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.
From the menu bar, select : Off