It would appear that the common word of wisdom is that car alternators provide 50A of output. I actually looked at the supposed specs for my 2008 Jetta 2.5L on the internet -- a compact car -- and it would appear that it might be rated for 140A 14V (in ideal conditions?), which is a lot of power -- 1960 watts!

  • How much of this 140A ends up charging the 12V 61 Ah 330A DIN battery of the car?

  • Is there a way to determine whether the battery is getting a sufficient charge from the alternator in various operating conditions?

For example, it would appear that my alternator has passed the AutoMeter BVA-230 test at O'Reilly Auto Parts just fine, same for my 6-year-old OEM battery (which within the subsequent week has refused to start the engine after an overnight at 7°C).

However, after I pressed them for what's going on (during the above visit where everything has passed all of their tests and they were ready to leave me alone, assuring me that I have absolutely nothing to worry about), and asked for a re-test with both of the heated seats turned to the max, and same for AC, with the car at idle, they did immediately say that my alternator is failing several of their tests and I must replace it ASAP; when I explained to them that it's not supposed to power so much stuff at idle, and asked for a third re-test at higher RPM, it seems like it did again pass the voltage test now, but supposedly still failed a test called "diodes".

In manual testing with the 12V car voltmeter, it does appear that I have about 14,1V to 14,3V at idle with everything off for a few minutes after starting the car, but if I do turn both seats and the AC on, then it goes down to maybe as low as 13,1V or so.

  • At such 13,1V at 600 RPM idle with lots of stuff being on, is the battery still charged, left neutral, or is it actually being discharged?

  • Does it help to rev the engine (maybe to 2k RPM?) prior to turning the key off, if it was left idle at 0,6k RPM for more than 10 minutes, with all the accessories being on? How long should one rev it for and to how high an RPM? What's better -- revving the engine for a while, or simply turning off all accessories, and not revving?

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    You're over thinking it: a 6 year old battery is probably ready to be recycled. And, deliberately turning on as much as possible, to "stress test" your charging system, is just irresponsible. Your car's charging system is designed to "top off" the battery, and run a reasonable amount of accessories, wile you drive - NOT to be a full on battery charger. A properly working and properly used system, will normally be around 14 volts at the meter (while running) - any higher or lower, indicates a problem, or too much load.
    – tahwos
    Dec 9, 2014 at 2:49
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    Further more, changing test conditions, to simulate a fault, does not indicate a fault - it just negates any results from that test.
    – tahwos
    Dec 9, 2014 at 2:59
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    @tahwos How is this even a stress test? The charging system should easily handle all the accessories turned on at once. If it stopped doing it's job whenever you turned on heated seats, heated rear window and the AC at the same time then the people in colder climates would be quite pissed. Aug 18, 2016 at 8:26
  • @IhavenoideawhatI'mdoing, I think the idea is that the batteries are generally of sufficient capacities such that idling with all accessories turned on is not supposed to leave much extra juice for still charging the battery itself; if anything, they could in such situation be discharged slightly, which will be compensated once you take off, and have the increased RPM to cover all the load+charging.
    – cnst
    Aug 19, 2016 at 3:54
  • 2
    a stress test is not irresponsible, it's better than stalling out unexpectedly with a flat battery while waiting at a traffic light at night in heavy rain, BTDT.
    – Jasen
    Mar 9, 2021 at 4:49

4 Answers 4


On older vehicles the alternator would give out a high rate until it became warm. This used to be known as its saturation point. The alternator output would then drop off to supply the demand put on it plus maybe a couple of amps. A later 'smart' alternator and its control system can carry out conductive tests while operating and will better re-charge the battery. Never the less the continuous charging and discharging of the battery will eventually sulphate its plates and diminish its performance. If you wanted to you could buy an 'amp clamp' and follow the charging rate without disconnecting any cables. Or charge the battery at around 1 amp, then discharge the battery fully with a 1.2 watt bulb across it. Repeat this until the battery is de-sulphated. The more usual and easier option is to simply replace the battery regardless on its third birthday, especially if the vehicle is used for critical operations. In any event any alternator will only give out its maximum rating under a full stress test.

  • Using an "amp clamp" tool over an insulated wire sounds like a good suggestion and idea, however, with my battery being so old, and my alternator supposedly past its prime, too, there's really no way to know what the specs are supposed to be. What's the optimum RPM for the alternator? How fast is the battery supposed to be charged within a compact car like '08 Jetta?
    – cnst
    Dec 9, 2014 at 16:05
  • About 750 engine RPM is a usual cutting in speed (turning it on). Dec 9, 2014 at 21:01

Alternator charges the battery at a regulated 5 A. It charges at a much lower rate than specified on the batteries usually 1/10 of their capacity. This is because charging at full pin would overheat the battery, generate high levels of hydrogen gas and dramatically reduce the longevity of the battery.

61 A/h battery at 5A charge rate would fully charge from dead in 12.2 hours. The alternator is capable of generating 1.96 KW. If it did all day both the alternator and regulator would probably self destruct. The battery only purpose is to start the car and run accessories when the engine is not running. Once started the alternator runs the whole show.

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    Where are you getting this "5 A/h" figure? Any number followed by "A/h" is not a charge rate at all! (Nor if it's followed by "Ah") Mar 29, 2017 at 0:15
  • That’s exactly what I saw in my measurements and I thought I was wrong. The bmw alternator is rated at ~150A and I see it generating ~38A at idle according to the diagnostics module but when I actually measure the current following into the battery (both at idle and when it’s revved up) using a multimeter it only shows about ~5A (negative indicating charging)
    – rboy
    Oct 14, 2021 at 21:04

People need to realize alternators have to charge batteries at a low amperage or the battery would prematurely fail. So 5 or 6 amps is a good estimate.

  • Welcome to Motor Vehicle Maintenance & Repair! I'm not exactly tracking with you here. Could you provide reference to better explain your hypothesis? Nov 10, 2017 at 23:44

TL;DR: The alternator charges the battery as fast as the battery will let it, or as fast as the alternator can, whichever is lower, at a constant voltage (usually 13.8v, or 14.2v). The terminal voltage going down to 13.1v suggests the battery is not charging, and may be discharging, unless the battery is very low. The alternator or the battery is probably in poor condition.

The alternator will charge the battery at a constant voltage (usually 13.8, or 14.2), and electively never a constant current. The amount of current that goes to the battery will steadily naturally decrease as the battery charges. Immediately after starting the car it may charge at a high rate, like 50 amps, and then quickly go lower, like 5-10 amps, and eventually very low, like below 1 amp, as the battery is charged. The voltage should remain about the same at all times.

Battery university has an article that describes the voltage and current as a lead acid battery is charged. The voltages it shows are based on one cell so you’ll have to multiply by 6 for a 12v battery. Amperage is dependant on the capacity of the battery. https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

With all the car’s accessories off the voltage at the battery terminals should be between 13.7v and 14.3v within a minute of starting the car. The voltage at the terminals should remain about the same with all the accessories on. If the voltage stays at or below 13.7v when the accessories are on and the engine is at idle, then it’s likely that either the alternator or the battery is in poor condition.

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    headlights are like 15A load, wipers another 20A windows heater another 15, seat warmers , cooling fan(s) power seering... it all adds up quickly.
    – Jasen
    Mar 9, 2021 at 4:55
  • Yes, the accessory loads do add up quickly. The 140A rating the OP noted doesn't seem unreasonable for an alternator. Most of that will be used on accessories, not the battery. The alternator must be able to accommodate all the accessories and charge the battery.
    – timeSmith
    Mar 9, 2021 at 19:04

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