Unifun via Amazon offers the Unifun Waterproof 10,400mAh External Battery Charger, model no. U821, for $16.99. Coupon code "MDTVY4WW" drops it to $13.99. Plus, Prime members bag free shipping. (Non-members can spend $49 to bag free shipping; otherwise, it adds $6.74.) That's the lowest total price we could find by $6. This waterproof charger features 2.1-amp USB and a 1.0-amp USB plugs.