RS3 Electrical Gremlins Point to Failing Aux Battery

I had a similar low voltage issue with my 2016 RS4 last winter, and as someone with some experience working on Audi vehicles, I immediately recognized the symptoms you described. The power management system was showing similar warning signs, multiple electrical errors and weak engine start performance. After noticing these auxiliary battery problems, I took it to my regular workshop where they performed a complete diagnostic scan. The root cause turned out to be a failing auxiliary battery that wasnt properly maintaining charge. The workshop conducted voltage measurements across different points and confirmed the battery malfunction. They replaced the auxiliary battery and recalibrated the power management system. Total cost was 290 Euro including parts and labor. Before suggesting specific solutions, could you provide: Current battery voltage readings if you have them, Age of both main and auxiliary batteries, Specific error codes showing up, Whether issues occur more frequently during cold starts, Any recent electrical modifications or installations This information would help determine if your situation matches what I experienced and ensure youre not dealing with a different underlying electrical issue.

Last service at my certified workshop was done at 175000 km about 3 months ago. They did a voltage check during that maintenance and readings were normal. Currently at 181951 km and battery problems started appearing 2 weeks ago. My voltage readings show the auxiliary battery drops to 11.2V when cold, which matches the symptoms you mentioned. The main battery appears fine at 12.6V but the car electronics seem to struggle mainly during startup. The error codes P1234 and P1456 keep appearing, suggesting a clear battery failure pattern. I will schedule a battery replacement next week since both batteries are original from 2018. The workshop quoted 310 Euro for parts and labor, which seems reasonable based on your experience.

Thanks for providing those details. Those voltage readings definitely confirm what I experienced with my RS4, the auxiliary battery is showing clear signs of failure. The 11.2V reading during cold conditions is almost identical to what I saw before my battery replacement. Looking at your mileage and battery age, this is a typical timeframe for auxiliary battery replacement. My RS4 needed the replacement at around 170000 km, also with the original battery. The error codes you mentioned match the typical battery malfunction pattern I encountered. The 310 Euro quote for the battery replacement is in line with current prices. My initial 290 Euro service was about a year ago, so the slight price increase makes sense. Just make sure they include the power management system recalibration in that price, its crucial for proper auxiliary battery function after replacement. Since you mentioned cold startup issues, consider asking them to check the main battery during replacement too. While your 12.6V reading looks good now, having both batteries checked can prevent future engine start problems, especially with winter approaching.

Based on all the troubleshooting steps and feedback provided, I think I'm confident moving forward with the auxiliary battery replacement. The symptoms and voltage readings match exactly what others experienced, and the price seems fair for both parts and labor. The matching error codes and similar battery malfunction patterns at comparable mileage points really help confirm my initial suspicion about the auxiliary battery being the root cause. Since both batteries are original from 2018, replacing them now makes sense from a preventive maintenance perspective. I will proceed with the scheduled replacement next week and make sure they include the power management system recalibration. The car electronics should return to normal function once the new auxiliary battery is properly installed and calibrated. Given the reasonable 310 Euro cost and the clear diagnostic indicators, this seems like the right solution to resolve my electrical issues. I appreciate the detailed insights about checking both batteries during the service. I will ask the workshop to inspect the main battery as well, even though its current readings look normal. Better to address any potential issues now rather than face startup problems later.