VW Touareg Auxiliary Battery Issues Causing Power Fail

I faced similar car electronics issues with my 2015 VW Touareg last winter. Having some experience with basic maintenance, I initially tried diagnosing the voltage drop problem myself using a multimeter. The symptoms matched yours exactly, slow engine start, unusual error messages, and general electrical gremlins. After my initial checks showed low voltage readings, I took it to a specialist workshop for a proper diagnosis. They found the auxiliary battery was completely dead and causing voltage fluctuations across multiple systems. The complete fix included: Full battery replacement for the auxiliary unit, Testing of the main battery, System reset to clear stored error codes, Verification of the charging system Total cost was 290€ and solved all the electrical issues. Zero problems since then. Before suggesting specific solutions, could you share: Have you checked both batteries with a multimeter? Are the issues more prominent after the car sits for several days? Does the problem worsen in cold weather? Have you noticed any correlation with using specific electrical features? This information would help determine if your situation matches what I experienced and whether you might be dealing with the same auxiliary battery failure.

Latest service was done at 67000km. Your experiences sound very similar to mine. Since the initial issues started, I did a quick voltage test on the auxiliary battery showing only 11.2V, which seems quite low. The problems definitely get worse after the car sits unused for 3-4 days. The car electronics act up especially when using multiple electrical features simultaneously, like heated seats and mirrors. The battery malfunction warning now appears more frequently, particularly during cold starts. I will get a thorough battery replacement check done this week to verify if replacing the auxiliary battery resolves these issues. Thanks for breaking down your repair experience, it helps confirm my suspicions about the auxiliary system being the root cause.

Great that you checked the voltage, 11.2V definitely indicates a failing auxiliary battery. This matches exactly what I saw when my systems started acting up. The low voltage reading combined with your symptoms of increased issues after sitting idle strongly suggests auxiliary power degradation. Some tips from my experience with the exact same problems: The 11.2V reading under load points to a battery failure similar to what my car showed, Get both batteries tested professionally, my main battery showed fine but was actually borderline, The correlation with electrical features usage matches the voltage drop pattern I experienced, Cold weather amplifies these issues because battery capacity naturally decreases in lower temperatures After my auxiliary battery replacement, all those unusual electrical malfunctions disappeared completely. The engine start returned to normal, and the error messages cleared up after the system reset. Just make sure the workshop checks the charging circuit too, in my case they found slight corrosion on one connector that could have eventually caused issues even with a new battery. Given the identical symptoms and voltage readings, I am confident you are dealing with the same auxiliary battery problem I had. The 290€ investment was worth every cent to resolve all those electrical gremlins.

Based on your confirmation and details, I think we are absolutely dealing with the same auxiliary battery issue. The 11.2V reading matches the problematic values I saw, and the behavior during cold starts is identical to what I experienced. Your description of problems worsening with multiple electrical features running confirms the power management symptoms I had. Since my latest service was at 65000km, quite close to yours at 67000km, it seems these auxiliary batteries tend to fail around this mileage point. The battery malfunction showing up more after the car sits unused mirrors my situation perfectly. My mechanics confirmed this is typical of auxiliary battery degradation, as the parasitic draw from various systems slowly depletes the weakened battery. I will proceed with the battery replacement check you suggested. Your repair cost of 290€ gives me a good reference point for what to expect. Thanks for sharing your experience, it really helped validate my initial diagnosis and gives me confidence this is the right path forward.