Battery percentage keep changing numbers and die too fast

The first thing you may want to to is bring your iPhone into an Apple store. They can help you determine if it's a hardware issue.
If it's not a hardware issue, below are a few things you can do to preserve battery life.
One of the biggest battery drains is screen brightness.
To make sure your brightness is not too high, swipe up from the bottom of the screen. You should see this pop up. From here, you can turn down the brightness if it's too high.

The screens below are all available from Settings:

Now, you'll want to see which apps are draining the most juice.
Apple has battery usage stats built into the OS.
To see them:
1. (first to to Settings, as shown at the beginning above) Go to Usage

2. Then click "Battery Usage."

3. Voila! You can see which apps are using the most battery.

To turn off apps' ability to auto-refresh in the background:
1. (from Settings, as shown above)

2. Thengo to Background App Refresh.

3. From here, you can toggle indivual apps off, or just turn all of them off, based on what you just learned about which apps are hogging battery.

GPS is also a huge drain. Some apps are on all the time pinging your location. To adjust your location settings, and restore some sanity on your phone:
1. From settings, go to privacy

2. Then, go to "Location Services."

3. Finally, you can scroll through the list of apps using your location. I noticed that Dropbox was a battery hog, so I turned its setting from "Always" to "Never." (all Dropbox needs my GPS for is to begin a camera upload; not a critical function, I can just open Dropbox every once in a while to begin a camera upload).

My last suggestion is to turn off automatic downloads, app suggestions, etc from the App Store.
1. From Settings, go to iTunes & App Store

2. I turned all of these off. No automatic downloads of music, apps, books, or updates will happen. And Spotlight will use less juice hunting around on the App Store. I left Apps alone; this way the spotlight can still find installed apps on my device.

If anybody from Apple is listening, stop the madness!!!! Either spend more energy on getting better batteries to us, or have your UX and engineering teams handle more of this in the background.
It's crazy how fast my battery drains without doing these things.

not good

Have you tried this: completely drain your battery until the iPhone shuts off automatically. Then charge the battery to 100%.

THis helps sometimes to get the battery % showing the correct value again.