Other than the issue of stacking another ring terminal on the Alternator (+) output stud, I am unsure why everybody seems to think they have to pull power from starting battery to charge house battery
The OEM alternator circuit was never intended to pass the additional depleted house battery current, and acts as a bottle neck, and the OEM fuse might blow, and this load dump might take out the alternator diodes. No to mention the always undersized and problematic OEM grounds. If all thisthe OEM circuit has to be upgraded, why not bypass the OEM circuit, use a dedicated new shorter fatter less resistive circuit instead?
Also if one takes power from the engine battery, there should be a fuse on that circuit, close to the battery, adding even more resistance, and more voltage drop, inhibiting amperage flow into depleted house battery even further.
Take power right from the Alternator output stud and bypass the too thin OEM wiring, and the engine battery, the extra fuse, have a likely much shorter circuit as well.
Another consideration is the usual frame grounds. How many engines actually have a Stock frame to engine ground? Few is my bet, and then house battery current then has to flow through firewall to engine battery ground then engine battery to engine OEM ground. How much longer is that circuit, compared to running a physically parallel ground cable back to alternator mounting bolt? At the minimum ground the frame to the engine rather that relying on stock grounds for passing house battery current.
With a thick pipe to depleted house battery, perhaps the voltage regulator might allow higher voltages for longer, if thermally based perhaps much shorter before it drops to a rather disgusting 13.6ish.
Seems to me the whole DC to DC booster or the inverter to converter could be, depending on vehicle platform, as much, or more work as modifying alternator voltage regulation, and tricking engine computer, to seek and hold 14.7v whenever house bank is still less than 100%.
My alternator temp data shows as long as vehicle is moving 65mph the alternator case when maxed out will not get above 140f, but idling maxed out and temp skyrockets to 200F quickly. Since i can twist a dial to seek any dang voltage I choose, I can almost always max out my alternator, making this data valid, at least for my engine and underhood airflow idling vs as speed.
I'll soon be installing a secondary battery system in a 95 chevy g20 conversion van. I am using the Colehersee 200 amp silver tungsten contact solenoid and 4AWG, back and forth from alternator to house battery. I do not trust frame grounds, an no way will I take power from engine battery unless access to alternator output stud(+) requires hours of labor.
I am not sure what the chevy's VR will allow, and for how long, but I am also putting 200 watts of solar on the roof, and they are getting a PD9245 for a single G31 AGM and an easy way to plug in to the grid, when that is an option.
