First of all, there are many kinds of batteries out there, the most popular are Li-ion, stands for Lithium Ion. And Li-po, stands for Lithium Polymer.
NOTE: This post applies to Li-ion and Li-po batteries ONLY. If you wanted a video, scroll down to this post.
First lesson is the charge percent of your battery. Your battery "may" lie about the current charge percentage that it currently have. This occurs when you are charging, the device is computing it's "predicted" percentage based on how powerful and many the amount of electrons that flow through your device. To return this to the accurate percentage, you have to do a "battery calibration". It is done by:
1.) Charge your battery up to 100% and let it stay plugged in for about 1 hour to ensure that it's really 100%
2.) Pull the AC power then discharge your device down until it turns off itself.
3.) Charge your device up to 100% full again. You can use your device while it's charging.
That's it! Your device is fully calibrated!
Second lesson is, what does the charge percent of your battery tells you. You want the charge level of your battery, as much as you wanted, near 100%. We say that it's cool because the battery will stay long. EEEEENK! You're wrong! A Battery at 100% is actually a way of saying that the battery is somewhat reducing it's lifetime capacity. Whenever we use the battery, we can't deny the fact that the battery, little by little, is reducing it's lifetime capacity; As much as any other things do(damn you physics!). What you can do is not to charge it up to 100% "But you said that when calibrating, charge it up to 100%" That is if you're thinking of calibrating your battery, you can disregard this tip. The sweet spot of the charge percent is around 40% - 80%. Why this percents? Because the electrons can flow freely around the battery whereas when it's full, some of them needed to discharge. The critical level of the battery is 20% because at this percent, the electrons inside becomes unstable. There's much space to move, so, the electrons may jolt inside.
Third lesson is, charging your battery. When you are charging your battery, make sure not to leave it charging all up to 100%. When you leave it plugged in, chances are, the battery will deform, getting bigger and bigger, that's why your battery is small one day and when you checked it again, it's bigger.
And possibly opens or pops out some of parts of your device
You never wanted that to happen. The battery gets bloated because the electrons inside is overflowing, forcing the physical form to deform and the other electrons will fit in. And if the physical holder is too bloated and stretched out, the battery may explode! Some chargers have a feature that when the battery is fully charged, it will turn off itself.
Fourth and last lesson, the charge speed. You may notice that the charge rate of the battery gets slower overtime while it's charging. That's because the charge rate changes on how much percent the battery is charged. It charges faster when is below 10% and slower when it start getting up. This happens because it's one way of protecting the battery from overcharging.
Batteries! As explained by Linus(and also my reference)