A lot of people don't know this, but Secondary Class Batteries (I.E, Rechargeable batteries for electronic devices, etc.) Have a memory system.
Problem: It's only natural for secondary batteries to lose their charge over time. The better the battery, the longer it'll last. However, there is something you can do to ensure you get the maximum lifespan out of your batteries, so you're less likely to have to spend money on a new battery for your device, or a new device entirely when replacing the battery isn't an option. (Note: in most situations, this does not apply to store bought batteries, rechargeable, or not. I.E. AA, AAA, D, C, etc.)
Solution: Draining your battery of it's charge completely about once a month will ensure that your battery in your electronic device will last much longer, and hold a full charge much longer.
You see, secondary batteries have a memory system. When you keep your batteries 70% or higher all the time, the battery only accesses 30% of the stored energy, and overtime will have difficulty accessing the other 70% of the battery, until eventually it doesn't access that at all.
So when you've got an electronic device, and you're wondering why you just fully charged the device, but the battery dies 2 hours later, when it used to last 8 hours or more on a full charge, it's because the battery fails to access the additional cells.
TL;DR - Discharge your electronic device to 20% or lower of a charge, about once a month to extend the battery life, and keep the battery healthy and using the full capacity of it's cells.
This method isn't recommended for vehicle batteries, as they are designed to work differently.