Audi S4 Battery Drain Mystery Plagues Electrical System

Had a similar electrical problem with my 2018 Audi S5 last winter. With some experience working on cars, I recognized the symptoms matched yours, weak starts, flickering lights, and unusual warning messages. The voltage regulator in my charging system was failing, causing inconsistent battery charging. Initially thought it was just a bad battery, but testing revealed the deeper issue. Took it to a workshop where they confirmed the charging system was delivering unstable voltage. The fix included replacing both the voltage regulator and the battery since the irregular charging had damaged the battery cells. Total repair came to 290 Euro, but it solved all the electrical problems completely. Before suggesting specific fixes for your S4, could you share: How old is the current battery? Are the starting issues more frequent in cold weather? Have you noticed any battery warning lights on the dash? Did you get specific voltage readings during the test? A proper diagnosis of battery health and charging system performance would help pinpoint whether your issue stems from a failing regulator, alternator, or just a worn battery. Let me know these details and I can provide more targeted advice based on my experience with the similar platform.

Thanks for the input. My S4 had its last service at 60000 km just 2 months ago. The voltage drop readings were showing 11.2V at idle, dropping to 10.8V when cranking. The battery is original from 2020, and the starting problems happen regardless of weather conditions. No specific battery warning lights, just unusual electrical system warnings that come and go. The voltage regulator suggestion makes sense given the symptoms. The low voltage readings match what you described. Would testing the alternator output help confirm if its a regulator issue before replacing parts?

Since were dealing with the same platform, those voltage readings are definitely concerning. In my S5 case, healthy readings should be around 14V at idle and not drop below 12V during cranking. Your readings mirror what I experienced before my repair. Testing the alternator output would definitely help isolate whether its an alternator issue or just the voltage regulator. My workshop tested both, the alternator was actually fine, but the faulty voltage regulator was causing the inconsistent charging that stressed my battery. Given your battery is original from 2020, its about the age where these issues start showing up. The irregular charging from a bad regulator can significantly reduce battery health. When they tested my battery after fixing the regulator, it had dropped to about 60% capacity due to the charging inconsistencies. Based on the identical symptoms and voltage readings, Id strongly suggest having both the voltage regulator and battery health checked. The repair cost might be similar to what I paid since its the same charging system design. The unusual electrical warnings should clear up once the charging system is working properly. My experience shows these symptoms rarely fix themselves and usually get worse if left unchecked.

Thanks everyone. Your feedback has been really helpful, especially regarding the power supply issues. After the voltage drop readings you shared, Im more confident its likely the voltage regulator causing my problems. Will get both the regulator and battery health checked this week. The alternator issue symptoms you described match my situation exactly, the inconsistent readings and system warnings make more sense now. Not taking chances with further electrical problems, so getting it sorted right away before it affects other components. Will update once I get the proper diagnosis and repair done. At least now I know what to expect regarding the repair costs and potential parts that might need replacement. Low voltage seems to be the root cause here, so hopefully addressing the charging system will resolve all these electrical gremlins. Really appreciate the detailed technical insights from someone who went through the same problem.