"The battery and charger (sold separately) is an expensive option but economical in the long run. I was churning through 6 to 12 alkaline AA batteries a week with my Mindstorms explorations. I considered rechargeable NiCd AAs but I've had mixed experience with their longevity. This LEGO li-Ion battery seems promising especially because Li-Ion batteries are ubiquitous in their use to power cell phones, tablets, laptops and many other devices."