Slow-Cranking RS4: Battery Sensor vs Charging System

I experienced similar voltage issues with my 2011 Audi S4 3.0T. The symptoms matched exactly, slow cranking and dimming headlights at idle. The battery testing revealed it was more than just a basic battery charge problem. The voltage sensor was reporting incorrect readings to the ECU, causing the charging system to malfunction. This created a cycle where the battery wasnt receiving proper charging voltage from the alternator. The repair required replacing both the battery sensor and updating the control module software. Its a serious issue since incorrect voltage readings can eventually damage other electrical components or leave you stranded with a dead battery. A proper diagnosis needs specialized equipment to test the complete charging circuit, not just basic battery testing. The voltage sensor on these Audi engines is integrated with the battery terminal, making it a precise repair job. The fix completely resolved the low voltage issues. No more dim lights or slow starts, and the charging system now maintains correct voltage levels at all speeds. These symptoms definitely point toward a sensor issue rather than just battery age or alternator problems.

Thanks for sharing your experience. I had the exact same voltage sensor issue on my RS4 last winter. The battery health was fine according to initial tests, but the car kept showing weird electrical behavior. Did you get any weird warning messages on the dash before fixing it? My MMI would sometimes freeze and reset while driving which was pretty concerning. Would be great to know the repair costs in your case. I ended up paying around 650 Euro for the complete fix including parts and labor, plus they found and fixed a corroded ground wire while they were at it. Has everything stayed stable since the repair? Mine has been running perfectly for 8 months now but I still do regular battery testing just to be safe.

Your expertise with the voltage sensor issue was spot on. After seeing the mechanic last month, they confirmed and fixed the bad battery sensor that was causing all those electrical gremlins. Total repair came to 315 Euro which seems reasonable compared to your experience. The vehicle diagnostics showed the battery charge was actually fine, but the faulty car battery sensor was sending incorrect data to the system. Since replacing the sensor, all electrical systems are working perfectly, no more MMI glitches or warning lights. No additional repairs were needed in my case since the ground connections and wiring looked good during inspection. The new battery sensor has completely resolved the voltage fluctuations and charging system issues. Regular testing shows stable readings across all speeds and conditions. Good call on continuing with periodic battery tests. While the repair has been 100% effective, keeping an eye on electrical system health is always smart preventive maintenance on these performance vehicles.

Thanks for the advice. After dealing with this issue for a while, I finally got it sorted at a different repair shop. The technician found the failing battery sensor right away using proper vehicle diagnostics. My repair ended up costing 480 Euro including a new sensor, labor, and battery maintenance service. The mechanic walked me through the whole diagnosis process and showed me the voltage readings before and after the fix. The car starts perfectly now, even on cold mornings. No more dimming headlights or electrical glitches. The MMI system runs smoothly without any unusual resets. Best of all, the low voltage warnings have completely disappeared. Really glad I switched repair shops. The new place took the time to properly diagnose the issue instead of just trying to sell me a new battery. Been driving for about 3 weeks since the fix with zero problems. Just wanted to close the loop on this thread in case anyone else runs into similar battery sensor issues with their Audi. Getting the right diagnosis makes all the difference.