There are so many choices of extra external batteries for your iPhone or Android, where setting a criteria make the search so simple with price comparison in the end . Here a couple of ultra-portable thin battery packs slightly larger than a credit card and not larger than the cell-phone itself, and not covered with solid metal to be scratch-prone and could even hurt its serving phone while carrying it around .

below are both from Chinese-made Mammoth

  • CX2.5 size 97 x 64 x 7mm weighing 66 grams W5800
  • CX4 104 x 68 x 9mm 102g.W7200

there is also a larger CX5 102 x 66 x 12.4mm 120g.W9000
supplement after using them for a year : bought these to carry-around with minimum clutter as the short cord itself is part of the battery . was very easy to carry, slideable into any small pocket of your bag, and its rather-soft plastic surface did not get scratched nor do damage to any other device . so they are for emergency, as each barely make one full charge onto the latest iPhone 7 or Galaxy S7 at the time .

– Korea Tech BLog –