Bought a replacement alternator but I am sending it back as I measured and have seen it running over 16.5V. The manual says the regulator should keep it to a max of about 14.5V.

I put a brand new and fully charged battery in at the same time the alternator was changed, so I don't think the battery can be blamed. Also at the time I tested it, was running as little electrical load as possible, no lights, fan off, radio off, etc.

The seller has accepted a return on the alternator, but also sent me this advice:

"if you fitted this alternator to a vehicle with a flat or faulty battery it will perform eratically . most alternators are engineered to charge up to 18volts for short periods of time to replenish low battery voltage or cope with high electrical loads"

I am not sure I believe this, is it true?

  • I think the alternator is shooting up over 16V, and this is causing the ECUs to protect themselves and reset - dashboard lights all go off and the power steering stops working. Its an intermittent fault, at first I thought a loose wire. Commented Feb 19, 2017 at 23:03
  • @Paulster2 is exactly right, and his answer is the info you need. I'm just curious about the behavior of the faulty alternator. How much did the voltage vary with RPM? The 18V advice from the seller, sounds very fishy. Did he define what short periods of time means? Batteries have a maximum charge rate, above which you start to do things like boil the acid inside. DONT allow that to happen. Pushing the charging voltage to 18V seems like flirting with that problem. Not to mention the damage electronics in the car would take when run at 18V.
    – cdunn
    Commented Feb 20, 2017 at 14:10
  • It doesn't vary with RPM much at all, except occasionally shooting up over 16V. Weird behaviour, I can only assume something is loose inside the regulator, or the regulator is starting to break down. Only reason I asked on here is because maybe there is some type of alternator/regulator setup that goes up to 18V that I have never heard of before. As far as I know a regulator should provide max 14.5V and that is it. Commented Feb 20, 2017 at 19:35

3 Answers 3


I won't say a good alternator will "never" charge at 16.5vdc, but keeping it there for any length of time will boil the battery very quickly. Your line of thinking is correct in that a good alternator should be charging in the 14.1-14.5vdc as long as there's no major load on it. If there is, the voltage will be down some from this. When it starts continuously charging higher than this is when you start having issues.


It depends on the car and the age. You don't specify make and model, but on Jaguars the charge control is matched to the "newer" type of battery which is calcium based and the voltage can be higher 15.5v during parts of the charging cycle. Older charging limits varied between 14.2 to 14.8 volts depending on the state of charge and if the battery had just delivered a load i.e. starting the engine.

  • Its a Fiat Sedici - about 10 years old too. Interesting though what you are saying about a newer type of calcium battery. So there are regulators that give higher voltages but this is a relatively recent thing? Commented Feb 21, 2017 at 14:08
  • Well, my car is 12 years old, has a new battery fitted 2 months ago and has the "newer" charging system as it charges between 14.4 and 15.1 volts - led display checked with my fluke meter.
    – Solar Mike
    Commented Feb 21, 2017 at 15:15
  • But its a Jaguar? Nice car. Commented Feb 21, 2017 at 16:58

While the answer by Paulster2 is good, I have something to add. It matters where the voltage is measured! If you measure the voltage close to the alternator, it can be greater than when measured in the battery terminals. The reason is Ohmic voltage loss. To save costs, wires in cars are typically thin and the voltage loss can be great if there's a lot of electrical load. Good alternators measure the voltage at the battery terminals and use whatever voltage at the alternator that is required to obtain good voltage at the battery terminals.

So, as a summary, you should be measuring the voltage at the battery terminals. There the voltage should be between 14.1 VDC and 14.5 VDC. If I saw a voltage of 16.5 VDC at the battery terminals, I would return the alternator.

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