# Wiring for radio - permanent 12V connection



## Rosbotham (May 4, 2008)

Hi all

Any opinions welcome.

I'm fitting a car stereo in the rear of my motorhome but am undecided what to do about the "permanent" 12V connection to it.

Background on MH wiring;
The habitation part of my motorhome doesn't have a permanent 12V from the leisure battery...the builders configure it so that when the ignition is turned on on the base vehicle, the leisure battery supply to the habitation is disconnected.

For various reasons, I _do_ have a permanent 12V connection going through the habitation unit from the base vehicle battery (don't ask, the numpty that fitted my reversing camera wired it to unswitched 12V and the wire's still there...).

Onto the radio connections;
I'm going to be wiring the main power for my radio through the habitation 12V supply - which means generally it's on the leisure battery, but it can be switched over to be powered by the vehicle battery.

Now, the radio itself...like any other car radio...has two 12V supply wires; one which in a car would go to the ignition switched 12V to provide the main supply for the stereo itself, and one for a permanent 12V that just a trickle supply that keeps the memory settings etc when the ignition's off.

As I say, the switched 12V I'll take from the habitation 12V supply, ie it'll be driven from the leisure battery unless I over-ride that. When the vehicle ignition's on, it'll disconnect, and I can also manually disconnect/isolate when in store.

I've got 3 options for wiring the permanent 12V connection;

1) Wire to unswitched vehicle 12V battery
2) Wire to habitation 12V or
3) Wire to unswitched leisure 12V battery (which would require a new wire running through)

I'm tending towards (2), even though it means whenever I start the engine or isolate the habitation 12V, I'll need to redo the radio memories afterwards. I've concluded this because I'm wary of the unintended consequences of (1) or (3)...could any auto-electrician tell me if I'm worrying un-necessarily?

The concern I have is, say I go down route (1). Typically the vehicle battery sits at 12.5V or so on my MH. The leisure battery...because it spends most of it's life on hookup...is nearer 14V. Assuming the habitation is being driven by the leisure battery, this would mean that "permanent" 12V connection for the radio would be sitting at 1.5V lower than the "switched" 12V connection. Depending upon the internal circuitry of the radio, I suspect _fizzz_ will result as the designer would never expect anything other than switched/permanent at same voltage, or switched=12V permanent=0V.

For route (3), similar consideration applies, but only if I flick the switch to make the habitation be driven by the vehicle (by definition I'd only be doing that when leisure battery was dead - which would mean that the permanent supply would be 0V, main/switched power 12V).

Does this make sense, am I being unduly cautious, or have I totally bamboozled by not explaining very well....


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## sallytrafic (Jan 17, 2006)

Lots of comments

1 not all feeds to the habitation area have to be switched off when the ignition is on and often they are not. 

2. A mA or mmA feed to your radio is not going to upset your ABS so don't worry. The whole issue of interference between safety critical systems is in my view overstated.

3. I have looked at two radio circuits and I don't think it matters one jot if they are fed from different supplies at different voltages.

4. I would tend to wire both to the habitation battery just bypass the relay/switching for the permanent feed which I think is your option 3. I would also wave a meter around and see for example if there is an unswitched feed anywhere else - to your fridge area for example to run a compressor fridge.


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## Boff (May 10, 2005)

Hi!

In this forum we already have discussed several cases where exaggerated "safety" demands got into the way of simple reason. This is one of them:


Rosbotham said:


> ...the builders configure it so that when the ignition is turned on on the base vehicle, the leisure battery supply to the habitation is disconnected.


I can assure you that no, absolutely no MH builder on the continent would ever install such a relay. So there is an option number 4:

4) Throw out the relay that switches off the habitation 12V supply when ignition is turned on. And connect the radio just as normal to the (then permanent) 12V feed from the leisure battery.

Best Regards,
Gerhard


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## Rosbotham (May 4, 2008)

Thanks for the replies both.

Boff - I'd tend to agree with you (on a similar vein, I've desoldered the "sounder" on my towbar relay which alerts when the indicators are working on the trailer, because it literally used the same type of sounder used in smoke alarms, so woke the dead whenever I indicated). However, I'm reluctant to interfere too much with the electrics installed by the converter because the MH is only 6 months old and wouldn't want to give them the excuse to invalidate the warranty.

Sallytrafic : On your (1), agree, but that's how mine is setup (Compass Avantgarde, incidentally). On (2), agree, not concerned on that front, it was more concern for the internals of the spanking new radio driving me...which your answer to (3) re-assures a bit. I may go down your (4)...which was my (3)...but think I'll need to put the wire in myself...e.g. the fridge is further away from where I'm fitting the radio than the actual leisure battery is, and in any case that works mains/gas/vehicle battery. I've got a rudimentary wiring diagram for the habitation so I think I'll give it some study over a beer or two over the weekend...weather's looking nice to sit under the awning and contemplate...


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