I purchased my iMac with a wired keyboard for 2 reasons. One, I wanted the numeric keypad. Two, I didn't want to be bothered with batteries. I have a Magic Mouse and it does eat batteries though not as frequently as your keyboard. Some people use rechargeable batteries for both the keyboard and the mouse. At least you're not buying batteries as frequently.
Unless you have a pressing need for a wireless keyboard, you might consider switching to the wired keyboard. You might also consider the rechargeable battery alternative.
I suspect that the wireless keyboard and mouse are like children's battery operated toys in that they all eat batteries.
Thank you Tanya for your comment.
I have put rechargeable batteries in the keyboard now - see how long they last?
I did look this morning in the Apple store for a wired keyboard and could not see anything.
I am interested in the theory of magic mouse interferring with power of the wireless keyboard and if batteries go flat again, I will plug in my Apple wired mouse, see if it makes a difference.
Also the theory that the keyboard is not automatically powering down after use- a fault.
A bit difficult to test if it powers up on touching the keyboard keys!
Fully charged batteries fitted to I- Mac wireless keyboard on 15th Sept. 2012.
Now I am getting the message Keyboard batteries low.
I checked thru System Pref. and sure enough batteries shown as only 18% charged.
What is going on ?????
My friend says his wireless KB lasts for months , I need to ask him if he has a wireless mouse!