There’s a place for giant USB battery packs, but that place is not your pocket. For a quick charge while walking around, it’s worth owning a smaller USB battery as well, and this 5,000mAh model from Anker is down to $14 after you clip the 10% coupon on the page.
That’s a bit larger than most “lipstick-style” battery packs on the market, and should give any smartphone out there at least one full charge.