Owning a smartphone can often be a balancing act of usage and power conservation. If you're noticing that your phone is draining a lot quicker than usual, however, there is a way you can easily find the culprit. iOS and Android both offer statistics on what apps are using up your battery.
On iOS, just open the Settings app and tap on "Battery." You'll see a list of apps in order of how much battery they've consumed. There's two categories: You can compare usage from the last 24 hours and the last 7 days. There's also a clock icon that when pressed, will tell you exactly how long each app has been running on sceen and in the background. At the bottom of this screen, you'll see the total usage time since the last full charge and the time that the phone was in standby.
If you're on Android, you have a little bit more flexibility in being able to see what's taking up battery power. Open the Settings app from the app drawer and then tap the "Battery" option. This will tell you not only the apps that have consumed battery, but device services as well. It will even tell you how much power was used by the screen, Wi-Fi, operating system, Google Services, and more. There's also third party programs that will tell you even more about the battery life, such as GSam Battery Monitor.
There are other things that can explain battery drain that might not show up in the battery statistics (especially in the case of iOS). If the phone is in a low signal area and is constantly hunting for signal, that can drain the battery as well.
As far as Lithium Ion battery care is concerned, it never hurts to just keep the battery charged whenever possible. It's most important to not let the battery completely drain, but most devices will protect against that by shutting off before the battery is totally depleted.