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.
I tried:
Switching it off and on.
Resetting it.
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.