Many cyclists are running lights with a 3.7V lithium battery cell standard known as 18650. If you have those batteries and want a portable phone charger, here is a very brief review.
There are many different power packs available. I searched for one that was small and light and reused my 18650 cells. On ebay I found:
FLOUREON Power Bank pack PCB Case 18650 Battery Charger Box for Phone DIY Kit.
Without batteries the price is typically £12.99 for a UK stocked supplier. For about £3 more you can have a version that comes with 2 batteries. Some come with a free USB light.
Note that the spec sheet says it outputs 5V at up to 2.1 amps. Take note, some power packs can only output 1 amp which may not be enough to charge the latest phones and pads. Check the labelling on your device's charger or plug to see what it outputs.
I've had some questionable quality cheap Chinese gadgets in the past. Floureon's website looks legit compared to some other come-and-go brands. That's not an endorsement though, I've only had this product for a few days.
Unboxing reveals the charger box, a Micro-USB cable, instructions and mini USB powered light widget. The plastic feels robust enough and not too thin.
On the case are two USB sockets. The larger standard sized one is for output. Surprisingly, the Micro-USB socket is for input, charging the 18650 cells.
There are three lights and a button. Pressing the button when no cable is plugged in reveals the capacity of the batteries inside the box by lighting an appropriate number of lights. I discovered that the button is easily pushed while sitting in a bag and I couldn't bare the thought of wasting precious energy so inserted paper into the battery terminals so prevent the lights coming on when not in use.
Plugging in the cable, large end to the Floureon, micro end to my phone, the lights then animate like runway lights, one at a time moving towards the phone. In the dark the lights are bright enough to be distracting so need covering or turning over.
Before buying, check the length of your 18650 cells. Only cells 67mm or below will fit. The cells I'm using in the picture are unprotected cells retrieved from a laptop battery pack. Consumer ready batteries with protection circuitry added are longer. This page on Candle Power Forum shows quite a few are longer than 67mm.
So far I have charged my phones 1500mA/h battery from around 15% charge to ~80% about three times. My two 18650 cells are 2000mA/h each. With some expected efficiency loss, I'd expect two and a half full charges from two cells. Of course, with this format I can swap in extra batteries. The product works as expected and I'm happy with it for charging my phone.
I have not fully tested the input charging of the 18650 cells via the Micro-USB socket. I plugged it into my desktop computer's USB socket and the lights animate in the reverse direction. However, unprotected cells in particular need to be charged carefully so not to cause an explosion or fire. I'll leave that task to my dedicated charger.