See any problems with this? Use diodes to power radio memory wire from two different batteries?

Thread Starter

zirconx

Joined Mar 10, 2010
154
I'm installing a new radio in my 2006 Ford motorhome. The radio has two power wires, a red primary power wire that comes on with the key to turn on the radio, and a yellow wire that is hot all the time to provide power to retain the memories and settings. The problem is I disconnect my chassis battery when storing the RV for more than a week. I don't want to have to reprogram everything in the radio when this happens.

There is also a house battery on board. It will occasionally be disconnected for maintenance or storage, but less often than the chassis battery. Both are 12v batteries. The house battery is 400ah of lithum iron chemistry. The motorhome comes from the factory with a toggle switch to power the radio from either the chassis battery or the house battery. This switch toggles the primary power and the memory power wire.

My proposed change would be to send power from both batteries through diodes so that the memory wire is automatically power from either battery, regardless of the state of any of the switches. The memory wire typically draws 200 microamps, but it also surges to around 500ma when the radio is powering down (i.e. the key is turned off and the red wire no longer supplies power).

Do you see any potential problems with this? I know there will be a 1.2v drop across the diode, I think that will be fine. FYI the radio is a Sony XAV-AX150. I will of course put a small fuse on each wire as it leaves the battery.

Thanks.

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DickCappels

Joined Aug 21, 2008
8,710
That is an elegant solution.

Consider the voltage from each battery under “normal” operating conditions. For example, if the chassis battery’s voltage rises a little bit while the motor is running, that is probably fine, but would other differences between the battery from time-to-time be a problem?
 

Thread Starter

zirconx

Joined Mar 10, 2010
154
I think that would depend on how the radio's internal circuits were designed, right? You are saying - what will happen if the primary power wire and the memory power wire are at slightly different voltages?
 

crutschow

Joined Mar 14, 2008
29,818
A standard silicon junction diode has about a 0.5V drop @ 0.2mA, not 1.2V.
A Schottky diode will have about a 0.2V drop at that current.
Either way I doubt there will be a problem if you power the memory wire using either type of diode to the two batteries.

The one way for it to be a problem is if both voltages go to the same chip in the radio, which could possibly forward bias some of the internal diodes and generate a large fault current if one of the battery voltages is significantly larger than the other.
 
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MisterBill2

Joined Jan 23, 2018
11,913
The circuit will certainly work very well.
The only concern I offer is if the selected battery is disconnected while it's charger is active, since I don't know how high the no-load voltage from the charger might go. But it will be simple enough to switch over to whichever battery is not being charged.

You will need to select diodes rated for more than the momentary current surge, and I also suggest fuses near the battery connections for both, because the memory backup wiring will be a thinner gauge and will overheat if there is a shorted circuit incident.
 

Thread Starter

zirconx

Joined Mar 10, 2010
154
What do you think about this idea? - Use a 8 AA battery holder to make a battery pack. Then wire it in isolated by diodes so it doesn't get charged and only supplies power to the memory wire. At 200 microamps, the battery pack should last 520 days (assuming I can get the full 2500mah out of the AAs). It would only be supplying power when the chassis battery is disconnected.

This is a little more simple because I won't have to run a new wire from the house battery up to the radio. There is plenty of room under the dash for the battery pack.
 

MisterBill2

Joined Jan 23, 2018
11,913
If you can locate a battery pack that will not come apart with the typical automotive road vibration then it should work quite well. And iif you used rechargable cells you could arrange to have them recharged with a diode from the power antenna control line, if the system has one.
But it is also possible that the drain is more than claimed. I have a friend whose boat battery was being run down, so much that the engine would not crank, by the current taken by the radio in the boat. It seems that the built-in clock was also powered fro that memory power circuit..
 

Thread Starter

zirconx

Joined Mar 10, 2010
154
Well I'm getting closer to installing this, and doing some testing to make sure that different voltages on the red & yellow wires do not cause a problem. And they don't. But I discovered something else I must have missed in my earlier testing. The yellow wire supplies all of the current. The red wire is simply the "turn on" signal.

I don't want to run all the radio's power through those diodes all the time. The radio might consume something like 10A when the volume is turned up.

So now I'm not sure if there is a simple solution. I was thinking something like this (below)... but that lets power backfeed from the yellow wire into the red wire when the red wire does not have power... maybe that is not an issue, since that would only happen when the key is turned off. And at that point the red wire is disconnected from everything (because the accessory power relay is off).

Hmm but if the house battery had a higher voltage than the engine battery, and the key was on, then power from the house battery could backfeed from the yellow wire into the red and start supplying power to other circuits in the car?

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Thread Starter

zirconx

Joined Mar 10, 2010
154
To complicate matters further, my new radio does not have a way to turn itself off! If there is power on the red wire, the radio is always on. That means if I flip the house/engine switch to House, the radio will drain the house battery. I cannot put the switch in the house position to keep the memory settings while I disconnect the engine battery. Unless I go through the effort of installing an additional switch inline with the red wire.

Starting to think I'll just have to reprogram the radio occasionally. This project is getting too complicated.
 

Thread Starter

zirconx

Joined Mar 10, 2010
154
How much current does it take?
I did not measure the red wire, but I did have an overall measurement from the power supply for everything, and a measurement on the yellow wire. They were about the same. So my guess is the red wire carries less than 100ma. Probably closer to 10ma.

The yellow wire carries around 1A. But I don't have any speakers hooked up. When the volume is loud, I assume the radio will consume much more. I believe the fuse is either 15A or 25A.

Actually I don't think it will be too much work to install a push button switch in the dash for disconnecting the red wire. If I did that, then I could flip the house/engine switch to house, use the new push button switch to turn off the radio, then I could be able to disconnect the engine battery and let the house battery maintain the power settings. The house battery is 400ah of lithium, so it has very low self discharge. Should have no problem keeping the memory settings for months. I will just need to remember to put the switches in the appropriate position.
 
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Thread Starter

zirconx

Joined Mar 10, 2010
154
Wouldn't that also drain the engine battery?
The red wire comes on with the ignition key. So it would only drain the battery if you left the engine/house switch in the Engine position and left the key on.

But if you left the switch in the House position, it will drain the house battery.
 
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