In this guide, you will find instructions on how to change the additional battery (also referred to as the small, computer, auxiliary, or secondary battery) on Mercedes-Benz ML-Class W164, R-Class W251, and GL-Class X164. A complete list of applicable models is shown below.
The most common symptom that the small battery needs to be replaced is the car starts and runs fine, but you get a battery warning light on the dashboard. When a diagnostic scanner is used to scan the codes, you will get a fault code pointing to a faulty G1/7 auxiliary battery. One of the primary purposes of this battery is to allow you to shift the car in and out of Park if the main battery fails.
You will need a 12-volt battery. The original battery is lead acid, but AGM batteries will also work. Here are a few options.
Instructions on changing the auxiliary battery on the R-Class are slightly different than the ML and GL instructions. On the R Class, you can replace the battery without removing the main battery or passenger seat. These instructions apply to models such as R350, R500, R320 and R63 AMG.
Turn off all the accessories and lights. Move the shifter to Park position and remove the key from the ignition.
Move the plastic door molding on the passenger side.
Lift up the carpet. It will help to remove the carpet on the passenger side completely. At minimum lift up the carpet enough to get to the 10mm plastic nuts.
Remove the 10 mm nuts.
Lift up the fiberglass panel.
Locate the auxiliary battery and unplug the positive and negative connections.
Remove the two 13mm bolts. Once you remove the two bolts you can slide the old battery and the bracket out.
Remove the auxiliary battery.
Install the new auxiliary battery 000000004039 in reverse order.
On the ML and GL models, you will need to remove the passenger seat bolts and tilt the seat forward. You will need Female Torx Socket Set to remove the Torx bolts.
Another option is to move the passenger seat forward and up. Then remove the main battery under the passenger seat. This will allow you to access the auxiliary battery.
In some cases, the battery warning light may stay on after you replace the battery. If that is the case check the voltage provided by the alternator.
If the voltage is between 13 and 14.8voltst the alternator is working ok. If the voltage is low there may be a problem with the main battery, ground connection, alternator or voltage regulator.
Next check the auxiliary battery relay.
Also, check that the small negative wire is connected to the negative post of the main battery.