Kept changing my C300 key battery every week and seems not to have a solution. what is happening and how can i solve it?

Interested as well as I'm having the same issue. Bought Energizer and Duracell. The last time I put the brand new battery in and started the car and it said low battery immediately.

Ensure your key is stored outside communication range.

Since I started keeping my keys in a 'faraday" box I have had no more battery issues.

Win! I think that is key (no pun intended).
Both of my keys are on the same batteries now for at least 4 years.

Is it fine to store the spare key near my normal key? I don’t think this matters, but figured I’d ask since this thread is here. I doubt the keys will “talk” to each other.

Since owning the car, THIS has been the most annoying thing about it lol

I'll check this when I get home

Agreed. Ill look at the metal clip - don't think mine is an issue with it being in range as it gave me the warning as soon I put the new battery in (literally changed it in the store and walked out to the car). Thanks for the tip!

Keep both sets (actually 2 sets x 2) on the same key rack without issue.

They are all went 2 years before needing a battery replacement; free from our dealership.

HI...When you replace new battery that nearly 3,30volts its ok. But i established that every pressing the button, battery lost 0.01 volt... When the battery decrease to nearly 3,05 volts;

the massege appears in the car,
the red light on the key stops flashing
you can still open/close the doors or use the start button,
-BUT you can NOT close or open the windows using the key.(ex: press and hold the lock key to close all windows but you cant).

So we have to replace a new one in a quick period but its unacceptable because volt is still 3 and it should be 3...
I heard that this is a software fault and service has got a software to fix that situation. If someone has any experience about this software or something, please inform...