Apple iMac (21.5 and 27-inch, Late 2009) - Manual (Page 47)

Chapter 4 Problem, Meet Solution 47
Replacing the Batteries
Your Apple Wireless Keyboard and Apple Magic Mouse each come with two AA
batteries installed. You can replace them with alkaline, lithium, or rechargeable AA
batteries.
Keep the battery compartment cover and batteries out of the reach of small children.
To replace the batteries in the wireless keyboard:
1 Press the On/off (®) button to turn off your keyboard.
2 Use a coin to remove the battery compartment cover.
WARNING: When you replace the batteries, replace them all at the same time. Don’t
mix old batteries with new batteries or mix battery types (for example, don’t mix
alkaline and lithium batteries). Don’t open or puncture the batteries, install them
backwards, or expose them to fire, high temperatures, or water. Keep batteries out of
the reach of children.
Battery compartment cover