AccuBattery is a really good app on the play store that monitors your phone’s battery, how fast it charges, discharges, how much the battery has degraded, what apps are using the most battery, and so on. Over time, you start to realize what takes up a ton of battery, and what barely takes any battery up. It also monitors the amount of time your phone is in a deep sleep mode and gives statistics on whether the screen is on/off. It can be useful for spotting problems.
You can download it from here.