Hi all,

What should the battery voltage be when the car is running? I took my 02 MC to my mechanics the other day after I installed a new battery. He told me the voltage reading was around 13.8V to 13.9V when car was on. Does that sound normal? Someone else told me it should have been always above 14V... does that mean my alternator is not putting out enough voltage to charge the battery (weak alternator?)?

Please advise...

Thanks,

Ron

 

did you have stuff on, headlights, a/c, anything that might draw from the alt, if you did that might account for the 13.8-9 typicaly if I remember right its not much over 14v some think like 14.3 and under hope that helps

 

The voltage depends somewhat on temperature. 13.7-13.9 is normal in hot weather. Temps in the 50s will usually get you voltages in the low-mid 14s. This is normal, and it a function of the voltage regulator adjusting for ambient temps. Since the battery does not need as high a voltage to maintain a full charge in higher temps, the regulator scales the output back to prevent overcharging. The opposite is true in cold temps, as the battery needs a higher voltage for a complete charge.

Take readings using a voltmeter directly on the battery terminals. If using a scan tool to read the voltages via OBDII, the readings tend to be a couple tenths lower I've found.

 

I don't think I had anything on at that time... I had Daytime Running Light programmed but then I pulled my handbrake at that time so the light should be off. Since you mentioned a/c, I have another question: when I had a/c on and sat in traffic, the fan (high speed fan) would come on every 30secs and whenever it comes on, the rev would drop a little and go back up... is that related to alternator?

Please let me know...

Thanks,
Ron

 

It was quite hot (80s) with a little bit of humidity that day and I remember the bonnet was quite warm. My mechanics took the readings directly from the battery terminals using a voltmeter so it should be fairly accurate. So the voltage seems OK with that weather condition, right?

 

I'd say entirely normal, especially if that is the voltage while a good number of accessories are on. Put the headlights on high beam, run the a/c, this will put a sizeable load on the alternator. if you still have a solid 13.8 and it's pretty warm out, you are good to go.