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What is the amperage of a typical car alternator when charging the car battery, and how long would the alternator take to charge it (let's take a 60Ah battery as an example)?

I have seen multiple questions on this, but no definite answer, especially none mentioning any kind of authoritative source.

  • Some say most alternators can charge a battery very quickly because they can output > 80 A.

  • Others say alternators cannot sustain that amperage for long and, regardless, it would be harmful for the car battery to charge it at 80A.

If the latter, which seems reasonable, is true, then is it fair to say that an alternator isn't really faster than a typical Noco, Optimate, Ctek, Black & Decker charger, which charges in the 4 to 8 A range? If so, is it therefore wrong to say that driving for 20 minutes or so would be sufficient to recharge an almost flat battery?

[editing to clarify, as suggested in the comments] In other words, let's say that a 4A charger charges a 60Ah battery which is half full (12.2V, right?) in about 7-8 hours. Can an alternator charge the same battery in much less time? Don't batteries suffer if you recharge them too quickly at too high an amperage? To put it another way, the question is twofold:

  1. max amperage of the alternator when charging the car battery, and
  2. (related) max amperage at which the car battery can be charged; I mean, if a car battery shouldn't be charged at more than 7A with a mains charger, then it shouldn't be charged at more than 7A by the alternator, either, right?

I appreciate that exact details will change from car to car, but I am looking for some high-level indication.

PS Note I am not talking about charging a fully depleted / deeply discharged battery.

Pythonista anonymous
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  • This question is too broad. Alternator amp output varies widely. Alternators are not meant to charge a depleted battery and can damage the alternator or cook the battery, but are meant to restore full charge to the battery after starting the car and to run accessories while the engine is running. – Moab Apr 16 '20 at 00:13
  • Many alternators are now controlled by the engine ecu and have very detailed charging regimes, also the ecu will shut down the alternator totally under certain conditions. The only way to find out what your alternator does is read the workshop manual and it won’t tell you everything as it is basically a build or test guide. – Solar Mike Apr 16 '20 at 05:44
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    It varies - on a summer day most of the alternator power may go to the battery as lights, heater, wipers etc are not needed, but in winter, with lights, heater, wipers, heated screens then all or most of the alternator power is not going to the battery. Also, edit your original question to improve it, don’t use comments in lieu of editing. – Solar Mike Apr 16 '20 at 07:01
  • FIrst, alternator output depends on engine RPM. At ilde, an alt can only output about 25-30 amps max, regardless of it's max rating. Low and slow is the best way to recharge a battery to obtain maximum battery life. But if a battery is only partially discharged after a few weeks of sitting or lots of short trips, then a 15-20 highway speed drive will easily bring it back up to charge. If the battery is dead, that's a whole different animal. Then you're back to low and slow. An alt should never be used to recharge a dead battery. Great way to burn it up. – user9181 Apr 16 '20 at 14:02
  • What confuses me is that I have always heard that batteries should not be charged at too high an amperage, so typically should be charged between 4 and 7A. But then it is safe if an alternator charges them at much higher A? I don't understand. – Pythonista anonymous Apr 16 '20 at 14:04
  • You may want to check this answer. Your question is very broad, as has been stated. Please edit it with more clarity. – Pᴀᴜʟsᴛᴇʀ2 Apr 16 '20 at 14:11
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    This might help, https://batteryuniversity.com/learn/archive/what_causes_car_batteries_to_fail – user38183 Apr 17 '20 at 06:39
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    Followed by https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery – user38183 Apr 17 '20 at 06:54

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