[tundra_desert]

There's likely dozens of leads to chase, but I'm seeking ideas for what could cause voltage reduction roughly after 30-mins of driving.

Upon ignition, battery voltage ranges from 14.34 to 13.82; the state of charge is 78% and rising. After 30-mins, sometimes less, battery voltage drops to 12.29 and state of charge begins to drop. After another 5-mins, voltage drops to 12.03 and state of charge continues to dip. If I were to carry on, the yellow battery warning appears to indicate a charging problem.

I initially had the battery replaced and registered. No difference. I've also swapped out the 170a alternator with another I picked up from a salvage yard, and replaced the serpentine belt and tensioner. There are no codes available within BimmerLink; I don't have access to ISTA.

I'm having the new battery checked tomorrow, but suspect they'll let me know that it's not the culprit. If that's the case, I'll need to have a trained mechanic diagnose.

[HighlandPete]

Originally Posted by tundra_desert

I'm having the new battery checked tomorrow, but suspect they'll let me know that it's not the culprit. If that's the case, I'll need to have a trained mechanic diagnose.

From what you are describing, it would seem the energy management is involved, as if the intelligent alternator control is cutting back due to an acceptable running SoC, (75 - 80%), then failing to control the appropriate phases as the SoC drops.

Does the alternator charge on the over-run? Or do you lose that function as well? It clearly does charge at the beginning of your trip, both for the voltage you are initially seeing and for a rising SoC.

[tundra_desert]

Highland, thanks for the response. I'll mention the energy management feature when I take it to my shop.

I'm uncertain what your asking about charging on the over-run. I searched the forums and found that you used the term in another post with battery/alternator questions, but was unable to crib the meaning even with the additional context.

[HighlandPete]

Originally Posted by tundra_desert

I'm uncertain what your asking about charging on the over-run. I searched the forums and found that you used the term in another post with battery/alternator questions, but was unable to crib the meaning even with the additional context.

Over-run is when you are slowing down, completely off the throttle in this case, typically ahead of, or instead of immediately braking.

You should have some sort of indication in the dash display that 'regeneration' is active. Does it indicate regeneration is taking place?

If you are monitoring voltage, it would typically be 14.4 - 14.7 Volts. Usually more towards 14.7 Volts with a lower SoC.