A while ago, I bought a nice external battery, which was rechargable, and capable of powering any device that used a USB connection. It worked for a couple of months, then it wouldn't work.
I did nothing about it for a long time, and it inhabited my box of "things that don't work but might benefit from future attention", but today, I decided to have a look at it.
Opening it up wasn't easy; it had two halves of a shell, which I think were glued together. I prised it apart, and looked inside. It consists of a small piece of electronics, and four 18650 batteries, wired in parallel.
I separated off the batteries, and tested them. One was stone dead, the others had some life. The dead one is the reason the whole thing didn't work. I think they must have used cheap, unprotected batteries. The problem with an unprotected battery, is that there's nothing to stop it from discharging below the level at which it fails.
I tried the small electronics part with a good 18650 battery, and it worked fine, providing 5.15 volts at the USB outlet. So now what?
Option 1) I could resolder the 18650 batteries, but leave out the faulty one.
Option 2) I could make the batteries external to the device, and use an 18650 battery holder.
I've ordered a handful of battery holders (five for £0.99, Ebay), but I think what I'll do, is just resolder the three good batteries, so the capacity will be only 75% of what it was, but 75% is a lot better than zero!
Lesson from the story - if you buy the cheapest iPhone external battery pack, then it will come with the cheapest batteries inside, and most people won't be able to fix this.
Second lesson - buying a more expensive iPhone external battery pack doesn't guarantee that you'll get decent batteries inside it.
Upshot - buy the kind of external battery that lets you put in your own batteries. Then you can buy decent batteries to power it.