talleng2

My2018 Panamera 4S has been in the garage all winter. I always put an AGM battery minder on it. I went to take the car out today and noticed the battery minder was showing a load from somewhere and the battery was only at ~75% charged. The last time I looked, maybe 3 weeks ago, it was fully charged with no load. While driving, I noticed the charge rate jumping around. Normally it always held pretty steady at 14.5V. Today it is swinging from 12.1V to 15V. It is rather rapid and erratic. It doesn't seem to coincide with engine speed.

Has anybody else seen this happen?

Thebig1

Could be battery on its way out

sac02

(bold emphasis above is mine)

I was ready to suggest you consider that the battery charger may be going bad when looking at erratic charge rates, but I re-read and saw you said specifically in-car charge rates.

"Rapid and erratic" voltage swings sounds like a loose connection somewhere to me.

It could be actual bad voltages, and you may be able to diagnose this if you have PIWIS - you'll likely have history codes stored for voltage or communication faults in various ECUs. (Note you may have these history codes even without warning lights).

Or it could be bad voltage sensing.

Start at the easy and obvious places (battery cables, alternator) and then move to other troubleshooting.

Ericson38

I wouldn't rush to change anything or find a fault with the car just yet. We have a 2019 Base and have been running a Schumacher 15A charger/maintainer on it for almost 4 years now, since it has only been driven about 3.5 K miles a year.

Drive it normally, then park it and check the voltage at the battery terminals about 1 hour after shut down and then the next morning.

I also get occasional wild swings on battery voltage on both the 2019 Pana and the 2023 Cayenne, between the ranges you mentioned, and not engine RPM proportional as you noted.

Maybe someone had turned on the ignition or left the interior lights on or something when you were recommissioning it. The Schumacher's % charge value in my mind is not accurate. I have seen it go from 80% to 99% in 30 minutes, and 20% of a 150 amp hour battery ( = 30 amp hours) can't be supplied by a 15 amp charger in 30 minutes. That would take a 60 amp feed for 30 minutes to accomplish.

talleng2

After several more on/off cycles, it seemed to settle down. It will be going in for service in the next month or two, so I may have them run the codes.
Thank you, everybody.

E39Nutz

There is a charging algorithm in the car (hence it needs to know when one replaces with a new battery). The algorithm senses that the battery is low, then it allows higher throughput (>14V) to the battery. If it sees the battery is full, it limits the throughput and you see around 12.5V going through. I see the voltage goes erratically often when the battery is in low charge, ie resting state at below 12.5V. I won’t be too concern. I have a 2018 also with original battery.

Anchor2018

I also had a similar situation when driving to northern Wisconsin for vacation. Except before I arrived, I had transmission error codes which I have learned it is probably the battery. But I had just replaced my original 2018 battery in Jan. I went to O’ Reileys and they said that I have a bad battery and that the alternator tested fine. I replaced the battery and I had the same issues driving 5 hours home. But this time, I did not have any devices plugged in that could additional strain on the battery. I am going to mechanic this week to have him look at the alternator and sensors for the diagnostics. I will let you know what I find out.

Shuga

Did you code the battery?

Anchor2018

Yes, I did and I bought a battery charger, charged it to full. And had them check the codes again. After a 20min drive it was drained to 12.9. So I went with the easy steps to get back home and bought a new battery which was still under warrenty. 😁

huskeric

as noted within one of the above posts, it is normal for varying voltages, specifically in non-sport mode. These cars have 'smart charging alternators' that determine the voltage load required and manages as such. I also replaced my battery, coded properly, and still witness the same activity, yet all is fine with the battery, alt, connections, etc. When I switch to sport mode, the volts immediately rise to 14+ to compensate for more electrical requirements, but then go back down once out of sport mode.

Anchor2018

Thank you for the information. I was concerned when the "transmission error" you can no longer reverse (which I couldn't) came on. That was before I replaced the battery. Also, we had 3 devices plugged in and traveled for 5 hours, could that drain the system? It was also a 85 plus day out too.

Shuga

No, you should be able to charge your phones on a 85 degree day without getting a transmission error. The longer you drive the more your alt should recharge your battery