VW Phaeton Power Issues: Battery vs Alternator Alert

I had a similar electrical problem with my 2004 VW Touareg V10 TDI. After several years working on VW diesels, I can share some relevant experience with voltage drop issues. My symptoms matched yours exactly, weak starting, dim lights, and erratic electronics. The workshop did a complete charging system diagnosis and found the battery had dropped below critical voltage levels. Even though the alternator was still functional, the deeply discharged battery had damaged cells. The battery replacement solved all issues immediately. Total cost was 290€ including diagnostics. However, its important to verify why the battery discharged in the first place to prevent repeat failures. Before suggesting specific solutions for your Phaeton, could you share: Current battery age, Any recent periods of disuse, Whether you have access to a multimeter to check charging voltage, If the alternator warning light comes on This would help determine if youre dealing with a simple battery replacement or potentially a more complex alternator issue affecting the power supply system. Consider having a professional check the entire charging circuit. Sometimes voltage drop problems can be caused by corroded terminals or damaged wiring rather than just the battery or alternator.

Thanks for the detailed suggestions. I checked the battery terminals and they show some corrosion. The battery is original from 2002, which explains a lot. I measured 11.2V at idle which seems quite low. Had my last service 2 months ago at 46309km where they didnt flag any alternator issues. The alternator warning light hasnt illuminated yet, but with the battery being 21 years old, it makes sense to start there before investigating deeper electrical problems. Will have the charging system properly tested to rule out any alternator issues affecting the overall voltage. Will report back once I know more about the complete battery voltage situation after professional testing.

Yes, those voltage readings definitely confirm what I suspected based on my Touareg experience. An 11.2V reading at idle is a clear sign of serious voltage drop, a healthy system should show around 13.8-14.2V when running. The 21-year-old original battery is way past its service life. When I replaced mine, the voltage regulator readings immediately improved from similar low levels to normal charging range. The corroded terminals you found can also contribute to the voltage problems by creating additional resistance in the circuit. Given these symptoms match my situation exactly, I strongly recommend: 1. Replace the battery as the primary fix 2. Clean or replace the corroded terminals 3. Have the alternator output tested after installation 4. Get the voltage regulator checked as preventive maintenance My repair cost breakdown was: Battery: 290€, Terminal replacement: 45€, System diagnostic: 60€ The testing confirmed my alternator was still good despite the low voltage symptoms. Your Phaeton likely has the same charging system design, so expect similar results once the ancient battery is replaced. Keep monitoring the voltage readings after repair. If they dont stabilize above 13.5V, then investigating alternator issues would be the next step.

Thanks for following up on my Phaeton electrical issues. The detailed cost breakdown and system voltages are really helpful. A 21-year original battery definitely explains the symptoms. Based on your similar experience, I will proceed with: 1. Full battery replacement 2. Terminal cleaning/replacement to address the corrosion 3. Complete charging system test to verify alternator health The quote I received matches closely with your costs: Battery 295€, Terminals and labor 110€, Full diagnostic 65€ I feel much better knowing your Touareg had nearly identical battery voltage problems that were resolved with a new battery. Will schedule the work next week and do the alternator testing afterward to ensure proper charging system function. Thanks for walking me through the voltage troubleshooting process. Will update once I have the battery replaced and confirm the charging voltage returns to normal levels above 13.5V at idle.