Li-ion portable battery charger to power the Arduino?

I need some help regarding powering the Arduino with a portable cell phone charger. I once saw a video of someone using one and I bought a charger to try it out. Even though the charger specs said 5v output, I came to find out it is only the boost output voltage which last for a few seconds before stepping own to 3.5v or so. I thought maybe I read the specs wrong and later ordered a different one that talked up it's 5v output but it too does the same thing (why don't they just say 5v boost, 3.5v continuous?) . I ended up giving it to my wife for her phone. I can't find the video I first saw someone using the portable charger.

Is there such a thing? I am interested due to the much larger mah capacity of portable chargers and its relatively small footprint which would allow me to run the Arduino for a very long time in a compact form. I run the atmega328 in low power mode (average ~7ma with accesories) which seems to work well...just looking for a larger capacity 5v battery source.

Thanks for any help.

What load did you have on the charger? Normally they are 5v. You may have been drawing too much current for the charger to maintain the voltage.

Weedpharma

7 ma load is too low for most phone chargers. They contain a boost converter which has to be switched on when a load is connected otherwise the converter will slowly drain the battery when nothing is connected. Depending on how the load detection circuit works, what you are likley seeing is the 3.7 V from the LIPO cell in the charger. They commonly use 18650 cells either singly or in parallel.

You could slow it down to 8mhz, it will run just fine at 3.5v, will use less power too. All depends on your application.

Thanks for the input. Looks like it's not a good solution. I'll punt on this one..