To prevent battery damage due to over -disc barge, a low-voltage detector and switch should be included in the design of the battery backup circuit. The detector circuit should consume extremely low current. The switch should exlnbit a low-voltage drop and be easy to control. R1 and D2 provide a trickle charge for the battery. Chosen for its low forward voltage drop, Schottky diode D3 prevents forward polarization of the diode incorporated in Ql.
When the battery voltage is above approximately 8 V, the output of U1 is low and Ql is turned on. If the battery voltage falls below 8 V, the output of U1 increases and turns off Ql.