Is this the way a C350 voltage regulator works?
01-02-2021 | 05:10 PM
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From: Punta Gorda, FL
2014 C-350 1993 SAAB 900S
Is this the way a C350 voltage regulator works?
2014 C350 sport coupe (W204) with 33,000 miles and just installed a new AGM replacement battery about 100 miles ago. (Yes the original did last that long!)
Using Vehicle Data display for VB (battery voltage) and IB (battery current) while driving. Those values with engine off or at idle in garage are very close to what I read with my Fluke DVM and clamp on amp meter.
VB is about 12.7 before starting. During first 5 to 10 minutes of driving the VB is 14.8 and IB drops from ~50A to 10A. This is exactly what I would expect from an well done AGM charging system when the battery is almost fully charged at startup.
But then the strange stuff starts happening!
The VB suddenly drops to 12.7V and IB fluctuates wildly between -10A and +10A, while spending most the time at -4 to -8A. This fluctuation is independent of RPM, steering input, lights on/off, or anything else while driving. But, every so often (seemingly random intervals) the VB jumps to 14.8 or 14.9V and the IB jumps to +40A and then slowly drops to +10A at which point the VB drops back to 12.7V and the wild current fluctuation begins again. This continues endlessly during all my driving, the longest continuous stint being almost 45-minutes on the freeway at 85 mph.
When I return to my garage and put the car in Park the VB jumps to 14.8V and the current jumps to +40A, while slowly dropping to +10A, at which time the VB drops to 13.8V and the IB stabilizes at about 5A. (Just what I expect from AGM at float)
I know that AGM batteries bulk charge at up to 14.8V (mine is a DuraCell/Penn West and bulk charge is recommended to be 14.4 to 14.8 and terminating when IB is 4% of C20 battery capacity or about 3.2A).
I also know that an AGM is supposed to float at 13.8V with a very low current flow.
My questions are:
WHY is the voltage regulator dropping the VB to 12.7 for most (about 90%) of the drive time when it should be floating the AGM at 13.8V?
Why is the IB (current flow) indicating at drain (-A) when the alternator is at 12.7V and fully capable of putting out more than 60A as I see immediately after startup?
Why does the VB jump up to 14.8V when I put the car in Park?
What don't I understand about MB voltage regulation, alternators, and electrical measurements? I suspect things are working as they should but not in the manner I expect or understand. Please explain it to me!
Using Vehicle Data display for VB (battery voltage) and IB (battery current) while driving. Those values with engine off or at idle in garage are very close to what I read with my Fluke DVM and clamp on amp meter.
VB is about 12.7 before starting. During first 5 to 10 minutes of driving the VB is 14.8 and IB drops from ~50A to 10A. This is exactly what I would expect from an well done AGM charging system when the battery is almost fully charged at startup.
But then the strange stuff starts happening!
The VB suddenly drops to 12.7V and IB fluctuates wildly between -10A and +10A, while spending most the time at -4 to -8A. This fluctuation is independent of RPM, steering input, lights on/off, or anything else while driving. But, every so often (seemingly random intervals) the VB jumps to 14.8 or 14.9V and the IB jumps to +40A and then slowly drops to +10A at which point the VB drops back to 12.7V and the wild current fluctuation begins again. This continues endlessly during all my driving, the longest continuous stint being almost 45-minutes on the freeway at 85 mph.
When I return to my garage and put the car in Park the VB jumps to 14.8V and the current jumps to +40A, while slowly dropping to +10A, at which time the VB drops to 13.8V and the IB stabilizes at about 5A. (Just what I expect from AGM at float)
I know that AGM batteries bulk charge at up to 14.8V (mine is a DuraCell/Penn West and bulk charge is recommended to be 14.4 to 14.8 and terminating when IB is 4% of C20 battery capacity or about 3.2A).
I also know that an AGM is supposed to float at 13.8V with a very low current flow.
My questions are:
WHY is the voltage regulator dropping the VB to 12.7 for most (about 90%) of the drive time when it should be floating the AGM at 13.8V?
Why is the IB (current flow) indicating at drain (-A) when the alternator is at 12.7V and fully capable of putting out more than 60A as I see immediately after startup?
Why does the VB jump up to 14.8V when I put the car in Park?
What don't I understand about MB voltage regulation, alternators, and electrical measurements? I suspect things are working as they should but not in the manner I expect or understand. Please explain it to me!
Last edited by OldTiredSailor; 01-04-2021 at 02:21 PM. Reason: spelling, clarity
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