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Discussion Starter #1
A couple of weeks ago, my car wouldn't turn on in the morning, I swapped out the battery with a new one, and the car has been running without problems.

At the time I swapped the battery, I measured 12.7 volts between the terminals on the battery (when the car is turned off), and 14.3 volts when running.

Now I have measured 12.7 volts between the terminals (when the car is turned off), but only 13.0-13.1 volts when the car is running.
I started thinking about if it was the alternator, I've made the following measurements:
- Negative terminal on battery -> alternator case = 13.1 volts
- Positive terminal on battery -> alternator post nut = 13.1 volts

But shouldn't the measurements be higher when the car is running between the terminals on the battery?
And shouldn't the measurements be around 0.05 volts between the battery and alternator? Could this be a sign of grounding-issue?
 

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Those measurements don't make sense to me, but I might not have understood exactly where you are taking those measurements.

How many miles has the car done? I would suspeect the alternator is not doing too well and would suspect worn brushes on the slip rings, or the voltage regulator, but that's less likely.
 

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Discussion Starter #3
Which of the measurements don't make sense to you, or is it all of them?

The car has done just around 145000 miles, so it could be the alternator.
But when I made the measurement after the battery-swap, it apeared to be charging the battery, but now it doesn't anymore.
 

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Hi...I think the first reading 14.4 volts was a good alternator struggling to charge a bad battery....the next reading.13.1 is lower as the new battery is holding a good charge.Perhaps a test with the battery under load (lights ect turned on)and test again..I think the alternator is ecu controled
 
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