On my TD5 I have a second battery squeezed into the battery box under the passenger seat - they are very snug and held in place by a few bits of wood with bit of old inner-tube providing back-up insulation to prevent shorts to the box cover.
I forget what size the battery is. The battery is charged from the alternator when the engine is running (via a voltage-sensitive relay) and can run our thermo-electric fridge and a few LED lights for a couple of nights.
The house battery runs hard-wired USB chargers, external lights, radio, fridge and there is an Anderson connection on the tray. Wiring the radio was a bit fiddly because I want it to turn on/off with the car and auto-dim when the lights turn on etc as normal, but also be usable when the car is off. (This used up 3 relays!)
I have built my own cubby-box that has 3 levels - 2 for storage and the bottom one for the electrics. (See pictures.) I'm quite tall so I made it higher and also made bits for my coffee and assorted pens & glasses. The wiring is a bit messy and not (yet) well labelled, but I know what it all does! Some of the wiring is for additional reversing lights and front-spots that run off the main battery (and also require relays to avoid overloading etc).
Handy hints:
- Buy switches, relays, fuse blocks, USB chargers etc are super-cheap on eBay for about a third of the cost of Jaycar.
- I have run multi-core cable to the dash and the tray. This is heaps easier than running another cable every time you need something else.
- I run a single earth connection through the multicore to make wiring switches etc easier. For high-current devices I also earth locally.
- Label every cable as you go!
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