If you are unsure if your battery is behaving poorly, or if you have just been using your phone more than usual, or maybe you have an app that is abusing it. in iOS 12 there is a way to take a quick look.
1. First Open Settings
2. Open the Battery Menu
3. Click on Battery Health, if your battery is in really rough shape it will say "service" right next to the words Battery health
4. Results, Here Apple will tell you how much of your total battery is left, so here even if the phone says 100% it can really only go to 84%. Also, below that is Peak Performance capacity, Here you can see it is unable to meet peak performance demands, causing the phone to crash. Even if the phone said it had 40, 50 or more % left.