Unfortunately, if the battery is completely dead, tracking it in real time isn’t possible. Services like this website rely on the device being powered on to receive the tracking SMS and ping its GPS coordinates back to the system. Since a turned-off phone can’t process messages, the driver would definitely need to charge and turn it on first for the text link to do its magic. Right now, your best bet is to log into Apple’s Find My or Google’s Find My Device to see its last known location before the battery died, or call the taxi company directly. Good luck getting it back!