HELP PLEASE: Lights via Two Alternate Power Sources


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Learning as I go, I have searched forum threads and YouTube videos, but have not found a clear consensus on this type of project – using two alternate power sources to power multiple relay harnesses for respective LED spots / flood lights (small Nilight “light bars”).

While driving my camper van (i.e. off road at night), I want to power rooftop LED spots from starter battery power ideally via the ignition-switched fuse block, and those lights will be triggered via 4-gang rocker switches on front dash – also powered via ignition-switched fuse block. That starter battery main power source is “Circuit-A.”

When van is parked (engine off…), I want to power my rooftop LED spots via the cabin battery bank, lights controlled by a separate, midway-mounted 4-gang rocker switch panel. That’s “Circuit-B.”

Plan to use a 40-amp 5-pin relay for switching between Circuit-A power (via ignition/fuse block to relay) and Circuit-B (power from battery bank to Lynx to cabin DC distribution fuse panel to relay). Forum threads lead me to believe I can reverse-wire a 5-pin relay, connecting Circuit-A and Circuit-B to pins 87 and 87A – just not clear WHICH to which – and pin 30 can go to a positive bus bar that will power each small light bar or pair of lights via their respective fused relay harnesses. This is what my dangerously ignorant brain has come up with so far.

Given the extended length of my van (and location of roof ports), it works best to locate Circuit-A / Circuit-B power-switching relay, the bus bars (+/-) and light relays midway back on driver’s side. From there, respective power circuits run between the lights and their relays, and rocker-switch (trigger) lines to their respective 4-gang panels & individual switches.

I plan for the front dash LED rocker switches (for Circuit-A) to be powered via the ignition fuse block (add-a-fuse jumpers). A manual On-Off switch on the power line for the side-mounted Circuit-B 4-gang rocker switches will control power for those LED switches from the cabin DC fuse panel.

Will this work?

? – Will I need diodes (& where?), or will the main power-switching 5-pin relay (87 / 87A) plus all other switches be sufficient for preventing back-flow?
? - IF this plan will work -- HOW should I wire the main power-switching relay (i.e. – should van ignition power – Circuit-A – go to pin 87 or 87A, and so Circuit-B to the opposite….)?
? – Pin 85 (or 86?) to Ground?
? – Pin 86 (or 85?) be connected to…? I was thinking maybe that single On-Off switch for Circuit-B’s rocker switches could possibly trigger access to Circuit-B power when that switch is manually turned “On” (via Pin 86?), but that seems redundant (loopy) – since when Circuit-A power is removed (engine off), Circuit-B will already be connected in the relay…. (IF I understand all this….)

I’m throwing globs of wet noodles on the wall – so anyone who really knows what’s what is asked and welcome to help me finish this dish. Thanks for any help!


Forum Member
You could just use diodes after all of the switches. The small voltage drop might actually be beneficial to your lights. I.e. they say 12V but you'll have c.14v.

Switch A to anode of one diode.
Switch B to anode of another diode.
Join the cathodes and wire to light.


Forum Member
Thank you for your reply. Not sure we are on the same page, but....IF I understand you correctly -- I will insert one inline diode on each of my individual rocker switch lines that go from switch Pin 3 (5-pin rocker switches) to Pin 86 on that light's relay. There are two 4-gang switch panels (one 4-gang panel for each separate circuit -- Circuit -A has front dash panel, and Circuit-B is side wall panel). 4 rocker switches on Circuit-A will each have an inline diode (between each switch Pin 3 and its relay Pin 86), and same arrangement for 4 switches on Circuit-B -- and this should prevent any current backflow between the two circuits (? - just making sure for any clarification). I can certainly wire in these 8 diodes on all the switch / coil trigger lines (which are typically low amperage), but just want to be sure this is what you had in mind.
Wondering if your reference to "Switch A" and "Switch B" are just generic labels for two initial switch examples, or if you were referring to the two alternative circuits - Circuit-A and Circuit-B (?). In any case -- above is my interpretation of your tip.
Thanks again for your reply and help!


Forum Member
Onr good relay via hi beam triger lever on dash with a on of cut out, then a 2 way rocker to fire the relay from either engine bat or hab batts, so simple to do, no need to get over complicated.

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