If I had a dollar for every time someone complained about their iPhone's battery, well, I'd have a lot of dollars. Breakages aside, battery performance issues make up the bulk of the complaints I get about the iPhone.
I've been doing a lot of battery testing on the iPhone lately, and I've narrowed down the five main culprits responsible for draining battery life on the iPhone.
Must read: Apple products you shouldn't buy in 2018
Without a doubt the biggest battery drain. Both charging the battery in the cold, and using the iPhone in the cold. While hot weather can have a detrimental effect on performance and battery life as well, nothing zaps battery life like the cold can.
Room temperature -- around 20°C/70°F -- is the best temperature for charging equipment. However, since we don't all live in climate-controlled rooms, we can extend this range out to 5 to 45°C (41 to 113°F). Anything to either side of this and things can get bad for the battery. This is especially true for charging the battery at temperatures below 0°C/32°F, which can permanently damage the battery.
I find modern displays to be crazy bright, and that backlight is a huge drain on the battery -- especially if you use your iPhone a lot of you get a lot of notifications that activate the display.
I recommend dimming the display and turning on Auto-Brightness to get the best battery life.
You could add poor Wi-Fi to this too, but poor coverage has a far greater impact of battery life because the iPhone is constantly hunting for a better signal.
Sometimes a poorly coded app can tear through your battery. I've even seen very popular apps such as Facebook or the YouTube app have bugs that cause them to rip through a fully-charged battery in a few hours, even when they are not in use!
This is not that common, but I have come across a few people where the culprit for battery drain was just endless notifications coming in while their iPhone was locked was causing the display to switch on constantly.