Alternator's rate of charge on car battery?

The message from Bill Woods contains these words:

My big heavy diesel recharged itself from dead flat (not even a click from the relays) to quite happy to manage a frosty morning in about ten minutes.

Ok, if you assume that your alternator will be putting out most of its rated output (unlikely but just for example), then in 15 minutes you will have replaced about a quarter of your batteries capacity. To fully recharge would theoretically take about an hour. But every start doesn't take much of the capacity (especially with a modern car in reasonable nick) say 100Amps for 5 seconds. So even on a quarter charge you could get lots of starts, using the figures I quoted up to 90 starts.

Given that you won't have put that much back obviously you won't get that many starts but you can see why you can start easily.

So we're probably talking 10A load? If that flattens the battery in an hour, battery is less than 10Ahr capacity remaining, which means it's quite close to end of life anyway. Secondly, car batteries are not deep discharge types, and by flattening it, you will have further punished it.

Many years ago, I had a Mini and fitted an ammeter to it when I was having some battery trouble. The Lucus alternator on that Mini could charge the battery at 38A if it was low, although this drops off as it charges up (I doubt a battery would survive charging at that sort of current for any length of time, but the current will drop naturally as the battery voltage rises).

A battery ammeter will show you that for a first time start, the alternator has replenished the charge by the time you have driven about 200 yards down the road. Such a start really doesn't use much from the battery.

It can probably put all that into the battery if there's nothing else drawing any significant load. Given your battery would seem to be less than 10Ahr capacity remaining, it would fully charge in ~20 minutes. It would probably charge enough to start the car again in just a few minutes.

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Depends. Alternators are usually governed by a voltage regulator. The lower the charge the higher the charge. This is or was measured by resistance of the battery. As the resistance climbs the charger backs off until it is trickle charging. Also speed is a factor. I do not know the design specs on your car. I am pretty certain that if you drove 20 mph for 20 minutes you would get less of an charge than if you were doing 55 mph for the same time frame.

If your alternator can put out 55 amps then, in theory, it will recharge your flat battery in 45/55 hours or about 49 minutes. In practice it may be limited to somewhat less than that so perhaps an hour to recharge. You ran it for 15 to 20 minutes so it had about a quarter charge when you got home. After a flat battery it is best to take it for a medium speed drive for about an hour.

Or even better for the battery, hook it up to a small 4amp charger for about

10-18 hours.

Andy

I just said that. Still nice to know we agree.

Hi Rusty, I am the OP.

Presumably the figure in the spec for the alternator assumes a certain rpm (which wasn't listed in my Haynes manual).

I would guess that the spec for the alternator is at its MAX output and that would need to be at a relatively high number of revs.

I probably never came close to that value when driving for 20 mins!

Some minimum RPM, which would be higher than a normal idle for a warm engine. It might, however, be about the same as the fast idle for a cold engine; hence, if you cold start an engine and let it run for a couple minutes before driving off... the battery is nearly fully recharged before you are even moving!

It is indeed the maximum output of the alternator, but it probably does not require a "relatively high rpm" to achieve it. "Relatively low rpm" is probably a better description in fact!

Also that maximum output is *far* more than it will ever be allowed to dump directly into even a totally discharged battery for more than a few seconds. It can put out that much... when all the lights are on, the horn is blasting, *and* the battery is accepting a high charge rate.

Probably the battery itself will never take more than about 20 Amps, and even that would only be for a short time.

My guess is that in 20 minutes of driving you would get about

19.9 minutes of charge at the highest rate the regulator would allow! At a stop sign, with your foot on the brake, the engine will probably have too low an rpm to generate full output. With a 55 Amp alternator you might not be charging the battery if your headlights are on. If the lights are off, almost certainly some charge will be going to the battery.

Even without gauges you can pretty much verify what engine rpm provides some minimum output from the alternator! At night, with a warm engine, park your vehicle in a dark area with your headlights on and illuminating a wall or other object that allows you do easily judge the brightness of the headlights. Leave the transmission in drive, put your left foot on the brake, and allow the engine to drop to idle rpm. Then use your right foot on the gas pedal to very slowly increase the engine rpm. If the lights get brighter, the alternator was not producing enough output to charge the battery. At the point where increasing the rpm no longer increases the intensity of the lights, the regulator has taken over and is limiting the output from the alternator. At that point the battery *and* the lights are both getting all the current they can.

Generally, if you notice your headlights get dim every time you bring the vehicle to a stop, it means your alternator can't charge the battery at idle rpm. With a fully charged battery (which then requires very little charge to maintain) you might not even be able to notice the lights get dim at idle, which means the alternator is capable of supplying at least the current required for the lights, even at idle speeds.

What about the heating effect from both I^2R and being in the engine compartment. Part of the 'decent charge' may be the temperature change of the battery.