Typical charging rate of a car battery (from alternator)


Good analysis, altho you have to be careful of what no load terminal voltage of a battery really "means". A fully charged battery, afaiu, quickly develops "surface charge", equal to the charging device, so theoretically relatively little current actually passes thru, despite Ohm's law. I'll have to measure this myself one day to make sure....
But at some discharge level, the above sounds about right. I solved/simplified this whole thing with a bit of manual labor: I just hump the marine batt/inverter inside the house, and charge it with a charger, about once a week. Saves a whole lot of mucky-mucking around with car wiring/hole drilling, but..... you just can't believe how HEAVY those marine batts are..... holy shit.....
One thing I"ve learned from these varied discussions is, There is no free lunch. Having the car alternator charge a second battery is neat and nifty, but not at the trouble/expense of a new alternator. So I don't really mind humping that heavy marine battery, at least once a week.
And, it seems these newer cars indeed draw a lot of juice. My buddy has a DC clamp-on ammeter (yup, DC), and holy shit, with nothing running but the engine, the current is 20+ amps, and can quickly jump up to 40. So a second battery, esp. a discharged one, could really add to the load.
They make these isolator switches for second batteries, but really, too much electrical drama for me.
Electrical load can greatly affect mpg's, afaict. Consider that cruising 60 mph for a small car can occur at a mere 10 hp. Adding 1 hp worth of electrical load can then reduce your mpg's by a full 10%, over old non-electronic cars. Technology giveth, and tech taketh away....

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