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I agree with others that a Kindle shouldn't need charging every day. I read several hours a day, and I recharge when the battery level gets down to between 20-40%; my experience is that I have to recharge around once a week or thereabouts. I generally don't charge to 100%; the design of a Kindle makes it less convenient to know the actual charging state. Instead I charge until the LED turns green which is around 95%.
Incidentally, others have suggested that leaving the WiFi off will save battery. That's probably true. I leave WiFi on most of the time, but most of the time my Kindle is at home where it is continuously connected to WiFi. I know that cell phone batteries discharge more rapidly when you are out of cellular range, and I suspect a similar phenomenon applies to Kindles and WiFi. So I do put my Kindle in airplane mode if I'm going to take it away from home.
I think its an oversimplification to say that lithium ion batteries don't like to be fully charged. What they don't like is elevated temperature, and fully charging a lithium ion battery to 100% can cause the temperature to rise unless the charging circuit is designed to taper the charging rate as the battery approaches full charge. Some smart phones are are designed to offer tapered charging, but they are also more expensive than a Kindle. You get what you pay for.
Batteries are electrochemical devices, and elevated temperature causes changes in some of the chemicals used in a battery. So the other issue is that if the ambient temperature is elevated (say, in a desert environment), the battery won't last as long.
Lithium ion batteries do display a 'wear out' characteristic that is related to the number of charge-recharge cycles; a cycle is defined as charging from 0% to 100%, so charging from 25% to 75% is half of a charging cycle. Amazon probably has a design target for the number of charge-recharge cycles a Kindle battery is expected to support, but they haven't made that information public.
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