ATC LLC via Amazon offers the Masione 14.4-volt 2,500mAh Replacement Battery for iRobot Roomba 500 Series for $20.99. Coupon code "FZXJYPU9" cuts it to $16.99. With free shipping, that's the lowest total price we could find by $2. This Ni-CD battery is compatible with select Roomba 500, 700, and 800 series vacuums (check the product page to see a full listing). Deal ends January 31.