Kindles use Lithium Ion (LiOn) batteries.
Lithium Ion batteries lose CAPACITY (the amount of charge that they will hold) as they age. This is measured in mAh (milli-ampere hours). It determines how LONG the battery charge will last.
When Lithium batteries age, they get increased "rock content" (lithium that can no longer hold a charge). This happens even to "new-in-box" batteries, which means that it is not a good idea to buy a spare battery to use later. Later, the unused battery will be dead too.
Many LiOn batteries have a limited number of charge cycles before they lose too much capacity. They can also be affected by temperature, and by maximum charge voltage. They last longer if they are charged to only 70% of their maximum "consumer" capacity.
Batteries intended for life-critical and mission-critical applications are derated to 70% max voltage of what the same battery would be when used in consumer applications. That way they have maximum total lifespan instead of maximum time between charges.
You can read more here, if interested in knowing how and why Lithium batteries behave this way:
http://batteryuniversity.com/learn/a...ased_batteries
So, HarryT is right...