2007 730d - voltage issue

[Blvck7]

Summer was ok, but when cold weathers came - I noticed that's something not right with the voltage.

When I start the car, battery voltage drops to 10.9-11.5V and after like 15-20mins engine tone changes a little, becomes a little louder and voltage goes up to 14.5-14.7V, and absolutely everything seems to be ok.

Any ideas?

[Anas76]

I bought my 2007 750i in the fall, so can't tell what my voltages were like in the summer, but I am also noticing an issue with the voltages for the past couple of months.

My battery was behaving better than yours; 11.8V before starting, drops to ~10V while cranking and then goes up to 14.7V to 15V while driving right away.

I ended up replacing the battery because the local store had a sale and installed it myself (I have a Foxwell N710). This made me realize that while the old battery was a replacement, it was never registered nor coded properly. The car was showing a 110Ah battery that was never replaced while the battery installed is a 95Ah AGM. So, not only was the car charging the battery at a higher voltage than it should, but it was also trying to overcharge it.

I coded the 95Ah AGM in my settings and registered the new battery. I do hook it up from time to time to a trickle charger.

The voltage is still low, 12.2V at rest, but I know it's a new battery, so I am not worried anymore.

So, in your case, the charging voltage is in the normal range for an AGM. Bear in mind that charging voltage should change with temperature although I am not sure if the car adjusts. With all the technology in the E65, I would assume it does vary the charging voltage based on temperature.

It's really weird to me that your battery voltage remains at 10.9-11.5V while the car is running and doesn't go up until 15-20 mins later. Are you driving during that time or just sitting still?