A properly working charging system puts out more than enough current to run the car without the battery. Years ago, the quickest test of a charging system was to start the car and disconnect the battery. If the car died, there was a problem with the charging system. If it kept running, you were okay. This test is not recommended today because of computerized ignition systems. When you disconnect the battery, they are not grounded properly.
The old-time DC generators didn’t put out much current at idle. This is one reason they were replaced by AC alternators, which have very good output at idle.
I’m not sure this is still universally true.
I can’t dig up much evidence, but there are at least some alternators out there that don’t produce much current at low RPMs.
The engine controls will also be more sensitive to voltage levels than a new car. It doesn’t take much to fire a spark-gap ignition. In my current car, the F/I won’t turn on without at least 13.5V.
I’d be really surprised if a stock alternator, spinning at 700-900RPM, made enough current to power the F/I pump, cooling fan, ignition, DRL’s, and other gadgets in a modern car for an extended period of time.
Sorry. That should read “…voltage levels in a new car.”
My first car had a tiny 1.6L engine and a carb. The mechanic who looked it over before I bought it pointed out the results of the electrical system test: it didn’t make enough juice to keep the battery topped up at idle. But he said that was normal in many models with very small engines. I drove that car for 7 years, and I never replaced the battery and never needed a jump start. The wipers would run very slowly when I stopped at a light, though.
My current car has a signifigantly larger engine, and computer-controlled port injection. It seems to have plenty of juice at all speeds, although I don’t have any measurements on this one.
So I suspect that some cars would have battery trouble after extended idling, and some would not. I would guess that more modern ones, with their computer-controlled idle speeds, would be more likely to handle the extra drag from the alternator gracefully than my first car could.
How does your car start then, I assmue you have the standard 12V battery? If this battery was a bit run down the system voltage might not make 13 till the battery recharges, also during cranking the voltage may be around 10v, so if the F/I doesn’t turn on, no fuel enters, car doesn’t start?
What am I missing?
I’m with Doc Nickel on this. You can argue that some electrical systems won’t put out enough jiuce, but this arguement boils down to “poorer designs will fail sooner”. Duh.
So how 'bout we stipulate to a generic “well designed” engine, including computer controls to maximize efficiency and engine life under a variety of conditions, including idling.
What we would need to look at is the mean time between failure rate of critical engine parrts. And answer other questions like “Are the A/C, lights, stereo, heat, defroster, etc, or or off as the car idles”. What load is being imposed?
My WAG is some kind of belt failure. Months or years.
Given that our original poster mentioned an SUV, it’s fair to assume that the vehicle posited in the OP has a medium to large, to possibly XXXL engine. Based on the information you just shared, we SHOULD be fine on voltage in this experiment.
My car has a 350 cubic inch chevy engine, and the same 140 AMP generator the police package that year had. When I start the thing, the voltmeter widget goes pretty far into “charging up” territory.
I think it’s time we all sent a mass e-mail to the guys at Mythbusters. Just tell them, “There’s a guy in Illinois who’s been led to believe that his 2001 Chevy Tracker will idle for twenty years so long as it has a constant supply of fuel. Prove him wrong!”
Of course, I doubt that Mythbusters has $10,000 in the budget for a 2001 Chevy Tracker, but maybe they’ll kick out $900 for a 1989 Crown Vic and extrapolate the results…
I e-mailed Tom & Tray, the Click & Clack brothers, about this topic (and this thread). I await results.
It wouldn’t just be the $900. Based on our lowest estimate (mine), excluding the electrical posting, you’re talking 140 days, or $5540 worth of gasoline alone. Nonetheless, if I had the greenbacks, I’d DO that experiment.
Man them Fords must have some crappy alternators. According to the graph on page 2 of your link at 1,000 RPM both the stock and the “High Performance” alternatiors had ZERO output. Either their graphs are shit, or there alternators are. On the cars that I teach on I have observed 50A output at idle. Idle speed on our cars is 750-800 RPM
Since a fully charged battery puts out 12.6 volts how do you ever get the car to start? 
Every FI system I have every looked at has a shutdown voltage of about 9 volts or so. During cranking battery voltage can drop as low as 9.6V @70F on a good (but not great) battery.
Answered above.
Alternators are overdriven, i.e. they spin faster than the crankshaft. No reasonably well designed car will drain the battery at idle except MAYBE if every single accessory is turned on.
My point exactly. If you check the graph on page 2 of link P. Nym poosted it shows 0 output at 1,000 RPM.
Had to track down an electrical fault in my 110 ci diesel fiesta last year, where the battery would drain overnight. I used a clamp meter and found a charging current of around 50A at idle. Unfortunately, the alternator drew 3A from the battery when it wasn’t spinning, which explained my problem.
Charging current of the new alternator was around 30A at idle.