I heard it's a pain in the butt to replace the aux battery in the trunk. what are some signs to battery is getting weaker? I had my 2012 ML350 Since Oct 2019 and had no issues whatsoever. The mileage is now 71k miles.
also I bought the flower shape wheel nut and 17mm nut socket. however It's still hard to remove the wheels. what should i do? I don't need to remove the wheels now but i just want to be able to remove the wheel and install spare in case of emergency. thanks
So far my ML is running great!

 

I wouldn’t wait for signs of a weak battery. With a 9 year old vehicle you probably are experiencing some now. Replace the main battery and aux battery now, on principle alone. Batteries are consumable items and don’t last forever.

The aux battery replacement for W166 is well documented on this site. Ditto for the main battery under the front passenger seat.

The flower-type bolts don’t add value in my opinion. Order a set of MB locking bolts and use one per wheel. Simple and secure.

 

LOL, I have nearly 100,000 miles on my 2014, and as far as I can tell, still running on the factory AUX battery.

 

Hi Sara. Yes, it is a pain in the butt to replace the aux battery, but it depends on which way are you choosing to go.

There is a video (can't find it right now, and also it is in Russian), but the biggest point is that you don't need to rip apart all rear and bumper assembly in order to reach the battery, in that video which is 2-5 mints long the guy was explaining that you need to cut 10-15 cm of the material in the trunk on the right side just about above the battery and if you will fold it, you will be able to reach the battery easy.

Good luck.

 

Aux battery replacement

I posted this in August. Getting an aftermarket aux battery off Amazon and cutting the carpet is absolutely the way to go.