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My (VW Polo SE TSI Mk5, 2015, 1.2, bought secondhand) car just had a new battery fitted (EFB for start-stop cars) and on checking the battery voltage with a multimeter at idle (engine on, car stationary) it comes up to about 13.1, occasionally 13.2. Revving the car bumps it up a bit. I worry if I don't have an issue with the alternator, as reading up on it online it says at idle it should be about 13.5, so wondering if the alternator is struggling to charge it.

Is there a canonical reference for my type of car as to what the voltage should be in these circumstances? Anything else I can do to settle the issue of whether there is a problem with the alternator?

Edit: Could the lower voltage on the new battery be down to the fact that I might have a smart alternator and since the battery is new, it doesn't drive as high a voltage as it would on an older battery?

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    Welcome to Motor Vehicle Maintenance & Repair! Sep 18, 2023 at 14:52
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    What is the battery voltage when the engine is not running? And how good is your multimeter (does it have a fresh battery in it)?
    – Jon Custer
    Sep 18, 2023 at 16:43
  • @JonCuster That was the mobile mechanic's multimeter reading, but you gave me an idea, I just took my multimeter out and tested it and got a much more encouraging reading of 13.41 when idle (and 12.6 when not running), so I reckon his multimeter's battery might have been old. I might borrow a decent multimeter from a neighbour as mine is crap and see if I can get a decent, accurate reading there.
    – Nobilis
    Sep 18, 2023 at 17:10
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    Sounds about right. Revving the car bumps it up a bit. Perhaps up to about 16 volts? Is there any other evidence that the battery is poor - like difficulty cranking the engine, or not holding a charge, or not being charged? What exactly is your concern? The alternator doesn't supply much charge when idling (it's driven by the engine, which is idling). It charges when under power – being driven. Sep 18, 2023 at 21:56
  • @WeatherVane My main concern is that voltages just seem far too low for what I expect I should be seeing, so is it possible there's a bit of an issue with the alternator? I took the car for a 20 minute drive on a fast road tonight - no problems starting or driving, but the voltages seemed a little on the low side even after the drive - around 12.8 when not running and 13.4 when idling the engine, revving it at most gets it up to 13.9. The battery was only put today, so perhaps not fully charged, hence the lower voltage?
    – Nobilis
    Sep 18, 2023 at 22:19

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