Tenergy via Amazon offers its Tenergy 3,500mAh Ni-MH Replacement Battery for iRobot Roomba for $22.49. Coupon code "DNS11702" cuts that to $15.74. Plus, Prime members bag free shipping. That's the best price we could find for a similar battery by $8. It's compatible with a range of Roomba models (listed on the product page). Deal ends June 25.