Frye FONIX FP40D - Manual (Page 41)

General Operation 33
You may still be able to operate when the gauge shows empty, but you are on reserve. Battery oper-
ation shuts down when the battery voltage reaches approximately 10 volts.
If the software fails and the battery discharges to 9.2V, there is an automatic hardware shutdown to
protect the batteries.
Be sure to recharge the batteries within 24 hours if the instrument shuts down (having reached
10V), to prevent damage to the batteries.
Replace the batteries when you are dissatisfied with the longevity of the charge, or when the bat-
tery does not hold a charge for more than an hour.
If you find it impossible to turn off the instrument using the OPERATE button in units with a
Battery Option, hold OPERATE down and then push RESET twice.