Connecting CB and Stereo to Aux battery power and ACC switch

[Dusty T.]

My aux battery project is again underway after a year-long hiatus!

Right now I'm in the process of connecting my stereo and CB. I want them to run on the Auxiliary battery power, but also turn on and off when the key is turned in the ignition. I'm also running a SPDT relay to override this switched power so the keys aren't in the ignition when stationary- so I have that figured out.

So basically, the "constant" power leads from these units would be connected to the Auxiliary battery fuse block, while their "switched" power leads would be connected to the factory harness ACC switched wire which is powered from the MAIN battery. There is also a display "dimmer" lead that hooks up to the stereo also connected to the factory harness and MAIN battery as well.

So, has anyone connected their electronic accessories this way, and is it safe practice to basically have the main battery providing the on/off signal, while the auxiliary battery powers that same unit? Or, should I invest in some relays to completely isolate the two systems. I'm thinking this is okay since the switched, constant, and dimming power are all coming from different sources anyway, and any backfeeding within the stereo and CB would have already been a problem.

Input would be hugely appreciated since I'm in the middle of connecting everything up right now. Thanks and yes, I probably do have far too much free time :Wow1:

 

[ntsqd]

There are a couple ways to do this that pop into my head, and others probably have more ideas.

You can turn the relay on with power from either battery if the whole system is all 12 volts.

What I would do is to use two relays. One wired as you propose, to switch power to the radios on and off. (Any memory power wires will want to get power upstream from this relay.) The second relay makes sure that there is no connection between the main and the aux systems.

There may be a simpler way, but I'm tired and it eludes me.

 

[Rando]

The way you are doing things 'should' be okay. The only possible problem that I can foresee is if either of the radios were to common the acc wire and the main power wire inside. If this were the case, and your aux battery was isolated and low on charge, it would draw charging current through your radios! The relay method that ntqsd would certainly prevent this. Another option would be to put diodes in the lines to make sure that power can only flow INTO the radios and not out. At the very least put a small fuse on the acc line (1A).

On the other hand if the radios don't ever common the power and acc lines you should be fine (but a fuse is always a good idea).

 

[Dusty T.]

Yeah, I feel like I'm wandering into uncharted territory hooking these units up this way, but I'm sure someone out there must have the same setup with an aux battery powering their equipment, with the switched power coming from the main. I called a very reputable auto electric shop in the area, and they said this method would be fine and feed-through would not be an issue. This assuming they understood my question because this is a unique setup I have.

Ntsqd-

Thanks for the schematic. Been thinking about doing what you described.

Rando- you hit the nail right on the head with my concern about power feeding through while the batteries are isolated. What I decided to do is go ahead and wire up a SPST relay that is triggered by the factory ACC circuit. This will then switch power from my added auxiliary fuse box that will work as the "ACC" power for the radios. That way they will be receiving ALL of their power from the same aux battery source.

Now, there's still that "Dimmer" signal lead that connects to the stereo (which is a nice feature). Since that is hooked up to the instrument cluster lighting which I know has always been on a seperate circuit than the stereo, I can't see how any of that signal power could be combined with the stereo power within that unit. So that chance I'm going to take. As an added precaution, I connected a 2-amp fuse between this signal wire and the stereo.

Thanks for the help. I think I'm covered, but would still love to hear from someone else who has a setup like this!

 

[ntsqd]

I considered the diode option, but didn't mention it because they tend to be mechanically frail. Not electrically, but in the way they mount (or don't as the case tends to be) it can be easy for them to get broken.

They're also not all that common in added-on automotive electrical, so diagnosing some sort of failure first requires awareness of their presence and knowledge in how they work.

So I was looking for the simplest suggestion rather than the cleanest.

If you have a source for those used by GM that plug into an ATC fuse location, and have such a fuse block, then a diode is a pretty good option.