Could I check my use of diodes in the following circuit?

I'm wiring a new car stereo head unit into my motorhome (RV).

I'm going to use most of the standard radio harness connections. However I would like to run the 12V power from the leisure battery so we can listen to the radio whilst parked up without the engine on and not drain the main battery.

The question I have is around the wire going into the head unit from the ignition circuit to turn the radio on when the ignition is on. I want to add to this a separate connection from the leisure battery with a separate switch. I want the radio to come on when I either turn the ignition on or switch the leisure battery switch.

I was going to use two diodes so that the leisure battery didn't interfere with the ignition circuit.

Have I got my wiring correct?

Will single 6A diodes do the job?


Motorhome stereo wiring diagram

  • \$\begingroup\$ How does the leisure battery get charged? What is the Yellow wire for clock, memory? 6A is enough for about a 50W device, if you have a power amp it will blow. Remember that there will be a 0.3 to 0.7V drop across the diode. A Relay or two may be more appropriate if you want to avoid forward voltage drop. \$\endgroup\$
    – KalleMP
    Oct 27, 2014 at 10:18
  • \$\begingroup\$ Thanks @KalleMP the leisure battery is charged via the alternator and also via mains hookup. Yes the yellow wire is for the memory. Thank you for the relay tip! \$\endgroup\$ Oct 27, 2014 at 11:06
  • \$\begingroup\$ In addition to series diodes, a TVS diode from the input to "head unit" to ground is a cheap way of saving the electronics from all manner of spikes. \$\endgroup\$
    – Lundin
    Nov 22, 2019 at 9:11

1 Answer 1


As it is difficult to determine exatly how the batteries are charged from the alternator it is not possible to be totally certain what would be the best way to proceed. If there is a double (diode bridge) output alternator or some other existing battery disconnect system then diodes later may not be needed as the head unit could connect to the Aux battery all the time and if this runs flat you would have to start the engine to charge it and this may automatically connect the two batteries already.

If you want to operate it from the main battery with engine not running you couldconnect the ACC line to control a relay that will supply Main power (NO) to the head unit (COM) when the ignition is on and then feed from the Aux battery (NC) when the ignition is off. You will have a power glitch when you cycle the ignition.

If the batteries are connected together at the alternator then none of the extra stuff afterwards realy matters as both will drain together.

You can have a look at the large selection of solutions that have been invented for your situation in the list of pictures blow. There are smart products and DIY solutions, some of those solutions will not work and you can ask here if you want to confirm a specific case. The important thing is to establish how the two batteries are currently separated from each other at the moment when not charging and if the second battery would be nice to have available for starting if you let the main battery run flat with the headlights illuminating your campsight.

Some of these are intended for high currents and others for slower charging only. Some will sense the main battery voltage and only hook up the other if the main battery if mostly charged.


  • \$\begingroup\$ Thanks so much @KalleMP great answer. I will explore further exactly how the leisure batter is charged. On another note, I didn't have the head unit when I came up with the plan above, I do now and it has a built in amp. Which can draw a max 15A current. So I've decided to just have an on-off-on switch instead and toggle between the batter either being powered from the main battery, no power or powered from the auxiliary battery. Then do away with the diodes all together. \$\endgroup\$ Oct 28, 2014 at 16:52

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