While I currently use an iPhone 8+, a still have the iPhone SE that I bought in March, 2016. With all of the attention to batteries on the iPhone, I decided to check this device’s battery. It hadn’t seemed particularly slow to me, but the battery hadn’t been lasting a full day for some months before I got the iPhone 8+ and stopped using the SE.
Kirk uses iMazing, a Mac app, to check the iPhone SE battery health (current max charge vs max charge when it was new). If you are thinking about checking and, possibly, replacing your iPhone battery, this is an excellent case study.
Side note: You might also check the Coconut Battery app, which will tell you about your Mac battery, as well as the battery in any USB connected iOS device.