Rakuten offers the New Trent PowerPak Xtreme External Battery, model no. NT120RB, for $35.95. Coupon code "VISACHECKOUT10" cuts it to $25.95. With free shipping that's $13 below our July mention and the lowest total price we could find. This 12000mAh, water-, dirt-, and shock-proof, battery is compatible with most smartphones, iPhones, and 5V tablets.