The GLS550 has a second battery that powers comfort features when the engine is off
The Mercedes-Benz GLS550 comes with an auxiliary battery in addition to the main engine battery. This smaller battery sits in the trunk and powers features like the climate control, seat heating, and infotainment system for a short time after you turn off the engine. It also helps the main battery during cold starts by providing extra electrical support.
The auxiliary battery is not something you interact with directly — it charges automatically when the engine runs and discharges slowly when the car sits parked. But like any battery, it wears out over time and eventually needs replacement. Knowing what it does and what signs point to trouble can save you from being stranded with a dead car.
Key Takeaways
- The auxiliary battery powers comfort features like climate control and seat heating for several minutes after the engine shuts off.
- A failing auxiliary battery often shows up as flickering dashboard lights, slow window operation, or the climate system shutting down when ready when you turn off the engine.
- The auxiliary battery typically lasts between three and five years depending on how often you use post-engine-off features and how hot your climate is.
- Replacement involves removing the trunk lining to access the battery in the rear wheel well, a job most owners take to a dealer or independent Mercedes specialist.
How the auxiliary battery powers your car after shutdown
When you turn off the GLS550's engine, the main battery when ready stops charging. But you can still use certain comfort features — the climate system can run for a few minutes, your seat heaters stay warm, and the infotainment screen remains lit. The auxiliary battery makes this possible. It holds a charge separate from the main battery and releases it to these systems on demand.
The auxiliary battery also helps during cold starts. When temperatures drop, the main battery's power output weakens. The auxiliary battery provides a boost to the starter motor, making it easier for the engine to turn over. This is especially useful in winter or in very cold climates where a single battery might struggle.
The system is designed so that the auxiliary battery never fully drains. If it gets too low, the car's computer cuts power to non-essential features to protect it. This is why you might notice the climate system shuts off abruptly or the windows move more slowly than usual — the car is preserving the auxiliary battery's charge.
Signs that your auxiliary battery is wearing out
A failing auxiliary battery usually announces itself through small electrical oddities rather than a sudden complete failure. The most common sign is that comfort features stop working when ready when you turn off the engine, instead of running for several minutes. Your seat heaters might not warm up after shutdown, or the climate system might cut out right away instead of maintaining temperature.
You might also notice that dashboard lights flicker or dim more than they used to, especially when the engine is off. Windows and sunroofs may operate more slowly than normal. In some cases, the car's computer displays a battery warning message on the instrument cluster, though this does not always happen with an auxiliary battery issue.
Another clue is difficulty starting the engine, particularly in cold weather. If the main battery is in good condition but the car still cranks slowly or hesitates to start, the auxiliary battery may not be providing its boost. However, this symptom can also point to a weak main battery, so it is worth having both checked.
How long an auxiliary battery typically lasts
The Mercedes-Benz GLS550 auxiliary battery usually lasts between three and five years. The exact lifespan depends on several factors: how often you use post-engine-off features, how many short trips you take (which means less charging time), and your climate. Hot weather accelerates battery wear, so cars in Arizona or Florida often need replacement sooner than those in cooler regions.
If you frequently use the climate system or seat heaters after turning off the engine, the auxiliary battery cycles more often and may wear faster. Conversely, if you rarely use these features, the battery might last closer to five years. Regular maintenance — keeping the main battery in good condition and ensuring the charging system works properly — also extends auxiliary battery life.
What happens during auxiliary battery replacement
Replacing the auxiliary battery requires removing the trunk lining to access the battery, which sits in the rear wheel well area. The job is not complicated, but it does involve some disassembly. Most owners take their GLS550 to a Mercedes dealer or an independent specialist who works on Mercedes vehicles, since the procedure requires disconnecting the battery safely and ensuring the car's computer recognizes the new battery.
The replacement battery itself is a smaller unit than the main engine battery — roughly the size of a large paperback book. Once installed, the car's charging system automatically recognizes it and begins the charging cycle. Some dealers may run a diagnostic scan to confirm the new battery is communicating properly with the vehicle's electrical system.
The entire job typically takes one to two hours at a shop. If you are handy with cars, you can do this yourself, but you will need to follow Mercedes' specific procedures for battery disconnection and reconnection to avoid triggering warning lights or resetting vehicle settings.
Maintenance steps to extend auxiliary battery life
Keep your main engine battery in good working order. When the main battery is weak, the charging system has to work harder, and the auxiliary battery may not charge fully during normal driving. A main battery that is more than three years old should be tested to confirm it is still strong.
Avoid leaving the car parked for very long periods without driving it. The auxiliary battery slowly discharges when the engine is off, and if the car sits unused for weeks, both batteries can run down. If you know your GLS550 will sit idle, consider disconnecting the negative terminal of the main battery to prevent slow drain.
In very hot climates, park in shade when possible. Heat accelerates battery wear, so keeping the car cooler extends the life of both batteries. If you live somewhere with extreme temperatures, have your batteries tested annually rather than waiting for symptoms to appear.
Frequently Asked Questions
Can I drive the GLS550 if the auxiliary battery is dead?
Yes. The auxiliary battery is a helper, not a requirement for the engine to run. A dead auxiliary battery means you lose post-engine-off features and may experience slower cold starts, but the car will still start and drive normally. However, you should have it replaced soon to restore full functionality.
Is the auxiliary battery covered under the Mercedes warranty?
The factory warranty typically covers the auxiliary battery for the first year or a set mileage limit, depending on your purchase agreement. After that, replacement is an out-of-pocket expense. Check your warranty paperwork or contact your dealer to confirm what is covered on your specific vehicle.
What is the cost to replace the auxiliary battery?
The battery itself costs between $200 and $400 depending on where you buy it. Labor at a Mercedes dealer usually runs $150 to $300. Independent shops may charge less. Total cost typically falls between $400 and $700, though prices vary by location and shop.
Can a weak main battery cause auxiliary battery problems?
Yes. If the main battery is failing, the charging system cannot fully charge the auxiliary battery during normal driving. This makes the auxiliary battery appear to fail prematurely. Always have the main battery and charging system tested before replacing the auxiliary battery.
Do I need to reprogram anything after replacing the auxiliary battery?
No reprogramming is required. The car's computer recognizes the new battery automatically. However, some dealer shops run a diagnostic scan after installation to confirm the battery is communicating properly with the vehicle's electrical system.