[General Questions and Answers] felixdd:
At this point, I would advise you do a full charge and deep discharge of your phone. Just to allow the phone to calibrate itself to the battery again.

However, I would advise against running programs to intentionally drain it at a high draw. So instead of playing 5 hours of FPSECE to drain it flat, just let it sit on standby and use it as you normally would, until it dies. Then charge it back up in one continuous charge (refrain from unplugging until it goes back to 100%). Hopefully that will restore the accuracy of your battery meter by a bit.

And IMHO, the article you included discusses things about batteries that are no longer true....

2. Don’t wait for full discharge, charge it frequently. Keeping the battery near to 80% always, gives better life. This is also what many vendors claim as “memory effect”.

No. "Memory effect" is, in laymen terms, the battery not being able to hold above a certain charge after being repeatedly discharged from the same capacity. E.g. being discharged at 80%. It doesn't "give battery life" -- in fact it kills your battery's capacity.

This is a term more relevant for old NiCd batteries. LiONs and most NiMH batteries have very weak/no memory effect.

3. Every battery has limited Full charge-discharge cycles. Of the order of 300+. In other words a typical phone battery can be fully charged/discharged 300 times. Doing more frequent charges, as specified in point #2, will increase overall life.

Partially true. Every battery can be charged a number of times before its ability to hold a charge deteriorates. Usually this is around 300, though the exact number varies between individual batteries. However, the concept of a "cycle" isn't exact -- just because you charged from 60-80% (or whatever) doesn't mean you didn't use a cycle. It's really a continuum, and should only be conceptualized as the battery losing its ability to be charged and hold a charge, the more times you charge it.

4. During first time use (when the battery is new) don`t use it till its fully charged. This is why it is always written on manuals “let the device charge for 2-3 hours”.

The necessity of preconditioning is controversial now. There is little evidence to suggest that devices nowadays benefit from preconditioning, and likewise there is little evidence that not doing so harms battery longevity. Many manuals these days simply omit to mention preconditioning.

5. Best way to increase battery life is Not to use it. If you keep AC power plugged in on your phones, keeping the battery at 80% (as in #1), your battery will last longer. Though discharging it once in a month would be must in such cases.

Partially true. Not using your battery does indeed help its longevity, but not by plugging into the AC. Heat is a LiON battery's enemy, and plugging it into your wall will generate heat that is ultimately bad. This translates to laptop batteries as well -- if you want to store a laptop battery, the best bet is to discharge it to 40%, then put it somewhere cool (even the fridge if you want).

Also I would say doing a deep discharge once a month is too frequent. Once every 2 or even 3 months is more appropriate.

Finally, LiON batteries themselves have a shelf life. So even if you let them sit there, they will lose their function after a few years. Just so yo uknow.

6. Surrounding temperature contributes a lot. Colder weather gives better battery life. So make sure your cellphone doesn’t overheat, if it does, find ways to keep it cool.

True. And that's why you shouldn't leave it plugged in.

Learn more about batteries here: http://www.batteryuniversity.com/