But you take a risk, that and once the voltage drops a little your mpg suffers because the battery is no longer sending as much juice to the ignition coil(s), hence your spark plugs deliver a weaker spark.
This may be true for a 92 VX (I don't know for sure) since it uses a distributor, but on cars with modern electronic ignitions the ecu reads battery volts and adjusts the coil charge time so the coils put out just as much energy regardless.
What is the minimum battery voltage that would correspond to a 50% charge and thus would not harm the battery? 11.5 volts?
Since my car has a carb and a mechanical fuel pump, I could imagine that it could run for several hours during the day (no lights on) on the interstate without draining the battery much.
I was thinking of putting a switch on the dash that interrupted the alternator field wires. I would connect a digital voltmeter to an energized wire and monitor voltage, when it dropped down to the point where it would begin to shorten the life of the battery, I could flip the alternator field wire on for a few minutes and charge it back up to a "safe" level.
With my CVCC engine, since the spark plug is igniting an extra rich mixture, I also might be able to get away with a weaker than normal spark.
And on a rainy night, with headlights and wipers and at least some fan?
You might make it 200 miles, maybe not... Maybe 100, maybe 50...
About 30 miles is about it... and that was with the assistance of an "emergency" alternator belt, which didn't work worth crap... rain wouldn't let up, had to have the wipers and blower on full... just hovering above freezing... soaked to the skin from standing out in it trying to rig the emergency belt... crawled it about an hour on the back roads... before it started bogging really bad at low speed... AND I was turning the headlamps off every chance I got, just running sidelights until I saw another vehicle, then popping them on lowbeam again... so I parked outside a small town garage for the night... coldest damn night ever... so I kinda have this habit of regarding people willing and eager to do away with their alternator as severely disturbed
2nd instance different vehicle, this time with the alternator dying, did 1 1/2 hour of highway travel in good weather before it stalled out and wouldn't go, had DRLs and nothing else running. Had cellphone, had CAA, got towed, phew.
IMO, trying to run off a regular car battery with no alternator or provision for charging, is like getting all the disadvantages of an electric car, while keeping all the gasoline bills of of an IC car... by the time you're carrying the 4 deep cycle batteries that I would consider a decent reserve source with no charging system, you probably aren't saving any gas.
Unhook the field coils, use an electric clutch if you like, but as soon as you feel the thing stumble you can hook it back up and rev the motor at 2500 for a minute or two, and not have to sleep a night in the back seat with wet clothes and only a mars bar for supper in 1-2C temps.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice