[991PCAR]

Looking at system voltage through the hidden menu, at idle, I'm getting 14.1-14.5V readings, however when driving I see the voltage drop into the 12's. Also, while coasting and in the "efficient dynamics" mode/the blue bar on the mpg gauge, I'm back above 14 Volts. I just replaced and registered my battery and started monitoring the voltage a bit so I don't know if this is normal behavior or if my alternator is not keeping up. Any thoughts very much appreciated. TIA

[Surly73]

Originally Posted by 991PCAR

Looking at system voltage through the hidden menu, at idle, I'm getting 14.1-14.5V readings, however when driving I see the voltage drop into the 12's. Also, while coasting and in the "efficient dynamics" mode/the blue bar on the mpg gauge, I'm back above 14 Volts. I just replaced and registered my battery and started monitoring the voltage a bit so I don't know if this is normal behavior or if my alternator is not keeping up. Any thoughts very much appreciated. TIA

The 12s isn't "normal" but the peaking while coasting (Efficient Dynamics) is. You may want to connect another voltmeter somewhere like the lighter socket (they sell lighter socket voltmeters for a couple of bucks in places) to get a better reading. Vehicle electrical systems are long, branches, and can have different voltage readings all over.

The target voltage depends on many factors including how charges the car believes the battery to be. If it believes the state of charge to be on the low side you'll see 14.8VDC (the max charging voltage for AGM) almost all the time. If it believes the battery is at 100% you'll see low 14s at the highest and fluctuations all over.

So TL;DR - 12.x is low, but fluctuations are normal. I recommend using a better instrument to see if it's really dipping below 13 before becoming concerned.

[Unspec]

IIRC, BMW intentionally keeps the battery at just above "healthy" voltage while driving (12ish volts), and then charges the battery up to 14ish volts while coasting and you see the blue bar in the MPG indicator. Essentially a tiny regenerative braking system. I see the same thing for my car, and it's running on a relatively new battery.

[Surly73]

Originally Posted by Unspec

IIRC, BMW intentionally keeps the battery at just above "healthy" voltage while driving (12ish volts), and then charges the battery up to 14ish volts while coasting and you see the blue bar in the MPG indicator. Essentially a tiny regenerative braking system. I see the same thing for my car, and it's running on a relatively new battery.

I've measured voltage in the 13s, with "charging" voltages at 14.1 and 14.8 (for full beans). I haven't seen 12s in my vehicle whenever I've been watching.

[Unspec]

Originally Posted by Surly73

I've measured voltage in the 13s, with "charging" voltages at 14.1 and 14.8 (for full beans). I haven't seen 12s in my vehicle whenever I've been watching.

I might be remembering the float voltage wrong, it's been a while since I've measured the system. 13s sounds more accurate for AGM type batteries.

[991PCAR]

Thanks for feedback on this. Watched volts a lot closer today throughout my drive to and from work. Basically, it hovers around 13.0 +/- .2 volts when running, except coasting or braking where it goes to 14.7. If it helps to know my battery is brand new and is the 105AH 950 CCA AGM type.

Is this an alternator issue or something else electrical related? Any ideas of things I can check? Or if there is an issue, just drive it until it gets worse/easier to identify...

Thanks again for the input.

[ezaircon4jc]

Originally Posted by 991PCAR

Thanks for feedback on this. Watched volts a lot closer today throughout my drive to and from work. Basically, it hovers around 13.0 +/- .2 volts when running, except coasting or braking where it goes to 14.7. If it helps to know my battery is brand new and is the 105AH 950 CCA AGM type.

Is this an alternator issue or something else electrical related? Any ideas of things I can check? Or if there is an issue, just drive it until it gets worse/easier to identify...

Thanks again for the input.

Read up on BMW's charging protocol. IIRC, the alternator seeks to keeps the charge at around 80%. Something about battery longevity and increased MPG.

[Surly73]

Originally Posted by 991PCAR

Thanks for feedback on this. Watched volts a lot closer today throughout my drive to and from work. Basically, it hovers around 13.0 +/- .2 volts when running, except coasting or braking where it goes to 14.7. If it helps to know my battery is brand new and is the 105AH 950 CCA AGM type.

Is this an alternator issue or something else electrical related? Any ideas of things I can check? Or if there is an issue, just drive it until it gets worse/easier to identify...

Thanks again for the input.

I don't think you have an issue. Variance is by design and 13.x is "OK" when the battery is fully charged. Do short trips for a week or two and you may see 14.7 all the time - also by design.