlet on the upfitter's fuse box, so the vehicle's electronics may have been safe.  Still...

Fast forward, and I still use a little power pack, because we just don't need much else.  The electric sofa/bed is long gone, the original house lights are on a timer and not often used now that we have an LED strip and dimmer installed.

 

[MGfromBC]

Thanks for thinking this through further than I did.  When I first discovered this quirk I removed the radio fuse with the car running and didn't find anything other than the clock.  Since the outlet is only hot when the key is ON I used it as a handy way to have a radio on a separate battery.

I had also been thinking of continuing with this after I get the deep cycle and isolator hooked up.  Still thinking.  Since I have used it many time over several years I hope it's safe to assume that it is OK.  On the other hand, if I do continue, I think I'll put the smallest inline fuse that will support the radio and clock.

Thanks either way
MG

 

[highdesertranger]

Get a wire schematic and back trace the radio 12v feed. I really doubt they have one dedicated circuit off the ign switch just for the radio. Also there should be two 12v hots at the radio one ign and one constant. Highdesertranger

 

[MGfromBC]

Right,  the clock and radio get power for memory from somewhere.  I assumed that it was somewhere else.  The fuse for radio, clock (separate units) and ciggie outlet is simply labelled AUDIO. After earlier posts I do intend to check diagrams to see exactly where this comes from.  At this point the question has become a puzzle that I want to solve whether I go anywhere near using this method to power the stereo when I'm stopped.  When I get the deep cycle installed maybe I should go looking for a good AM/FM/SW radio with an AUX input.  Any suggestions?

 

[XERTYX]

You know I have thought of this myself. I don't want to energize the ciggy outlets. Dodge has a fuse that can be moved from IGN to always on. That runs the 2 ciggy plugs and the radio. I could have them always on but I had this idea. Bear with me.

The 2 12v ins on the stereo wiring install a diode. No backfeed. Then thru a switch to the aux battery from the hot 2 diodes joined to the 2 12v ins on the radio. No backfeed to the aux battery.

That way both would be isolated and both could power the stereo. I'd just want to switch off the aux battery when the car was running.

 

[MGfromBC]

Yeah, I've thought of a few ideas with diodes and switches and unplugging things.  These all rely on me remembering to do things.  I got hit in the head with a car a few years ago and remembering new things is difficult.  Fortunately I was already in the habit of pulling up my fly.  

The simplicity of what I've been doing is that I left the 7 Ah battery plugged in; it was always charged and I could always use the stereo without depleting the car battery.   I do intend to trace this system back through wiring diagrams out of curiosity but I think that my likely solution will be to buy a radio/stereo, possibly with satellite, and hook it up to the deep cycle battery.

MG