Q: Battery Percentage!

It uses both voltage and the sum of the amount of current drawn from the battery since it was last charged.

It is not that accurate really.

Also there is a micro memory chip inside the battery to communicate...

If you have ran the battery flat, it may never hold a full charge long, after.

• About Mac notebook batteries - Apple Support

• How to store Apple notebook computers and their batteries - Apple Support

Maximizing Performance - Batteries - Apple:

http://www.apple.com/batteries/maximizing-performance/

• Apps can affect Mac performance, battery runtime, temperature, and fan activity - Apple Support

• Apple Portables: Troubleshooting MagSafe adapters - Apple Support

Depending on the MacBook Pro model you may need to consider a battery

replacement, if the numbers and duration times do not recover. A visit to an

Apple Store or authorized Apple service provider may be helpful if testing or

a diagnostic is involved. User-replaceable batteries can be owner-replaced

and several iFixit repair guides show how to take out some models.

If battery or power issues present odd symptoms or issues, read SMC reset:

• Reset the System Management Controller (SMC) on your Mac - Apple Support

Note that if you take out a built-in battery, Apple service may decline to help;

some of these are said to be non-user replaceable. Been reported third-party

batteries may present an issue if you later revisit Apple, in such instance.

In any event...

Good luck & happy computing!