As one of its daily deals, Sunvalleytek via Amazon offers Prime members the RavPower 16,750mAh External Battery Pack in Black or White, model no. RP-PB19, for $23.99 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.67.) That's the lowest total price we could find by $8 and a very strong price for a battery pack of this capacity in general. It features 2.4-amp USB outputs and is compatible with iPhone, iPad, iPod, and many smartphones. Deal ends today.