Higher-megapixel cameras do a significant amount more processing of the image simply because the image is larger. Higher-megapixel cameras also typically have more features than lower-megapixel cameras, so there are more bells and whistles (and sometimes larger viewfinder screens, etc.) to chew up electrical power.

Typically rechargeable batteries don't run as long as disposable ones; their virtue is that they can be recharged and not simply recycled or tossed in the trash.

Perhaps you need a better battery charger that will re-condition your rechargables or you might need new batteries with a higher mAh rating. I purchased a charger and batteries from RipVan and find it works great. Here is a link

http://www.ripvan100.com/products_sanyo2700mah.htm