Does an alternator charge a car battery just idling?

I left the key turned half on yesterday in order to set the clock back. Battery is dead as a doornail today. One more thing I have against this time change nonsense. Would appreciate knowing if I have to drive around for hours once I jump start it.

It’ll charge idling. It will charge faster driving.

If I kill a battery I typically jump it 30 minutes or so before I need to go somewhere, let it idle, drive to my destination. leave it idling there, then drive back.

It can, but you’ll be generating an awful lot of heat and electrical stress in the alternator. Once or twice should be ok but you don’t want to repeatedly task the alternator with charging a flat battery.

Thank you. Just what I needed to know.

It probably doesn’t apply to this OP but I’ll add the following:
It also depends on what other electrical draws are present. For example, it’s not uncommon for traffic to grind to a halt for some hours here during blizzards or after a major accident with road closure. People with the headlights, fans blowing and heated seats on may find themselves with a deal battery since the idling isn’t enough.

Interesting. I’ll keep that in mind if that ever happens to me. Mindful of the advice above, I jumped my battery and went out on the highway for about 20 miles. It started nicely on its own when I got home.

In general not enough, many times idling takes power from the battery , though if you shut off everything it may but it would take lots of time. Alternators are designed for producing power at higher RPM’s and do very little at idle. They do make alternators that produce at low RPM’s but they are typically used in dual alternator setups where one produces at high RPM’s and the other low.

You really should get a battery charger on it, or at the least drive around for hours. Though if you chose the drive for hours, and the battery is old, it may be easier just getting a new battery as running it flat when old is basically going to kill it.

I drive a 2002 Saturn. I have a digital volmeter connected to the car’s cigarette lighter receptacle. As soon as I start the car the voltage goes to 14.3 V. And it stays at 14.3 V… it is at this voltage at idle, and it is at this voltage at 3000 RPM. And this is under full load (headlights, AC, etc.).

So in other words, for my car, the battery will not charge more quickly when I am driving it. It will also not charge more quickly if I turn off all the loads. As long as there’s 14.3 V across my battery, then the battery is charging fine regardless of anything else.

Am I wrong in assuming the same is true for most other cars?

No ,but what does it do when you’re battery is dead?

The voltage should stay fairly constant, but that doesn’t tell you anything about the amount of power the alternator is supplying. The faster the alternator rotates, the more power it can supply (up to the design output).

True. But there are folks on here who seem to be implying that the battery (after simply starting a car w/ good battery) will charge more quickly at driving speeds, and/or if loads are shed.

On this matter, my point is this: if the voltage produced by the alternator is sufficiently high (> 13.5 V) and is constant regardless of RPM and loading, then the battery will charge at the same rate regardless of RPM and loading. I assume this is true for all modern vehicles (and also assuming a good battery and good alternator).

Of course, there’s the special case of an alternator trying to charge a dead battery. As others have pointed out, this could be overly-taxing on the alternator. I am also not sure if an alternator can maintain a sufficiently high voltage when charging a dead battery.

This is not true. Alternators are not like the archaic DC generators from the 1950s; alternators can provide significant power output at low RPM by having the voltage regulator increase the field current. DC generators were hopelessly tied to engine speed and were known to fail to provide any charging current at idle or low speed. That’s why old cars with generators had ammeters, so you could tell which way current was flowing.

Dual alternators are meant for applications like ambulances and vocational trucks where they have to supply power to things like tons of lights, pumps, inverters, etc. they’re not “optimized” for particular RPMs, they just double the available current.

Most older cars and many newer cars (especially the less expensive ones) don’t have that good of voltage regulation. They’ll typically idle at somewhere around 13.5 volts and they’ll go up to 15 volts or so (sometimes even higher) at full engine RPM.

Some newer cars have fairly fancy charging systems, and you may not see the voltage go up much at all at idle unless the car thinks that the battery needs to be charged. On some of these, you have to turn on the headlights to get the alternator to go into charge mode so you can test to see if it is properly working.

The PCM controls the alternator field on modern cars, and the charging strategies can be very complex.

Apart from the alternator question, I suspect that if your battery goes flat in the few minutes it takes to change the clock, it is about to let you down anyway.

Bolding and underlining mine.
I think this is where your understanding gets confused. The charging circuit on modern cars (built in this millennium) is usually computer controlled. That is, by adjusting the current in the stator windings of the alternator, the output voltage is going to be constant. This has been true since they had electronic voltage regulators on alternators. What the computer does is control the current going to the battery so a battery needing charging does not drop the output voltage. This prevents the light dim at idle that was common in cars of last century. To put it another way, your cig lighter voltmeter is not measuring the voltage across the battery, it is measuring the voltage at the cig lighter. The computer is adjusting the current to the battery to maintain the 14.3 system voltage.

As jz78817 mentioned up thread, an external, plug-in-the-wall battery charger is how you should charge a dead battery. Charging a dead battery is very hard on the alternator. In particular, it can kill a brand new alternator.

I find this interesting. It implies the voltage set point is either a function of RPM, or the set point is 15 V and the field current is at 100% at idle (and thus it can’t produce 15 V at idle). I have never heard of the former. And the latter implies a terribly insufficient design.

I scratched my head at the OP also.
So he turned the ignition on to set the clock then left it in that position (leaving the keys in the ignition) overnight?

I am confused. Isn’t the output of the alternator, the battery, and the cigarette lighter all connected in parallel? And if so, how does the electrical system control the current to the battery? I thought the battery was charged by simply providing a regulated voltage to it between 13.5 V and 14.5 V. And the current to the battery was simply a function of battery’s state of discharge. I have never heard of a circuit on a car that “controlled the current to the battery,” but I admit my knowledge on this topic is somewhat dated.

the only control over the charging system is modulating or switching the alternator field current. if the system can’t maintain target voltage at idle, the PCM should open the throttle more to spin the alternator faster. but there’s typically* nothing in the system to limit the current being delivered to a charging battery; it’ll pull as much as it needs within the limit of the available current.

  • hybrids/EVs don’t have alternators, rather a DC-DC converter between the high voltage system and the 12 volt system. Those can control the current delivered to the 12 volt system.