Anker® Astro3 10000mAh Dual 2A/5V USB Output & 9V/12V DC Output External Battery Pack Charger.
Edit Please note this is for the older version, NOT for the new E version (despite what the Amazon page says). Also I corrected my maths, as pointed out by Ms L Atkins. Thanks for that, I missed out some brackets!
So how many times can you charge your phone? This battery is 70% efficient (nothing is ever 100%). According to their website (ianker.com) use this formula.
Capacity of the External Battery * 70% / Capacity of the Mobile Phone
So for my phone, HTC One V (1500 mAh battery) - (10,000 * 0.7) / 1500 = 4.66, or 4 full charges from 0% to 100% & 1 more charge from 0% to 66%
In testing I achieved 4 full charges from 0% to 100%, & 1 more charge from 0% to 54% before the Astro 3 was totally flat.
It took 2 1/2 hours to fully charge my phone using the S USB slot. It took 3 hours to fully charge my phone using the I USB slot. (Using the wall charger that came with my phone, it takes 1 1/2 hours to charge my phone from 0% to 100%)
2 USB slots can charge up to 2A (@5V). USB Slot I is for Apple devices, USB Slot S is for Samsung (read Android) devices. Both slots work for any USB device I have tried that is 5V. This also has a DC out slot that is switchable between 9V & 12V.
Your device will only draw UP TO the Amps that it is designed for. In other words if your wall charger is 1A 5V, it is ok to use this external battery, your device will only draw 1A. The external battery itself can supply up to 2A depending on your device, just make sure its 5V.
I also tried charging my netbook, an EeePC (1000HE) made by Asus, with an 8400 mAh battery that lasts for around 7 hours. The wall charger for this is 12V 3A. I was a little dubious about this as they say this device is not compatible with Asus tablets.
Anyway after 1 hour of charging, my netbook went from 3% to 32% & had 2 hours 15 mins of time on it. The external battery had 2 LEDs lit up after this, so between 50% & 75% charge left (I suspect just over 50%). The external battery was also rather warm, presumably my netbook was trying to draw 3A. I wasnt willing to try anymore because I didnt want to damage my Astro 3, but it will be ok for an emergency.
4 LEDs on the External battery to indicate how much charge is left. 1 LED 0% to 25%. 2 LEDs 25% to 50%. 3 LEDs 50% to 75%. 4 LEDs 75% to 100%. This external battery comes with loads of different connectors that should fit most devices.
My phone (HTC One V) is about the same size an an iPhone 4. This external battery is a bit higher, a bit more wider, & twice as thick as my phone. Also considerably heavier. (Other reviews have measured this, I cba)
It arrived quicker than I expected, ordered on 7th Sept, was told estimated despatch of 13th for delivery 18th. It was actually despatched 9th, tried to deliver 12th but was too big for the letter box, & I had to pick it up from the Post Office 13th Sept.
I've added some pics showing the device with my phone.