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SUV with 4 interior light pods in a row from front to back. All light up with stock interior light switch. All have LED replacements for the old stock bulbs.

I have an auxiliary 12v battery that I want to use to light the two rear light pods with 1 switch at the back of the truck. I still want to be able to light them from the stock switch and main battery also

I know I need a diode to keep the auxillary battery /rear switch from lighting the two front light pods (and possibly damaging the stock switch?).

So, front switch lights everything from main battery. Rear switch lights 2 cargo bay lights from auxiliary battery.

  1. What size diode do I need? Assume the LEDs are the normal replacements you can buy at any auto store. Small pad of multiple LEDs about 4x4 (2 of those) and two spot light LED bulbs. I don't know volts/amps

  2. Do I need another diode on the other side to protect the auxiliary battery / rear switch as well? If so, same size?

SUV =1999 Ford Explorer XLT

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Assuming the LED pods are at most 12W that's 1A. For lots of margin you could use 1N540x (where X is 4, 5, or 6) 3A diodes, two for each of the rear lights. The front ones just get wired directly. That's super conservative and will withstand worst-case automotive electrical system transients according to JAE/SAE standards and a fair bit of heat.

Unfortunately, I can't draw a schematic diagram myself here, but it will look like this, where R is the LED lamp assembly and the 'line' on the diode schematic symbol will be the band on the physical diode. Wire the two A connections to one source and the two B connections to the other source.

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6 of these 1N5406 diodes (4 + a couple spares) will run you a bit over $2 plus shipping at Digikey or Mouser.

This is what you asked for, however there is a potential flaw in this ointment (sic)- if you have the back source turned on at the same time as the front source it may draw power from your auxiliary battery rather than the main battery or vice versa (it will draw power from whichever source is higher voltage), so you could either use a relay as the other answer suggests or use a single switch to physically interlock so that can't occur.

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I would separate the supply to the rear lights from the front and use a relay - the two inputs being from either battery, the output to the lights, then there would be no issue of the batteries being connected together. However, you need to charge the auxiliary battery so check out split charge systems - usually some relay that only connects when the engine is running and the alternator is producing sufficient charge.

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