The lock will remind you when the battery runs low both from the voice assistant within the lock and from the app. If the batteries do run out, there are two plan Bs: 1. USB port: you can power the lock with a USB to USB-C cable from your charger/ laptop/ power bank. Note that the USB-C to USB-C cable is currently not supported. 2. Backup keys: every lock comes with 2 backup keys. We recommend keeping one in your car or with someone you trust.