Battarang can send alerts to your chosen device whenever your tablet's or secondary phone's battery is low or full.
To set it up, watch this review on https://youtu.be/xthkvsnNb-8?t=238 or visit https://battarang.anissan.com
This service is completely free of ads and trackers.
You can even download this APK for free from https://github.com/ni554n/battarang-notifier-android if you're willing to update it manually.
Feel free to contact me at battarang@anissan.com if you encounter any issues.