The product can be useful later in the day when smartphone battery is almost empty. But this external battery (a 18650 with a small electronic circuit) is small capacity about 1000 mAh, and the efficiency of the charging do not hope fully charge a smartphone but rather a third.
If this external battery 2600 mAh really was I would probably be 4 or 5 star but this is not possible. There are other external battery models in this format there.
