2014: https://news.ycombinator.com/item?id=7833010 (60 comments)
2013: https://news.ycombinator.com/item?id=5995422 (13 comments)
You can make a chain of these.
Watthours and Joules is difficult to know without measuring the voltage on the device, which would also have to involve the change in voltage over time to get a more accurate number. I'm probably wide off once you account for the reduced power over time.
We probably have to invent a standard that lasts one decade first.
Obviously, the capacity has improved vastly, but the size, terminals and voltage has been with us since your great grandfather wore diapers.
What you're proposing exists, it's just expensive because demand is low.