Why does my iPhone 5 say that I've used it for a few hours, when in reality I've only used it for a few minutes? The battery life of my iPhone 5 became so bad that it would be dead before I got back from work. So I went and bought an iPhone SE. Because I didn't migrate everything over, I've been using them side by side ever since.

Even though I'm barely using the iPhone 5, it's draining the battery much faster than the iPhone SE. I decided to change the battery of the iPhone 5, mostly because I wanted to learn how to do it, but even with the new battery things are only slightly improved.

I charge the phones over night, so they are both at 100% when I leave the flat in the morning. Then at some point in the day when my iPhone SE has 80% battery left, the iPhone 5 has 40% battery left. This is with the new battery.

I've analysed the battery usage and disabled all the background activities, that were draining the battery, but the problem persists. The most worrying thing is that it reports that I've been using it for several hours, when I've only been using it for a few minutes.

Does anyone know why this happens and how I can fix it?
Check your charging cord. The "time since last full charge" implies it was taken off charge that many hours ago. Possibly because the charging cable became disconnected or is failing.

Then of course you can try a factory reset of the phone, after a full backup. If it works then try restoring from your backup. I know this can be a bit of trouble.

But honestly an iPhone 5 battery, if original, is likely to be at the end of its life. Try replacing the battery before getting rid if the phone.