Renogy 20A dc to DC report

theoldwizard1 wrote:

pianotuna wrote:
Hi All,
I'll be "feeding" the unit with #8 wire.

I have a choice of powering from the alternator, or powering from the starter battery (which is of course charged by the alternator), but only with the ignition key turned on. Doing the battery route would eliminate surge on the alternator, so I'm leaning in that direction.

You are OVERTHINKING !

#10 wire is more than enough (I always recommend pre-tinned marine wire - no corrosion).

You want an input that turns off when the key is off. Don't worry about surges. The charger will prevent that.

10 ga may be "heavy enough" for the amperage, but depending on the length of the run may not be "heavy enough" due to the resistance of the wire.

When working with "12V" systems even a .1V can make a huge difference between working well and not working at all.

This is one place where you just need to throw away the amperage capacity charts and start looking at the resistance per ft charts.. And while at it toss the "3% voltage drop rule", that only applies to 120V and higher, with "12V systems", 3% voltage drop is way too much loss..

Math.. 12.0V x .03 (that is 3%) = .36V loss...

To get around that loss, one MUST do one of two items, shorten the run and/or use a much heavier gauge wire..

Shortening the run while cost effective may not be in the cards due to no good location exists to allow the items to be moved closer to the power source.

8 Ga in the case the OP is using is a good starting point, but it may still prove not heavy enough to minimize voltage drop to their batteries..