I have this USB voltage tester that I place between my usb wall charger and a USB charging cable. This device gives me the total current that went through over a period of time. Let's say after 2.5h of charging I get this:
IN: 5.22V TIME: 150min 0.835A
And that means over the duration of two hours and a half 835mA passed through at roughly 5V (that somewhat varies a bit)
The thing is, the device I am charging has a 3.7V battery with a total capacity of 800 mA. And this is where it all gets confusing to me:
The total power that went through should be ~5 x 0.835 = 4.174W
However, the 3.7V battery can take 3.7 x 0.8 = 2.96W = ~3W
That's over 1 Watt difference that I don't know where it went. The charging takes place by plugging a miniUSB cable directly in the device that holds the battery. The device has a display that briefly gets lit the first 15 minutes but then I managed to shut it down and remains black throughout the charging. There is also one red LED that stays on throughout the charging duration and then turns off.
Should I believe over a quarter (over 1W) of the power passed through is waste? Or is my math wrong?
Edit: full charging video available here https://www.youtube.com/watch?v=5kYaG2KwxDw