I’m trying to use a car alternator for 12 V bench power source, got it all going and getting 14.5 V with no load but once I apply a load to it the output drops to 8.5 V. Is it a faulty alternator or will it work only if it’s installed to a car battery? Or it may not make any difference with or without a battery? As anyone done this and be willing to help with suggestions? Advanced thanks for suggestions

  • 6
    How are you turning the alternator to generate your power ?
    – Criggie
    May 22, 2022 at 19:04
  • How big is the load you're applying?
    – marcelm
    May 23, 2022 at 5:56
  • The keyword I suspect to be your issue is external magnetization current due to most car alternators are not permanent magnet. Can you draw a schematic or block diagram of how you connected everything?
    – winny
    May 23, 2022 at 10:10
  • What's your use-case? You can buy industrial power supplies that will from mains (110-230V), produce 12V at 480W. This is probably far more than the alternator will supply (its about 1HP load)
    – CSM
    May 23, 2022 at 18:38

2 Answers 2


Several features.

Firstly, alternators have a magnetic field that is adjustable by adjusting the amount of current at the field. This causes the output voltage of the alternator to be adjusted.

The regulator that adjusts this magnetic field current may be inside or outside the alternator. Check that you have the regulator present in your setup too.

Secondly, if the regulator is inside the alternator, it may require a sense wire. There are setups that have a separate sense wire that's supposed to be connected directly to the battery terminals. The idea is that the sense wire has a very small current inside it, so the voltage drop is negligible. By the separate sense wire, you are sensing the voltage at the battery, not at the alternator output terminals. If there's plenty of current used in the car, the charging wire between alternator and battery may have significant voltage drop, and thus with a separate sense wire this voltage drop doesn't cause an error.

Thirdly, alternators are supposed to be rotated at some significant RPM. Even when the car engine is idling at 700 RPM, the alternator is probably rotating at 2-3 times that speed. It isn't directly connected to the engine, but rather connected via a belt that multiplies the speed by a factor dependent on the pulley diameter ratio. Also if the car engine is idling, the alternator may not provide enough current for any significant load.

Cars have a battery buffer. Therefore, it is possible that you have an alternator that would slowly cause the battery to deplete if the car idles forever and the load is very high. This isn't an issue, because cars don't idle forever, you rotate the engine at higher speeds occasionally too.

So I would start by

  • ensuring the alternator rotates fast enough
  • connecting a small lead-acid battery at the alternator output terminals
  • ensuring you have the regulator present in your setup
  • ensuring that, if the regulator needs a separate sense wire, that this separate sense wire is connected to the battery too.

If you don't have a battery, it may be tricky to get an output from an alternator at all: the alternator doesn't have permanent magnets but rather electromagnets that require current to function. If you don't have a source for that current, it may not work (try push-starting a car with a fully depleted battery and you see what I mean). It may or may not be the case that some residual magnetism could allow you to boot-strap an alternator even without a 12-volt lead-acid battery. I wouldn't rely on residual magnetism to save you.

  • 3
    The regulator can also be disturbed by the large voltage ripple when there is no battery and not enough capacitance in the circuit.
    – jpa
    May 23, 2022 at 17:14

A battery would make all the difference in the world. The battery does a few things, but first and foremost, when installed in a car, the power is drawn from the battery and the alternator replaces what is taken out. The battery also acts like a buffer, tempering voltage spikes and providing a cleaner power source. The battery also provides a power source to the alternator. The alternator requires a "field" source, which the battery provides. I think the battery will solve your issues.

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