C63S Battery for 2019

[Mistral]

Even though I use my C63S occasionally, it often sits unused in the garage for weeks at a time. It's probably because the car is over seven years old and still uses the original battery, but I'm well-acquainted with the "Starter Battery" alerts that come up from inactivity. I fix it by slowly charging it, and then it works for a time.

It wasn't the Starter Battery, but rather the "12V Battery" that prompted a fresh warning yesterday. It's likely that I haven't received this notification previously. At the moment, it's charging. It's due for a big repair next month, but I'm afraid they'll charge me a fortune to replace the battery, even though I've done it previously on other cars—my Jaguar XKR being the most recent—so I'm considering doing it myself.


Given that it mentions a Starter Battery and a 12V battery, I'm unsure about the total number of batteries in this vehicle.

According to my research, there is a single 12V battery located under the hood (or bonnet, in American English) on the passenger side; I attach the positive connector of the charger to this battery. It appears like there's an auxiliary device in the footwell that's not easy to access, but it's definitely an electrical gadget and not a battery.

So, I'm hoping you can shed some light on the following questions:

1. What's the deal with the two separate warning messages for the 12V battery and the starter battery? The "Starter Battery" alerts are resolved by charging the "main" battery, so why is there a difference?
2. When it comes to a 2019 C63S, which battery do you think would be the best replacement?
Thirdly, is it really essential to reset codes after changing the battery? A quick Google search reveals that many individuals have successfully swapped the batteries without needing to reset the passcode. They could refuse to reset the codes during next month's servicing because they didn't supply or install the replacement batteries, but I was wondering whether it was possible.

Much obliged.

[CyberNinja]

Behind the glove box, there could be an auxiliary battery in the form of a capacitor pack. On one of the entrance pillars should be a 3D barcode; scanning it will reveal the exact location of the batteries.

[Mistral]

I agree with you. After much Googling, I've come to the conclusion that it is recommended to change the Auxiliary Capacitor (voltage converter) together with the 12V battery anytime it is replaced. I had the car since it was 8 months old, therefore I assume both parts are original. They are now approximately 7 years old.

[CyberNinja]

A week after I swapped out the primary battery for my glc 350, the capacitor pack gave out. The previous owner sold it after using it as a school run taxi for their children, but getting only 18 mpg in over 7,000 km.

[TDudette]

My 2019 E63s' primary AGM battery was just changed.
After last August, the stop-start rarely works anymore.
Just 500 yards after starting today, it kicked in. Fluids are clearly not heated to the point where they can be seen below. When the green square with the A sign appears, it means that stop start is functioning properly.

Got the traffic lights working again, but then it stopped working again.
Starting next month, I will get the capacitor replaced.

[Mistral]

Is the 12V AGM battery something you changed out on your own?

[TDudette]

I was going to, but then I measured the area to be worked in and the various connections to the + and - terminals, and I let my neighbourhood handyman figure it out.
I fail to see why it would be necessary to link the new battery to the vehicle, even if it was mentioned.
It took around thirty minutes to get it on. That cubicle is incredibly cramped.