Will running engine stationary charge up battery?

Marginal, at best, in my experience.

Take it for a 20-30 minute run down a motorway. That'll do most of it.

I.

The message from Dan Tanner contains these words:

Yes, slowly, but it's not a nice thing to do to the engine. Get a battery charger.

Just idling is not enough to charge a battery properly. You would need to increase the revs to about 1500rpm, otherwise most of the current generated is used up just keeping the spark going (and other "always-on" elctrical stuff). It doesn't do a battery any good to charge it too fast, which is why battery chargers take several hours.

A car battery's capacity is rated in amp/hours. A small battery may have a rating of 40AH, which means, in theory, it would take 40 hours to charge with a 1amp current, 20 hours with 2 amps, etc. But although a modern alternator will have a current rating in excess of 100 amps (for running lamps, screeen heaters, etc), it won't charge a battery at that rate, and to do so would wreck the battery.

The best option is to use a charger overnight, and next best is to take it for a drive, out of town, in the daytime so that the lights aren't needed. Running at a fast idle will work as you suggest, but it's not very efficient.

That is a very good question. I hope someone with some real experience of this comes along with a factually based answer, not just a bunch of hearsay, guesswork and bollocks that these dongers have posted on this thread so far. :)

The message from Chris Bolus contains these words:

That was certainly true with dynamos, but all the alternator equipped cars I've ever bothered to test have always provided considerable charging current at idle. Not as much as at higher revs, but at least 10 amps beyond what the car was using.

OK, try this simple test. Take a car with a flat (but good condition) battery, jump or bump start it and leave it ticking over. In five minutes it will be charged enough that you can stop the engine and the battery will start it. This of course assumes that the alternator is ok etc, ie that the flat battery was the only problem.

And yes, a battery charger would have been more efficient, and yes a 30 minute drive would give it a nearer to full charge.

The answers to the 2 original questions are -

Yes,

and

It depends.

Steve

Ford Focus, idle with a partially discharged battery (60% remaining according to a sg test) charges at a rate of 2.5A according to my clamp meter.

Bring it up to 1500rpm and that went up to 12A.

Most cars won't charge much at idle. Switch on anything like lights, fans or other hotel power stuff and it can get close to discharging....

The message from "shazzbat" contains these words:

Though of course partly that's 'cos it's now got a nice warm engine!

But then, anybody who's stupid enough to do that when they're trying to get over a flat battery deserves everything they get.

Steve

VW Toureg has a two-speed electrically controlled gearbox in the drive to the alternator. If the ECU sees a battery discharge situation at low engine revs it will change alternator gearing to keep the charge high.

The alternator is rated at 190A continuous and 300A short term. It is also water cooled! As some variants have *3* batteries, I guess it's needed.

Chris

DC clamp meter? Where's that from then? I want one!

Google DC clamp meter for hits....

If a modern car with an alternator - yes it will charge the battery almost as effectively on idle as when moving. Older cars with DC generators generate very little current at idle (which is why Alternators were invented as more and more electrical power was needed).

If you are charging a flat battery - you'll need to keep going for a loooooong time - get a battery charger instead and charge it overnight. That way you won't use lots of expensive petrol, and won't have your car sitting there with the engine running waiting for someone to jump in and drive away!.

Not to mention it being illegal to leave your car engine running on the road while the vehicle is unattended.

... that's 'cos its an invite to thieves, apart from the safety aspect!

Yes.

Depends on how long it's been standing, especially if it has things which consume current with the engine off, as most modern cars do.

I've got a little but accurate LCD voltmeter fitted to the SDI, and that reads the same at start up (slightly fast idle) as at any other RPM. Due to the way the regulators work, they don't tend to 'fast charge' a flat battery. I doubt they ever exceed about a 30 amp actual charging rate.

So your household mains was DC in the '70s? ;-)

Car alternators became viable with cheap solid state components. And they're cheaper to make than an equivalent output dynamo and controller.

The main advantage is that they can be run at a much higher speed, since they have no high current rotating switch in the form of a commutator.