Is it possible to calculate remaining battery time in hours and minutes? Also, would it be able to calculate this time depending on if I'm using a certain app? Is it even possible to get such information? If you could please give me advice on how to do that, I'd be very thankful.
Estimating the remaining battery life is based on analytics. As the other people said you have to listen for battery level changes and in addition you have to keep track of them. After some time you will have enough data to calculate what is the average time the battery lasts. In addition you know when the battery drains fast and when drains slow so you can improve your estimation based on this. Also you will know in what time the user charges the devices. There are a lot of events that can be tracked and using the battery level. In addition you can also track when the screen is on or off. The algorithm of calculating the remaining battery life depends on you :)
I hope this explains (at least a bit) the idea of the estimation the battery life.