What battery does a Mercedes-Benz S-Class key take?

Mercedes-Benz S-Class key battery

1998–2005

Remote head key2 × CR2025
Takes two cells. Replace both at the same time, stacked the same way as the old ones.
Smart keyCR2025
TransponderNo battery
A key with no buttons has no battery. It starts the car using a chip that needs no power.

How to change a remote head key battery

Look for a small screw or a slot along the seam of the plastic head. Undo the screw if there is one.
Wrap the tip of a small flat screwdriver in tape or a cloth, put it in the slot and twist gently to part the head. Keep it off the circuit board and contacts.
Note which way up the old cell sits before you lift it out. It varies by key, so don't assume + is up.
Fit a new 2 × CR2025 the same way up, holding it by the edges. Leave any grease on the contacts.
Clip the case back together and test the buttons at the car. Most keys work straight away with no reprogramming.
Keep new and old cells away from children. A swallowed button battery can cause serious injury within two hours.

How to change a smart key battery

Slide the release catch and pull out the emergency key blade.
Wrap the tip of a small flat screwdriver in tape or a cloth, put it in the slot the blade leaves and twist gently to part the case. Keep it off the circuit board and contacts.
Note which way up the old cell sits before you lift it out. It varies by key, so don't assume + is up.
Fit a new CR2025 the same way up, holding it by the edges. Leave any grease on the contacts.
Clip the case back together and test the buttons at the car. Most keys work straight away with no reprogramming.
Keep new and old cells away from children. A swallowed button battery can cause serious injury within two hours.

Questions

W220 (1998–2005): 2 × CR2025 in the remote head key; CR2025 in the smart key.