I made a circuit to be able to switch my radio over to run on the house battery

[Vannautical engineer]

I decided to wire myself up a way to run my van's stereo off of the house battery if I want to use it for prolonged times when the van is parked. It ended up being slightly more complicated than I expected, but it works.

First let me say that you could probably get away with just running the power to your radio from the house battery at all times and avoid a lot of this complexity. Assuming your house battery charges off of your alternator, it is probably just fine to do it this way. But I didn't really want to have the radio draw off of my house battery all the time while driving, causing the charge controller to constantly have to top up the house battery, so I did it this way. Here is what I came up with.

Under normal conditions when the key is on, power comes from your starter battery, through pin 87a of the relay, out through pin 30, and into the power feed of your radio. The radio also gets turned on as normal from starter battery power supplied from your ignition switch.

However, whenever you flip a switch and send power from the house battery through the relay, it does three things... It supplies house battery power to pin 87 of the relay, it turns on the relay, which removes the starter battery power to the radio and flips it to pin 87, which is house battery power, and lastly it supplies power to the ignition switch input to the radio, actually turning on the radio even though the key is off.

The 1 amp diodes on each supply of power going to the radio ignition input keep the house and starter battery power systems separate. Without these diodes, power would backfeed between the starter and house battery systems and have some unintended consequences.

The 15 amp diode and the 1000uf capacitor were two components that I did not think I would need when I first did this, but I quickly realized I do need them. The one problem with switching power to the radio like this is you run the risk of wiping out the radio's memory (clock, radio presets, etc) every time you switch the power. Since the relay switches very quickly, I thought it would switch fast enough that the radio wouldn't lose its settings, but my radio did. (Yours may or may not.) Having that capacitor in there keeps a little power going to the radio for that millisecond when the relay switches, so the radio doesn't lose its memory.

Also now that I really think about this, the 15A diode I put in there probably is not needed. I added that to avoid power backfeeding from the capacitor to the rest of the car's electrical system, but because the capacitor is really only needed for the fraction of a second when the relay is between the two contacts, there's really no way for the power from the capacitor to backfeed during that time anyway. So, if you do this, you can probably skip that 15A diode.

Also all of this assumes your starter battery and house battery both use the vehicle frame as a ground. And be aware that the capacitor and diodes need to be wired up in the proper polarity to work correctly. I tried to show this in the diagram, but it's easy to miss.

 

[Sofisintown]

That's too complex for my hat.
Too complex by far
No way I'm disco dancing.

Good for you though. By the way, the radio consumes minimal power. 1-2 Amps?
Even better get a hand-crank radio. 0 draw from your batteries.

 

[PlethoraOfGuns]

Hat's off to you for being able to figure this out. But car radios draw so little power you can run them for days off a small battery. I wouldn't have worried about it. For simplistic and reliability reasons I would always try to avoid messing with a vehicles stock electrical system/wiring harness.

 

[tx2sturgis]

Re-inventing the wheel are we? Thanks for the write-up.

I would have used a simpler 'MBB' switch or relay.

MBB=make before break.

That way there is no interruption of power to the memory circuits of the radio.

BTW, most aftermarket in-dash replacement radios have a memory 'keep alive' power lead. As long as you have ground, and constant 12v supply to this lead (typically a few dozen milliamps) then the memory circuits will not get 'amnesia' during brief power drop-outs.

I found it much easier to simply carry a small portable battery-operated radio I can set outside with me...I kinda like listening to the radio at low volume levels at night...kinda catch up on the news.

But I know that you young 'whipper snappers' like to have loud tuneage!

 

[Vannautical engineer]

I have never seen a relay like that for 12V automotive applications. I looked around and couldn't find one. Do you know where somebody could source a relay like this? That would definitely eliminate the need for the capacitor.

I dunno, I have installed about four or five aftermarket stereo receivers in my cars over the years, and I can't recall any of them having a dedicated memory power circuit. All the ones I have come across just keep their memory from the main constant power. *shrug* If your radio does have this, by all means just keep that wire connected to your starter battery at all times and ditch the capacitor.

Portable radio? Yeah maybe that makes more sense for people, but I was bored, I have a lot of spare electronics parts, and I wanted to do something fancy in my van.

 

[Vannautical engineer]

That's too complex  for my hat.
Too complex by far
No way I'm disco dancing.

Good for you though. By the way, the radio consumes minimal power. 1-2 Amps?
Even better get a hand-crank radio. 0 draw from your batteries.

Yeah the radio itself seems to only draw 1-2 amps at a moderate volume, according to my battery monitor.

 

[Vannautical engineer]

Hat's off to you for being able to figure this out. But car radios draw so little power you can run them for days off a small battery. I wouldn't have worried about it. For simplistic and reliability reasons I would always try to avoid messing with a vehicles stock electrical system/wiring harness.

Good point, the draw is probably small enough not to cause issues. I just like seeing the power coming out of the house battery and not having any concern about the van not starting because I used the radio too much.

I mess with everything electrical. Have not been burned yet, but I do it right. Every connection soldered, every wire insulated and protected more than it needs to be. It's worth it not to cause electrical gremlins.

I do hate cutting into factory wiring though. But, I did all of these connections in the aftermarket harness between my factory radio harness and my aftermarket radio. So, no cutting into the factory harness was needed.

 

[Weight]

If using a relay, wire it so the radio has house bank when engine is off and starter battery when engine runs. Automatic. Solid state relay so little to no standby current.
A quick note to HDR. We old-snappers need turn the volume high as well.

