I would calculate not very long.

Typically a starter motor would draw about 400A from the battery. Say it was cranking for 2 seconds would draw 800A.

Typical Alternator output rated at 80A but that would be at full revs, so say about half this on idle speed.

So about 20 seconds in the above example. So not very long, take away load from other electrical items such as headlamps, ignition and ancillary circuits. Might be a bit longer but not much.

Edited by Cris_on_the_gas on 21/05/2021 at 14:06

""The second question related to this might be- how long does an engine have to run so that a battery is brought up to "full charge".

I've seen it stated more than once that car charging systems aren't very good at this, and essentially you never get a battery to full charge just by driving around, and so it is good for it to stick it on an external charger sometimes.

This probably refers to cars around the turn of the century (but well after dynamo's roamed the earth) so may no longer be true, if it ever was.

I dunno how one could confirm or deny offhand, since I don't have instrumentation that would measure state of charge. Certainly whenever I stick a car battery on an external charger the charger finds something to do.

Edited by edlithgow on 21/05/2021 at 15:52

I'd interpret that as tending to confirm that even (or especially?) modern car charging systems aren't very good at maintaining charge, especially with sub-optimal driving patterns post-exploding-cigar-disease-apocalypse

(All gone pear-shaped suddenly here too. Funny human-frailty story with lots of sex).

Seems car charging systems never were very good, and they've got worse because they have compromised basic function to save energy, which most of the time will be the right compromise.

Don't see how you arrive at that conclusion. Had alternators on every car I have bought since 1976 and with a few exceptions (more later) they have been well up to the job. But over that period batteries have improved enormously. In the 70's 3 years was probably the average life of a battery, by the late 80's I had my first battery last over 5 years and its improved a bit more since then, the Micra was nearly 8 years old when we sold it and the battery was still doing its job.

Lucas alternators in the 70's were in truth poor (but way better than a generator). They only charged at 13.7 volts (on a good day) and the bearings were not that great. the only saving grace was every small town had a re conditioner rebuilding them.

I said earlier that the Caterham was fitted with a late 80's Daihatsu Charade GTTi alternator. The Caterham factory fit was Motorola but they were huge units and with space at a premium people only fitted them if they were in the kit. Most (including myself) fitted the slightly smaller Lucas which worked fine but normally for a short period, heat and revs (even with the correct pulley) resulted in premature failure but the flickering dash light normally gave plenty of time to fix before total failure. Eventually I found a Valeo unit off a Transit, smaller than the Lucas, 14.4 volt charge and it took the revs fine for several years, that was until the casing cracked (possibly vibration). So I took the plunge and bought a kit including a new Daihatsu Denso alternator and that lasted 2 drives (about 30 miles). Alternator was exchanged and that lasted 25 miles. Chap refused to change it at first but relented when I said I would be asking Visa to sort, got a full refund.

In 2010 chap up the street was fitting me a set of tyres ween I mentioned the alternator, he used to have Charade GTTi and said he still had some parts and probably the alternator. He walked home and gave it me (I gave him a tenner for the kids once it worked). I made a temporary bracket and bought a suitable drive belt, 14.4 volts and 11 years later the temp brackets are still there and its still producing 14.4 volts.

Some time later I spoke to a chap who worked for Denso at a show and asked why a 30 + year old alternator works just fine and 2 new ones lasted a total of 55 miles. He asked where I bought them from and was told they were Chinese fakes. They had acquired one and the finish on the case was all wrong plus the product label was one they had not used since the early 90's when they dropped the "Nippon" prefix to the brand. He would not confirm if they had taken action and if so against who.

Our recent cars have had smart alternators, stop/start and appropriate batteries and with the exception of the recent comment by the garage about the Fabia needing a charge after 15 months of reduced usage all has been fine. To me that shows there are no real issues.