Battery Alerts let you know when a friend's phone is running low on battery. An alert is usually sent once the battery drops below 20%, but the exact timing can vary depending on the device model, its internet connection, and other settings.
When an alert is sent, you'll receive a notification like this:
You can turn on or edit Battery Alerts like the following:
iSharing app > Menu > Manage Alerts > Battery Alerts > Turn on/off for specific friends
If your friend uses an Android phone, you can also receive Power Alerts — extra notifications sent when their phone turns off or back on. Power Alerts are activated automatically once you turn on Battery Alerts for that friend.