 

[eDJ_]

At the point where the  1A Diodes are shown,   could a slide switch with a neutral center work?  (if a switch could carry the load)  Both the house and engine battery share the vehicle as the ground.   There may be good reasons not to do this but I haven't seen it mentioned.

I have a Dual AM FM radio (XDM17BT from Walmart $16) I'm wanting to install in my rig even considering it as a house sound system. Leaving the AM FM radio as is in the dash. This radio is not too large at all. (8" wide x 2 1/2 High X 2 1/2 deep)

 

[crofter]

I went with a digital battery powered radio (uses the same battery as my tools). In deep canyons has limited range but elsewhere ok. Nowhere near as good range as the dash radio in the rig though, possibly could be improved with antenna that is mounted outside.
-crofter

 

[Vannautical engineer]

At the point where the  1A Diodes are shown,   could a slide switch with a neutral center work?  (if a switch could carry the load)  Both the house and engine battery share the vehicle as the ground.   There may be good reasons not to do this but I haven't seen it mentioned.

I have a Dual AM FM radio (XDM17BT from Walmart $16)  I'm wanting to install in my rig even considering it as a house sound system.  Leaving the AM FM radio as is in the dash.  This radio is not too large at all.  (8" wide x 2 1/2 High X 2 1/2 deep)

Yeah you could put this in place of the diodes. The neutral center would go to the radio ignition input, one leg would go to switched house battery power, and one leg would go to the ignition switch.

Only reason I wouldn't do this is that it's just one more switch you have to flip to make the radio turn on. The way I did it with diodes, both the switched house battery power and the ignition switch power are fed into the radio at the same time (even though most likely at most only one of them would be hot at any given time), eliminating the need for a switch, and the diodes prevent the electrical systems from feeding back into one another and causing unintended consequences.

 

[eDJ_]

Thank you for taking the time to explain it.    I know switches may "leak" but most of us less tech savvy would "think" to use a switch.  But the diodes allow flow in only one direction so better for safety from an electrical fire.  I'm not that knowledgeable with electrical/electronics and I should get a breadboard and some books and start learning.

 

[MGfromBC]

I have a 7 Ah battery in a plastic box with 12v and USB outlets and a few lights that I keep charged at home, just in case. Once I had it with me on a trip and plugged it into a dash outlet to keep it charged. To my surprise, when I turned off the car and removed the key, the Grateful Dead just kept on playing. Exploration showed that the radio and the outlet in the ashtray are the only things on that fuse.

In the past I had not worried too much about use of the car stereo when camped. I had used it for days at a time without any impact on starting the car. When I discovered this way to reroute power to the radio I thought I had a great way to ensure that I didn't drain the car battery. Others thought otherwise. They did have valid points, which I could have addressed with diodes and relays, but I didn't feel like doing that amount of work to address what wasn't actually a problem.

One day I was riding my bicycle into town and saw something in the tall grass a few yards from the road. It looked like the back of a small solar panel I had used to build a flashing light on the end of a dock a few years before. Yup, a 1 foot square 5 watt solar panel. I took it home, tested it, it worked just fine. Not big enough for much of anything but too good to throw away.

I put a plug and a diode on the wire from the panel and the corresponding plug on the car battery. When I set up camp I pull the panel from under the seat, plug it into the battery and lay it on the hood, just to make sure.

MG

 

[Matlock]

Humph, I'm old but not old fashioned. I use my cell with a bluetooth speaker for radio and apps for music or whatever I've saved on the cloud.
Many phones have a FM chip that will pickup local stations so no wifi required. Need an antenna sometimes, depending on broadcast tower location.

But hey, what's the fun in new technology when you have a drawer full of electronic components and decent soldering iron, right! 

 

[Vannautical engineer]

A lot of the reason I did this is because my stereo has bluetooth. So, I figured why bother with a separate bluetooth speaker when I can essentially turn the whole van into one big bluetooth speaker with (almost) unlimited battery at the flip of a switch?

 

[tx2sturgis]

I have never seen a relay like that for 12V automotive applications. I looked around and couldn't find one. Do you know where somebody could source a relay like this? That would definitely eliminate the need for the capacitor.

Here is a cheap 2 position rotary switch (MBB) and no relay needed:

https://www.ebay.com/itm/184932862093?hash=item2b0edb888d:g:HzgAAOSwIe5faMvN

Mostly DIY with just a little effort and then add a knob.

Or if you wanna get fancy you can use a dual battery switch...just connect the common to the radio (and any other 12v loads) then you can switch back and forth between the starter battery or the house battery as the power source to feed the radio (and those other 12v items) These can be used to switch loads from one battery bank to another, and a bonus is that you can leave them in the middle position so that BOTH battery banks will share the load, and they will be tied together....so in the event you have forgotten to switch to the house battery a week ago, and your starter battery is depleted, you can provide a jump start from the house battery to the starter battery, assuming your house battery is capable of that and you have installed heavy cables that will carry 200 amps or more during the start sequence.

Battery switches are usually make-before-break. (but not always)

https://www.bluesea.com/products/9001e/e-Series_Selector_Battery_Switch

I looked at OMRON and they seemed to have 'obsoleted' their 12v make before break relays. Bummer. Well I suppose that's to be expected when the automotive devices controlled by these are now all controlled by a microprocessor....oh well.

Maybe someone else makes them....I didnt have much luck doing a google search. But the old-school rotary switch that engages one terminal before disconnecting the other, should work ok.

 

[tx2sturgis]

I missed the edit time-out but with the cheap little rotary switch and DIY...you will likely need a couple of fuses (one on each side of the switch to each battery) to eliminate high current flow thru the switch in the event of an imbalance of charge level, between the house battery and the starter battery.