The Oregon RFID readers will go into sleep mode when the battery is below a voltage threshold (factory setting is 10 volts) or when the sleep timer is enabled. The sleep timer is used to run the reader only at times when detections are expected (e.g. only run at night for nocturnal fish).
Sleep mode is indicated by a slow 1-2-3-4 light pattern on the status LEDs. The datalogger continues to run but the reader has been sent a command to stop scanning to save power.
The reader can erroneously go into sleep mode if the voltage calibration is wrong. Check the supply voltage and compare it with the value displayed on the PDA. If it is not the same then tap on the value on the PDA screen to bring up a dialog to correct the voltage. The same process can be done with the AV command where the supply voltage is entered in millivolts (e.g. for 12.5 volts type "AV 12500".
The shutdown and startup voltages are shown on the Setup screen of the PDA. Tap on them to change the values.
Lead acid batteries can be damaged if they are discharged below 9 volts. The startup voltage should be 2 volts above the shutdown value or the reader can enter into a phase of constantly shutting off an on.