The new Li-ion batteries do not appreciate being run down. Keep topped up and the battery will last much longer. Run down and you may lose life or even kill the battery if too far down.
Some people suggest dropping below 30% charge occasionally to reset the phone monitoring meter. Can't tell if or not that is an issue, but the recommendation is common around here.
Lots of information about these batteries at
Welcome to Battery University in the section on Li-ion.