My iPhone is not charging beyond 80%.

Protect & Prolong life of your iPhone's Battery:

1. Put on Low Power Mode
2. Put Auto Lock On (1 min):- with low power mode it will be 30 Sec
3. Put Auto Answer Off
4. Disable Push notifications for apps you do not use often
5. Enable Auto Brightness:- Settings > General > Accessibility > Display Accommodations > Auto-Brightness [Enable]
6. Manually lower brightness from Control Center
7. If you Want to watch continuous Video. Stream it on TV or a Monitor
8. Don't force close apps; Apps which are not on frontend screen are paused by the system to save energy. Apps require more energy to re-launch. Apps which are open in the background use large memory is a myth and quitting them is counterproductive
9. Location services to be allowed only while using the app
10. Set your iPhone to fetch email and calendar data from internet less frequently
11. Turning off Background App Refresh 
12. Charge before it drops below 20-30%
13. Try connecting to trusted Wifi where ever available in stead of Cellular
14. Enable Airplane mode in areas with bad cellular and wifi coverage. Your phone keeps trying for reconnection with the missing / weak network. This uses lot of energy
15. Switch off Wifi & Bluetooth while in Roaming or Not in Range
16. Enable "Wifi Assist" : Settings > Cellular or Settings > Mobile Data > Wi-Fi Assist [Enable]
17. Periodically Check your Battery Health (Settings > Battery > Battery Health)

iPhone Battery and Performance

https://support.apple.com/en-in/HT208387

Fast charge your iPhone

Use fast charge with iPhone 8 or later. You can recharge your iPhone up to 50 percent battery in 30 minutes.*

Fast charging gives you a quick and convenient way to recharge your iPhone 8 or later. Your iPhone fast charges up to 50 percent in 30 minutes when you use an Apple USB-C to Lightning cable and one of these adapters:

• Apple 18W, 29W, 30W, 61W, or 87W USB-C Power Adapter
• A comparable third-party USB-C power adapter that supports USB Power Delivery (USB-PD)

If you're not sure which Apple Power Adapter you have, check the wattage on the bottom of the adapter:

https://support.apple.com/en-in/HT208137