Dual Battery Trouble Plagues 300's Electronics

I had nearly identical symptoms with my 2017 300 Diesel last winter. Based on my experience with electrical systems, I can share what worked in my case. The voltage drop issues started similarly, dashboard warnings, occasional weak starts, and unusual electrical glitches. The main battery tested fine, but the low voltage problems persisted. After some basic testing, my workshop diagnosed a failing auxiliary battery as the root cause. The diagnostic process involved: full system scan for error codes, testing both batteries under load, checking charging system output, measuring voltage drops across various circuit.s The auxiliary battery replacement fixed all issues. Total cost was 290€ including diagnostics and labor. Since the fix, no more battery malfunction warnings or electrical problems. Before suggesting specific solutions for your case, could you share: Current battery age for both main and auxiliary? Specific error messages you are seeing? Whether issues occur more during cold starts? If you notice any pattern when the problems appear? This information would help determine if your symptoms match what I experienced and whether you might need a similar fix.

Last service was at 82000 KM, currently at 87623 KM. The auxiliary battery replacement resolved all electrical problems on my 300. Car electronics returned to normal function after the workshop diagnosed a faulty auxiliary power system. The total repair cost was 330€ including diagnostic work. The mechanics found significant voltage drop across the auxiliary circuits, confirming battery failure. No other electrical components needed replacement. An interesting note: the low voltage warnings disappeared immediately after installing the new auxiliary battery. The car starts perfectly now, with no more instrument cluster errors. Perhaps we could compare specific error codes? Mine showed P0562 and U0100 before the fix.

Thanks for sharing those details about your repair experience. On my 300, I had nearly identical error codes, P0562 and U0100, plus a U0401 communication error. Your repair cost was pretty close to mine at 330€. My auxiliary battery replacement also instantly cleared all electrical issues, just like in your case. The workshop confirmed severe voltage drops across the auxiliary circuits before replacement, matching your experience exactly. Speaking of mileage, my auxiliary battery malfunction occurred around 85000 KM, very similar to your situation. The timing suggests these auxiliary power systems typically start failing in this range. The immediate improvement in engine start performance and complete elimination of error messages really confirms the auxiliary battery was the core issue in both our cases. Good to know the fix has been reliable for you, my electrical systems have remained stable for over a year now since the replacement. Would be interested to know if your workshop mentioned any preventive measures to extend the life of the new auxiliary battery? Mine recommended regular system voltage checks during annual service.

The preventive measures my workshop suggested are quite straightforward. They recommended checking the power management system during each service and monitoring voltage levels quarterly. Keeping both batteries properly charged seems key to preventing future battery failure. Your experience with similar error codes and mileage is really helpful confirmation. The car electronics have performed flawlessly since the repair, which matches your long-term results. Our matching symptoms and repair costs around 300€ suggest this is a common issue at this mileage point. The voltage drop testing proved crucial for accurate diagnosis. My workshop explained that auxiliary battery malfunction often gets overlooked during basic diagnostics. Having them specifically test the auxiliary power system saved time and prevented unnecessary parts replacement. Thanks for the detailed comparison. Next service I will request those regular system voltage checks you mentioned to help extend the new battery life.