When not being used Trusted Contacts should not have any significant impact on your phone's battery. When your location is being shared your device's GPS is being used more than normal so more power will be consumed.
- If your phone runs out of power while you're sharing your location your contacts can still see the last location sent to them.
- If your location is shared with a contact while your phone is out of power (for example, if somebody asks for your location) then that contact will see the last known location of your device.
- People you've added as a contact will be able to see that your phone is out of battery.