Where is the auxiliary battery located in the Mercedes ML350?

[ONicole]

Mercedes ML350 auxiliary battery is installed next to the spare tire in the rear trunk of the vehicle. Function of Mercedes auxiliary battery: The auxiliary battery in Mercedes vehicles supplies power to the entertainment system, windows, and lighting system during startup. If there is an auxiliary battery malfunction, it is necessary to check the meaning of the fault code for repairs. Possible issues include low battery voltage or battery damage. Precautions for using the battery: In daily use, correctly use and maintain the battery to minimize or avoid using high-power automotive electrical equipment (such as air conditioning, audio systems, etc.) when the engine is off or idling. Before leaving the vehicle, ensure all electrical devices are turned off.

[OCharlotte]

Last time I worked on my car, I found the auxiliary battery location of the Mercedes ML350 to be quite hidden. It's usually tucked under the front passenger seat, requiring you to lift the carpet to access it. However, the position varies between older and newer models: in W164 models (2005-2011), it's typically mounted behind the footwell panel on the passenger side; whereas the newer W166 models (post-2012) relocated it next to the main battery in the engine bay, covered by a black plastic box. Special caution is needed when removing it—always turn off the engine and disconnect power. Once I forgot to cut the power and accidentally triggered the alarm. This auxiliary battery primarily powers the start-stop system and some electronic devices. If your auto start-stop fails or the battery warning light appears on the dashboard, there's an 80% chance it's due to this battery malfunction.

[BrookeRose]

As a Mercedes owner who frequently interacts with the brand, I've noticed that the ML350's auxiliary battery has two different locations. For the second-generation model (chassis code W164), it's mounted on a metal bracket under the passenger footwell, and can be removed by taking out two 10mm nuts. The third-generation (W166), however, positions it in the front right of the engine bay, right next to the main battery, housed in a small box with a ventilation tube. Don't underestimate this compact battery—though only about the size of a palm, it powers critical circuits like the door modules and emergency call system. When replacing, it's advisable to choose an AGM battery of the same specifications, as regular batteries are prone to discharging. If fault codes persist after replacement, use a diagnostic tool to reset the start-stop system.

[VanViolet]

Helped a neighbor with the ML350 auxiliary battery issue, whose location varies significantly by model year. For the 2009 model, you need to lie under the passenger seat and remove the insulation to see the silver battery bracket; the 2016 model is simpler—just open the hood, and the black box to the right of the main battery is it. This small battery typically has a capacity of less than 10Ah and is dedicated to the engine start-stop function. If the vehicle experiences start-stop failure or the SOS warning light stays on, it's likely due to a failed auxiliary battery. When replacing, remember to disconnect the negative terminal of the main battery first to avoid data loss. For replacement parts, stick to German VARTA or the original Mercedes part codes.

[CorbinDella]

Having disassembled three ML350s from different years, the auxiliary battery location is indeed a hassle. For pre-2011 models, you need to remove the side panel of the passenger seat and then unscrew the Torx screws on the cover plate. Post-2015 models place it in the engine bay, but it's hard to reach behind the fuse box. This tiny battery, barely larger than a smartphone, affects the window anti-pinch function and seatbelt pretensioner system. Always wait 10 minutes after turning off a warm engine before starting—the modules are still drawing power. Once, I got zapped when working on it directly; insulated gloves are highly recommended. Prepare a T20 wrench and plastic pry bar—metal tools risk causing shorts.

[Cali]

I remember once helping a fellow car enthusiast locate the auxiliary battery for an ML350, and discovered Mercedes' design is quite tricky. For the 2007-2011 models, you need to lift the passenger side carpet and remove the metal protective bracket to access it. For the 2012-2019 models, it's hidden behind the fuse box in the engine bay, requiring the intake duct to be removed first. This small battery primarily provides backup power for the electric power steering. If it fails, the steering wheel will become noticeably heavier. When replacing, pay special attention to the color-coded terminals—the red wire must connect to the positive terminal; reversing them can fry the ECU. It's advisable to use a multimeter to check the voltage first—if it's below 11V, replacement is necessary. After installation, don't rush to reassemble the trim panels; start the engine and test all electrical functions first.