How far can be driven without a charging system?

I was about to take my xk120 for a drive the other day but I noticed that the red IGN light stayed on. A few checks and I found the the field windings were shorted to the case. I’ll be tearing the generator down to investigate.
That got me thinking, what if was out on my drive when I lost the generator? I wondered how long the battery would be able to fire the plugs before the engine shut down?
Has anyone been in that situation before? How far did you get before it shut down? I think that would be good to know if it ever happens in the future!

My Dad got an E-Type from FL to NY using 2 batteries and charging both in the hotel room each night.

The math is fairly simple.

Install a fairly accurate (10A or less) meter between the battery and everything else (obviously NOT including the starter) and fire up the engine.

Turn off all other accessories and generator disconnected, see what the drain on the battery is. Run the engine up to cruising revs and save that reading too.

Divide Amps being used into Amp-Hours of your battery. That’ll give you Best Case scenario.

Probably over 100 hours. If you don’t run the heater, headlamps, wipers, etc. AT ALL.

A good rule of thumb is the battery will outlast a tank of fuel. Again, with ZERO additional electrical load.

Conceivably indefinitely if the plugs were fired off by a magneto and you could hang one of those plastic fuel reservoirs used for testing purposes from the upper corner of the windshield with a gravity fed hose snaking its way in to the carbs. Of course, being electrical, the fuel gauge would also be non-functional so you’d need a stick to determine fuel level in the main tank every so often while siphoning fuel into the smaller reservoir. This is exactly the sort of thing I did at 19 on an MGA to get me home from Palm Springs after a long-ago Spring Break.

Hi Cliff, There are so many variables here; capacity of battery, amount of charge in battery when generator failed, what ancillaries you have draining power (night driving with headlights?, normal lighting V’s LED lighting, a radio, electric fan) etc etc etc. I guess it’s a ‘how long’s a piece of string’ sort of question…

Yes: I have driven over 200 miles, no generator, but no using ANYTHING electrical.

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Makes you long for a good old mechanical fuel pump.

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you can calculate this using the “RC” figure printed on your battery, and checking current draw with an ammeter, but there is still a large number of variables

I would expect about 2hrs (with no lights) on a vehicle such as yours, maybe more if the battery is good

this happened to my son in the middle of nowhere…carry a spare alternator when 4wding

The main mount bolt that secured the Alternator to the engine broke… It’s an 8" long bolt… so I removed the Alternator! I drove from Mountainburg, Arkansas (halfway between Ft. Smith and Fayetteville in the Boston Mountain range), down to Ft. Smith and on into Tulsa, Okla, across Oklahoma to OKC, continued on south on I-35. I passed thru Denton, Texas (on I-35, 30 miles due north of Ft Worth). The sun was setting as hit the north Ft Worth City Limits, and I STILL had another hour’s drive, IF traffic was light!! At that time, I had “just enough” battery power to produce “legal” headlights (yellow glows, but no light cast onto the road!!) which I kept turned-off as LONG as possible!! I eventually made it home to east Ft. Worth (after “legal” sunset!!)… Total time of the one-stop (for gasoline) trip was around fourteen hours… on ONE “so-so” battery!! The engine was still running on all eight cylinders… BTW… That engine is a '67 429cid Cadillac V-8 with a beefed-up TH400 behind it and that’s bolted to a 4wd Transfer Case… All of this is stuffed into my '1974 Toyota Landcruiser FJ-40 (bought new Dec '73) This “beast” puts out nearly 400 horses and gets a whopping Ten Miles Per Gallon. On a good day!! I later added the stock Cadillac Gas Tank and I now carry 43 gallons on-board… If I had to replicate the Mountainburg, Ark to Ft. Worth, Texas trip today, (1) it would kill me, and (2) if I lived, I could do it non-stop!
I might note, a “standard electrical system” in a pre-widget vehicle, like our XK Jags, ONLY draws an amp or two to run the engine. Your brake lights will draw MORE POWER when you stop than the engine will while driving 50, 75, maybe a 100 miles!! Oh, yeah… The engine NEVER died from lack of battery power… but I did NOT dare shut the engine off as I knew it would NOT restart!!
Charles, Ch# 677556.

I know that there are many variables. I have an unmodified car so figured that if it would be in the daylight hours and the only draw would be the fuel pump, ignition system and occasional use of brake lights (I don’t have turn signals). Two hours sounds like a good number to shoot for, I’m normally not that far away from home with the 120.