Battery-operated equipment often requires a low-voltage monitor to warn the user when the battery voltage is too low or to perform other functions, such as power-source switching or device shutdown. Many specialized ICs are available to satisfy this need, but most target applications that use fairly low-voltage lithium or NiCd batteries. Thus, they’re not directly compatible with applications using lead-acid batteries, which may go as high as 14 V during charge.
If you need only a simple logic-level change in response to a low-voltage condition, you don’t have to overpower the design with sophisticated battery-management circuits. A comparator and a few additional parts can do the job instead.
The figure shows a monitor designed for inexpensive circuits powered directly from the battery, without an intervening voltage regulator chip. The monitor requires only a comparator, a voltage reference, four resistors, and a capacitor. When the battery voltage is greater than the design threshold, the comparator output is high. When the battery voltage drops below the threshold, the output goes low and remains low until reset by recycling the applied power.
The comparator, U1, is an open-collector (open-drain) type, and the reference is an LM136-2.5. When the battery is connected to the circuit, the reference voltage at pin 2 of U1 will rise rapidly to 2.5 V and remain there. The capacitor, C1, will pull U1’s pin 3 above the pin 2 voltage and the comparator output will go high.
As the capacitor takes on a charge, the voltage at pin 2 will assume a value determined only by the voltage divider comprised of R2, R3, and R4. This relationship can be used to set the battery voltage at the comparator trip point:
As long as the voltage at pin 3 is higher than VREF, the output will stay high. However, if the voltage at pin 3 falls below the reference voltage, the comparator will trip, sending the output low and clamping pin 3 to ground. The output will remain low even if the battery voltage recovers, as is often the case when the load is removed.