Ok, Something new to add with respect to low battery life.
I found my Kindle depleting in a day or less, with wispernet off. Charging it overnight didn't get the battery filled.
Switching it off and on.
Then thought to remove the battery to let it fully expire.
When I removed the battery, the plug, unexpectedly, slipped out of the socket without any effort. The wires didn't even stretch out.
That was odd!
Then I recalled having fumbled my Kindle the day before. The drop was from about waist high onto a solid floor. No damage, but I'm guessing the battery plug was loosened, just enough to cause a problem with the battery meter reading correctly.
So if you are have a sudden loss of battery life, I suggest checking to see that the battery plug is solidly seated.