The battery monitor in Fig 1 nominally draws 5 µA during normal operation and flashes an alarm indicator when the battery voltage drops below a critical value. The alarm-state current depends on the desired indicator brightness, but you can easily make the current average less than 500 µA. This monitor was developed as a change-battery warning in a digital panel meter with a battery life measured in years.
The circuit uses the unique features of a dual comparator and reference, IC1, to construct an input low-voltage detector to control an RC LED flasher, D3. The design uses the MAX923 as opposed to the MAX933 because of the 923's 1% on-chip reference. The components in Fig 1 are for use with a two-cell lithium supply that blinks the alarm LED for 25 msec twice per second when the battery voltage drops below 4.7V. You can adjust the component values for circuit operation over a range of 2.7 to 11V, which are the operating limits of IC1.
Divider R1/R2 sets the alarm indication threshold. The low-going threshold at the comparator input is nominally equal to VREF (1.182V) minus VHYSTERESIS (50 mV max). Divider R3/R4 sets the hysteresis for both comparator sections. The values in Fig 1 set the comparator thresholds approximately 50 mV above and below the reference voltage.
R5 and D2 shorten the LED's on time to significantly reduce the average current drain during the alarm state. Removing these components results in approximately symmetrical LED on and off times as set by R6 and C1. R7 sets the LED on current up to a maximum of 17 mA. D1, D2, and C1 must be very low-leakage devices. The 1N457A has a leakage of only 25 nA. C1 should be a high-quality multilayer ceramic capacitor. The component count for this circuit is somewhat high, but the advantage of low-idle current resulting in extended battery life was the determining factor for a particularly power-critical application. (DI #1685)