My car battery 45ah before engine start gives 12.6 Vol which means it is fully charged. Now, engine crank and the multimeter reading shows 14.4 v and continue to stay which simply implies that alternator is perfectly working and charging the battery. These data are ok.

The fact that when battery attains it's full capacity 12.6 volt, the alternator will stops charging it. In the present case, how long will the alternator outputs 14.4 v and stop charging the battery? Is current still flowing to the battery?? I'm confused this particular situation.


  • When the system voltage is 14.4V how much current is the battery getting?
    – Solar Mike
    Sep 1, 2019 at 10:23
  • I don't know how to find out that current!!
    – Thang Tons
    Sep 1, 2019 at 10:32
  • Does your multimeter have current settings? Do you have a shunt?
    – Solar Mike
    Sep 1, 2019 at 10:55
  • No shunt. It's just basic multimeter.
    – Thang Tons
    Sep 1, 2019 at 10:58
  • Would you know what to do with a shunt?
    – Solar Mike
    Sep 1, 2019 at 10:58

1 Answer 1


Here's an example how charging system works on a car with combustion engine. Power from the alternator goes to the battery and then goes to the car systems. Alternator is connected to the battery all the time and it's charging it all the time when engine is on almost all cars. See the scheme below.

There's a special voltage regulator (modern cars have it built in into the alternator, old cars have it as a separate block) or ECU controls how much energy alternator should output by changing the output voltage. It checks the load (the load increases when you turn on high beam, stereo, window heaters all the lights, etc.) and „says“ to the alternator should it output more or less.

Some conditions may also affect alternator output. When hot thermal protection decreases alternator power output, when cold power output will be increased. On high RPMs alternator produces more power, on low RPMs it produces less.

So basically when the engine is off the voltage should be somewhere around 12.5 V (12-13 V). When the engine is on the voltage should be above 13 V and below 15.5 V. In some conditions even when the engine is on the voltage may drop below 13 V (really hot day, both A/C are on, stereo is on, lights are on, you're charging your laptop \ camera etc, you're driving really slooowly in a traffic jam).

So, low voltage when the engine is on isn't a good sign. If you have 12.5 V when driving - something is wrong. Also high voltage when the engine is on isn't a good sign. If you have more then 15.5 volts it's better to check your alternator.

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  • Some cars have the alternator output controlled by the Engine Control Unit ECU, and under some conditions will exceed 15.2V.
    – Solar Mike
    Sep 3, 2019 at 16:26
  • @solarMike agreed, I'll edit my answer now.
    – JimJohh
    Sep 3, 2019 at 16:30

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