Portable power for the Raspberry Pi

I needed a USB power pack for a portable Raspberry Pi application. Following some previous bad experience with the 18650 battery format (fake batteries that had previously been used in laptops) I decided to buy a highly rated power pack and then open it up and test it properly.

I added a line to my crontab on the Pi (model B with no peripherals):

* * * * * uptime >> uptime.log

I discovered that it ran for almost exactly 6 hours. Next, I opened it up and unsoldered the battery for a proper discharge test.

I found that the Pi drew 500mA from the battery, which makes sense seeing as how the Pi draws about 300mA at 5V, and the battery is less than 3v even at the start.

I set the current to 500mA and the cut off voltage at 3V after a bit of research on what’s the safe lowest voltage for Li-Ion chemistry.

You can see the voltage rapidly dropping off around 3.25V.

I measured the charger’s low voltage cut off at 2.5V, which is perhaps a bit low.

More details and the CSV of the data are available on my github.


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