Although this thread is about battery calibration, these questions about battery charging always pop up.
Battery calibration is the process by which you set your device with your battery data.
Every single battery is slightly different. Although there are minimum stantards to every spec of the battery, each battery is different.
Calibrating your battery is nothing more than setting your phone to the real specs of your own battery (since the phone comes configured with the battery factory standards). For example, your phone may be set to use a battery which maximum charge is 1650 mAh, but your battery has a maximum charge of 1625 mAh. That may be within the acceptable factory standard, meaning you have a normal battery, but that also means that your phone will never indicate a 100% charge after you unplug the charger.
If you calibrate that hypothetical battery, you will "tell" your phone that its maximum charge capacity is actually 1625 mAh, meaning from that on your phone's battery indicator will be more accurate.
For that reason, it is necessary to fully discharge and fully recharge your battery for the calibration process, since your device must read the actual specs (minimum charge, maximum charge, charging current, etc) of the installed battery.
I don't know about how many times this full cicle (discharging and recharging) must be performed, but that is the reason why it must be performed for the calibration to succed.
A DIFFERENT THING is the normal discharging and charging process during normal use of the phone.
In this case, you should NEVER let your battery go under 20% and, what few people know, it also should never go over 95%. This stresses the battery and diminish its life time.
Also, and there's a lot of doubts about this, the lower the charging voltage, the slower the charging process, but the longer the battery life.
High charging voltages (like the one used by the charger sold with the phone, which is around 5V) stress the battery, diminishing considerably its life time.
That is true for all lithium-ion batteries!
Therefore it is better to charge your phone through USB (which uses around 4,3V) than to do it using the charger provided with the phone.
You may be asking "If the charging voltage of the charger provided by the manufacturer stresses the battery, why does the manufacturer provide such charger?"
For purely commercial reasons! Higher voltages mean shorter charging cycles, which please the consummer. Moreover, the battery capacity loss with the factory provided charger is around 15-20% a year (considering "normal" to "heavy" use). That is to say that after a year your battery will only reach 80-85% of the maximum charge it used to reach when it was new. And by that time your manufacturer expects you to consider buying a new phone, with more features.
Summing it up:
- calibration is just the process by which you "tell" your phone what the real specs of your battery;
- in normal use, charging your phone through USB is better than doing it with the manufacturer provided charger;
- try not to let your battery charge go under 20% nor above 95%;
- if you're interested, read more about it at batteryuniversity.com