What battery does a Volvo S40 key take?

Volvo S40 key battery

2004–2012

TransponderNo battery
A key with no buttons has no battery. It starts the car using a chip that needs no power.
Smart keyCR2032
Smart keyCR2450
Remote head keyCR2450

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 CR2032 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 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 CR2450 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

II (2004–2012): CR2032 in the smart key; CR2450 in the smart key and remote head key.