DIY / Electronics / Maintenance / Performance

When, Why & How to Replace Mercedes Auxiliary Battery

As the push for ever increasing fuel economy has pushed engines to be more efficient and use innovative technology we may not otherwise see, there are some configurations of Mercedes that run a dual battery set-up with one battery primarily, to my knowledge, to power start the engine and power most things, and another for start stop system reserve and to regulate voltage.  To see how to change the main battery check out this post.

Modern Mercedes use so much computing in the delivery of torque to the wheels, and in almost every sensor, computer, etc.  I figured taking a lesson from Range Rover Ownership that these cars may be sensitive to voltage and I could not be more right. Right out of the gate previously my brakes felt more powerful and confident with the big H8 battery.  With this smaller Aux battery in my car, a 200 CCA Duralast Platinum replacing a 7 year old OEM battery, I wondered what the impact would be.

If you are on a factory battery in years 5-7 you may want to preemptively change your battery without even testing it.  It drives that much better.

In a empty stretch of road I gently hit the gas to a known level.  And wow, she took off.  There was such a big difference I changed the plans for the rest of my day.  Literally, not just taking the long way home, but hey let's drive that beautiful back road to the UPS Store to send the cable equipment back.  Then let's return the core even though this new one isn't even an exact fit.

Wow, completely different.  So different the transmission really had to remap itself in eco pro or whatever, e mode stands for. A CLA 250 was driving very hard stop light to stop light and in eco with almost no effort I outpaced it but she was a bit jerky as the transmission learned.  Perhaps the transmission control unit (TCU) itself was operating differently with the additional torque.  Either way she put down power very nicely.

This drive was so nice. It really made ,e reflect on the fact that with ESP mode on there is a giant barrier of computer between the go pedal and the drivetrain.  I would not recommend running an ECU tune with ESP off.

When to Replace Your Auxiliary Battery

  • It says to
    • There can be a dash warning "Replace Aux Battery". It is ok to drive for a bit but not too long.
  • Your car is a certain age
    • I love the 2014-2016 Mercedes but if you are running original OEM AUX battery you may want to switch it.
  • It doesn't test to rated power
  • Start/Stop Stops Stopping the Engine
    • If your eco is always yellow, and your car isn't staying off as long or as often as it used to, it is a good sign your AUX battery is worn
  • Park Assist Stops Assisting
    • When you pull next to an open spot to parallel park, your car may have previously asked if you want to engage Park assist or something similar
      • If this stops working as often you may want to consider replacing your AUX battery

Why Replace Your Aux Battery

  • You're about to tune your car and the batteries are more than 5 years old
    • For me my car runs better tuned with the batteries updated
      • I would guess it has to do with voltage droop under load while trying to deal with aging batteries as part of the load
  • You want your park assist to assist, and your eco mode to stop your car
  • Overall you like your cars in top operating condition
    • Changing a worn AUX battery does make a palpable difference in my case, and for many reported on MBforums.
Tubing setup aftermarket AUX battery replacement

Here you can see my use of a cut down jura milk frothing adapter and the milk container adapter to connect to the factory tubing.  Using this Upgrade Provides an additional 10% capacity over factory, also using the part compatible AMG battery will yield similar gains. 

How to Replace Your Aux Battery

  • These batteries are often located in the trunk of your car or under the dashboard
    • For ones in the dash I cannot advise further but to point you to MBworld and suggest you ask there, which is not a bad choice when posed with a Mercedes question
  • For the ones located in the boot
    • Fairly easy, a 10mm wrench unhook the terminals
      • I loosely taped positive out of an abundance of caution
      • A 10mm bolt that secures a swinging holder in the case of the C218 likely W212 and similar
      • Unhook the vent
      • Install new battery
        • If you are using an aftermarket battery you will likely need to hold the aftermarket bolt loosely with your hand
        • If you are using aftermarket consider the venting solution if it doesn't fit stock
          • I ended up using two tips from a JURA Espresso Machine Milk hook up
            • I cut the black frothing tip down and used the tubing to connect while using the clear tip to connect to the factory tubing

Upgrade Mercedes Battery

  • AUX 14, and various 14 size batteries will work
    • Some car audio batteries may fit, these are hybrid batteries designed with large capacitors to capture and deliver power faster
    • You will likely need to modify the vent system or holding system in some cases.
  • AUX 14 Platinum
    • Will likely need to utilize vent adapter as above, and hold the bolts in
  • AMG Batteries are often bigger than non-AMG but will direct fit
    • CLS 63 battery almost surely fits perfectly into the CLS550 and provides ~12% increase in CCA from 170 to 200.

Ease of DIY: 1 for MB Battery 4 for non-MB battery

You will need a 10mm wrench or socket.  No extenders are needed.   If you are going to a different battery other than MB, may need to fabricate a vent adapter as above out of a cut down Jura milk adapter.

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