Hi there DaleH,
Thanks for using the Apple Support Communities. I understand you’d like to receive a notification for when your iPhone battery gets low when not in use. I’ll be happy to help with this.
While there is no built-in option for this, there are third-party apps that claim to do what you want. Just search the App Store for ‘Low battery alert’ or ‘battery alert’ and you should find some options.
Also, if your iPhone is completely running out of battery during the day, you can take some steps to help prevent this from happening. One way would be to follow the tips in our Maximizing Battery Performance article. This provides ways to not only improve your iPhone’s day to day life, but the overall lifespan as well:
Batteries - Maximizing Performance
You can also turn on Low Power Mode. When Low Power Mode is on, your iPhone will last longer before you need to charge it, but some features might take longer to update or complete. You can read more about using Low Power Mode here:
Use Low Power Mode to save battery life on your iPhone
Best wishes.