Audi S5 Battery Drain Mystery Plagues Multiple Systems

I had a comparable issue with my 2021 Audi A5 diesel last winter and have some experience with electrical systems. The symptoms were nearly identical to what you describe, flickering lights, slow starts, and voltage warnings. After monitoring the battery health through several starts, my workshop diagnosed a heavily discharged battery that was beyond recovery. The voltage regulator was operating correctly, and the charging system tested fine, but the battery itself had degraded significantly. The complete diagnosis and replacement cost me 290 Euro, including labor and a new OEM battery. The repair took about 2 hours, as they needed to reprogram several systems after the battery swap. Your symptoms strongly suggest a similar battery issue, but to provide more specific guidance, could you share: How old is the current battery? Does the problem occur more frequently after short trips? Are you using any aftermarket electronics that might drain power? What voltage readings are you seeing? These details would help determine if your situation matches what I encountered and whether you might need additional diagnostics beyond the battery.

Last service was at 85817 KM and the workshop performed a full diagnostic test. After reading about similar electrical problems, I am now getting more detailed voltage readings. The battery shows 11.8V when starting, dropping to 10.9V under load. Even with the engine running, voltage stays below 13.5V, which seems to indicate an alternator issue rather than just battery health. The charging system warning comes on more frequently during cold starts. My current aftermarket dash cam might be contributing to power drain, but the electrical issues persist even with it disconnected. I am particularly concerned about the voltage drop during idle, which seems more severe than typical battery aging would suggest. The original battery is still installed, so your experience with battery replacement is helpful. However, given the low voltage even during engine operation, I suspect the power supply problems might be more complex.

Having followed your case, the voltage readings you shared definitely point to a more complex issue than what I faced. While my A5 showed similar symptoms, the voltage stabilized above 14V once running. Your readings staying below 13.5V with the engine running, combined with that significant voltage drop under load, strongly suggests alternator problems alongside potential battery health issues. The voltage regulator in these systems should maintain around 14-14.5V during normal operation. Your readings indicate the charging system isnt meeting the required output, which explains the persistent undervoltage warnings even with the engine running. In my case, a simple battery replacement resolved everything because the alternator was still providing proper charging voltage. Your situation shows classic signs of alternator wear, the inability to maintain proper charging voltage and significant voltage drop under load. Based on the readings and symptoms, you might be looking at both alternator replacement and battery replacement. The repair costs will likely be higher than my 290 Euro battery fix. The alternator job typically requires more labor hours, and parts costs are substantial for these models. Consider requesting a full charging system diagnostic before proceeding with repairs. This should include load testing both the alternator and battery to confirm the exact scope of work needed.

Thanks everyone for the detailed responses. After comparing my voltage readings with similar cases, I think I need to address both the alternator and battery terminal issues. The charging system definitely shows more severe problems than a typical aging battery, especially given those consistent low voltage readings even during normal operation. The power supply concerns are clearly more complex than initially thought. While the workshop confirmed issues during the last service, the detailed voltage readings really help pinpoint where the charging system is falling short. Even after disconnecting aftermarket accessories, the battery health indicators and voltage patterns suggest multiple failing components. I will schedule a complete charging system diagnostic to properly assess both the alternator and voltage regulator before proceeding with any repairs. While the cost will likely exceed a simple battery replacement, addressing the root cause should prevent future electrical issues. Thanks again for sharing your experiences and helping me understand the likely scope of repairs needed.