The problem of underdeveloped batteries in phones and lost power became a big problem for people who own smartphones. You are completely stranded and that 20% power range can cause big stress. The power of the smartphone outstripped its battery capabilities leaving consumers in a difficult bind.
The external battery is recent on the scene and went through a number of stages (1).
1. Introduced in 2001 by a student who put together a few AA batteries to charge his phone.
2. Between 2004 and 2006 Victories launched mobile power.
3. Between 2006 and 2009 fully developed products hit the market.
4. 2009 to 2012 saw a growth stage and popularity of the product.
Not all battery packs are the same. Some can charge your phone once while others might be able to do it multiple times. The price range often impacts its ability. The price has come down and nearly anyone can afford one now.
For example you can get a RAVPower Portable Charger 10400mAh that can charge multiple devices at the same time for $24.96 with shipping. It can recharge a phone 4 to 6 times. Lots of options but this is a good one. You can also find other options and products at an Ebay Store