Yesw, what they probably meant to say is drain the battery completely [if you can turn the phone back on after it reports 0% and auto powers off do so and be SURE it is completely drained]. THIS DOES NOT MEAN THE BATTERY IS PHYSICALLY FULLY DRAINED Android shuts down before it physically is fully dead. How do I know this? If the battery were allowed to PHYSICALLY drain to 0% [not by what the OS says] you would need a special trickle charging device to charge it. Since those are not standard issue and not available to most users no device [be it ANY rechargable battery allows itself to be fully drained [yes it can happen if you encounter a bug on shutdown and the phone happens to hang and not fully shut down it can take the battery past the safe limits as it is hanged and not capable of stopping the battery drain [same as if you left it on a shelf and did not charge it at all for a LENGTHLY time the battery would not be able to take a charge with a standard charger and not be usable].
So, back to what I was saying, You need to fully [per the OS] discharge the battery, and be sure to try to restart the phone as Android can misjudge this and to varying degrees after an auto shutdown people can restart their phone and still get back to the OS with varying degrees of battery left. If this happens you definitely need a calibration [up to you to judge how severe like if you only get boot logo and then it dies again it is not that far off and you prolly dont need to go through the calibration unless you feel like it], but if it turns on and reports any amount of battery truly left I HIGHLY RECOMMEND you recalibrate.
What I do the short version
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Android Auto shuts down my phone after I think I have killed the battery
I try to restart the phone
If it makes it in to the OS and shows me my lockscreen or homescreen
I keep trying to turn it on until all I get is only the boot vibrate and then off
This can take anywhere from no tries to 10 depending on things
Once I get boot vibrate and power off [better than the word death I guess, my bad]
I fully charge the phone with the phone off [usually overnight and I mean with the charge only screen, charging with AC power without booting in to the OS].
In the morning [or no more then 12 hours later [should take only 5 hours [with most devices and their AC wall chargers [aka fast chargers], but want to be sure so I usually wait 6 - 7 try for no more then 8 hours, but definitely dont want it plugged in too long either so cut off is 12] I boot in to the OS without unplugging to be sure it also says 100% and does not revert to like 98 - 99% as I have seen happen. If it is 98 - 99 I wait until it says 100% then 20 extra minutes
Once I am satisfied it is truly 100%
Still plugged in restart to recovery and find wipe battery stats menu item and choose that
Once that is done on main menu hit restart device
As it begins to restart unplug it from charger
Now however you want use the phone be it streaming video to kill the battery faster or letting it sleep [screen off, regular usage whatever my schedule is best suited for] Trick here is kill it all the way WITHOUT plugging it in until OS auto powers off.
I then make sure to fully charge it back to 100% one more time and go on my merry way The more times up to roughly 4 - 5 you can fully discharge and re-charge the better but do at least the 2 cycles (one for the calibration itself and one more after you wipe stats)]
Best way to avoid the need for calibration is:
Always flash your ROMs with battery at 100% Android at install is meant to recognize battery at 100% when it starts running it stats. Of course sometimes you can't wait for whatever reason, and that is OK, just recognize over time it will drift and you will eventually want to calibrate.
Hope that helps better explain it all
