In the MBUX screen under Info > Engine, you can monitor the battery voltage. While the car is running with auto start-stop enabled, I observe the voltage fluctuating between 12.5V and 14.8V. However, when I disable the start-stop feature, the voltage remains stable at 14.7V.

I'm curious about why this happens. I think that when the car is running, the alternator is functioning as well. It seems the auto start-stop system may disconnect the alternator or halt battery charging to assess the battery voltage for stopping the engine. During this time, the voltage fluctuates between the battery's voltage and the alternator's output. This could mean that the start-stop function results in less efficient battery charging and may negatively impact the battery due to frequent short charges. I do not know if AGM battery like short charge or not.

Please correct me if I am wrong.

 

When the battery is being charged by the alternator, ~14.5 volts is normal. When start/stop is being used, the alternator isn't operating in the stop mode, and normal battery voltage ~12.5 volts is present.

The topic is controversial, but IMO start/stop accelerates the wear on the battery and starter. Mostly, I just find the feature annoying and of no fuel efficiency benefit to me, so I've permanently disabled it.

 

12.4v is considered fully charged for a resting 12v battery. When running the alternator provides ~14.4 - 14.8v to charge. That's perfectly normal and expected.

 

I have disabled the Start/Stop feature permanently. If you search, you will find that Start/Stop is not beneficial to the engine or transmission, but only provided by the manufacturer to help meet fleet mileage mandates. Since virtually all new cars have it, it is more of a novelty for the uninformed. Do your engine and transmission a favor and disable it. The fuel savings are negligible compared to the cost of the wear and tear on the engine. As long as the engine is running, there is a film of oil protecting all moving parts. Engine off and that oil film starts draining into the sump. The constant stopping and starting multiplies the amount of wear on the crank shaft, bearings, cam shaft, guides, etc.

 

Agree, there is more wear on the starter, the engine, the engine mounts, the starter battery, etc.

 

How is it permanently disabled?