AA to D Battery Converter

I have the need of buying 4 D size alkaline batteries every month or so to pump my airbed. As time grows, the amount of batteries I need to buy would have accumlated to the cost of the airbed itself soon. So I thought of buying rechargable D batteries, but they aren't cheap. Then I thought of buying the case converters which place one or two AA size alkaline batteries in the case and acts like a D size battery. Here is the solution: This “shell” opens up to reveal a slot for the smaller (AA) battery. However, there is one drawback, you won’t get much juice out of it, as AA batteries average 1500mAH (some rechargable nowadays go up to 2500mAH)— compared to 8,000mAH for a D battery. So it is not advisable for me to put 2 AA batteries which will underpower the motor of the pump.