Yee HAW! Hillbilly battery. Does it work? Dern tootin it works!
The DC-DC keeps about 13.8 the HB battery puts out 14.7 fully charged.
So as the battery voltage drops below 13.8, the DC-DC starts to pick up the load just as you predicted, Bob, when you thought of the diode arrangement. This way one avoids all the hassles with diodes though. I just happens that 13.8v is to the 14v battery as 11.8V is to a 12V. That means the battery will never drop into dangerously low discharge. It will still have a little bit of capacity left over but it's a 90% there.
At the end of the drive I measured the 12V for a voltage SOC and it was showing ~12.2V resting. That's about a 60% SOC. 50% is about as low as you would want to go with these batteries. We were driving for a couple of hours today but I don't know how fast it dropped.
I was able to get 3.5 L/100 on our usual highway run. Previous best was 3.8. Totally not scientific. I will have to try to do the a-b-a at some point.
This is sort of what you were talking about right?
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I was thinking about this at lunch and realized the deep-cycle battery only has to provide the 90% load over the time, vehicle base load, and let the vehicle inverter with diode handle the peak loads (aka., rear window defroster, windshield wipers, night driving lights and brake.) Anything over that would drop the output voltage and the inverter would then provide the make-up or peak power. For example, you measured ~20 A. in your first test. Add whatever you consider to be additional, standard load plus 5-10% and that would be your deep-cycle battery DC-DC converter design goal << 100 A. <grins>
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I'm going to look into some battery combination possibilities that give me that sort of range.