The battery is actually quite large. Our battery pack is 2600mah @ 11.1 nominal volts. 

We use three A Grade 18650 cells and that gives you over 50 hours play time between charges and will also give you plenty of power to charge your phone. Keep in mind that a small phone charger with 5v DC output is called 2600mah, but is often just one 18650 cell and a boost converter.