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Basics of testing a battery and alternator

  • 04-06-2020 12:16pm
    #1
    BnB
    Registered Users, Registered Users 2 Posts: 1,091 ✭✭✭


    Hi. These questions are probably basic for here but I would just like to understand it as I have a few older vehicles and have never understood this properly.

    I have some 24V and some 12V batteries (Cars and jeeps & tractors) and as they are older vehicles they sometimes give trouble.

    I have a Multi meter and a plug-in battery charger.

    Generally, when a battery is giving trouble, I am looking to find out if:

    a) The battery is just goosed and needs to be changed
    or
    b) The battery is fine but not charging when the vehicle is running so the alternator is giving trouble

    In general my questions are :

    For the battery, what voltage reading should a fully charged 12 V or 24 V battery have and how much variance away from that is still kind of ok ?

    Will a battery that is goosed still charge fully (but lose the charge) . Or to put it another way, with the tools above (Multimetre and charger) and the battery out of the vehicle, what process would I follow to test a battery to see if it is goosed.

    With the vehicle running, what Voltage reading should I get at the battery terminals (with 12 & 24V Battery) to test that the alternator is running as expected.

    Kind of a follow on from above, with the vehicle running would a goosed battery give the same reading as a good battery.

    Any other tips/do's/don't for checking a battery and alternator.

    Thanks in advance


Comments

  • #2
    mk7r
    Registered Users, Registered Users 2 Posts: 839 ✭✭✭mk7r


    There is no real way to test a battery with a multimeter and charger. Ideally you would have a battery impedance tester or even the older load tester. A battery can charge up fine and look fully charged but not be able to deliver enough amps which is what you really need from a battery.
    In general if a battery has been flattened for any period of time it wouldn't recover too well.

    *If you want a quick and simple test for the alternator then check is it outputting 14V+ when running. The battery health wont affect this so if its only reading 12 ish V then it's an alternator problem.

    *There are exceptions to this with more modern alternators employing smart charge systems that range from 12-16V depending.


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