The Passive TrackBox will switch into standby at 3.5V battery voltage indicating 0%. The box will then continue to run until 3.4V, which can be several hours. During this time at some point, the MODEM will stop working and the box may start blinking Red-Red-Red-Red-Red-Red-Red...Stored Passings will not be lost as long as the box is not turned off! Charging for 10 minutes should allow it to connect again.