[h2odragon]

Depending on the load, you can often skimp on the conductors and get away with it. Someone else mentioned undersized starter power cables. There's really only a few loads that are full draw, full duty cycle.

Old car stereo trick, to power big amplifiers in the trunk: use a second battery and big power lead to it, small power leads from the main battery to the secondary. That secondary can be backed or replaced by big capacitors, too; with commensurate increases in cost and possible risks when things go wrongs. But you can provide rich chunky amps and use skinnier cable than you'd think on the long run to do it.

[mschuster91]

Never forget to put an override relay (normal open, closing when the engine has run for a set amount of time or when manually bypassed) and a fuse in the path though (and that also applies for campers). There are a number of things to consider, and if done badly, also serious risks:

- you don't want a permanent connection between the main and aux battery to avoid accidentally sucking both batteries dry on the aux-battery side, and to avoid the starter overloading the cable between the two batteries

- you don't want to risk an empty aux battery charging itself on the main battery and engine with more current than the cable supports, hence the fuse

- you do want to be able to connect both batteries in a scenario where you accidentally drained the main battery (e.g. a light left on) to "self-start"

[h2odragon]

All very good point, tx. I liked diodes as well as fuses, especially on the inputs of capacitors. Automotive power is never clean.