Remote key
 

Does the key for my clk 320 1999 have a battery
The key starts car but no longer sets alarm
Can i get some help please

Of course it does, the battery is for the remote locks and arm/disarm - the start functionality gets power inductively from the ignition.

to change battery, remove the metal key blade and pop the rear cover off - it sort of slides off and you will find your batteries, plenty at your nearest pharmacy too.

I think its a pair of CR2025.

Push and hold buttons on the key. There's an LED near the LOCK button that should blink. If it does the battery is probably OK. FWIW those type of alarm sirens fail all the time.