Once damaged, most of these batteries will end up as hazardous waste and possibly in landfills. Li+ cells must never be discharged below 3V to avoid the possibility of irreversible damage. Supply 4.2V (unloaded), and a nominal average voltage of 3.6V. Any further discharge can potentially cause permanent damage to the cell. A fully charged NiMh cell is about 1.4V (unloaded) it supplies a nominal average voltage of 1.2V and can discharge down to about 0.9V. This is specifically true for nickel metal hydride (NiMh) and Li-ion (Li+) batteries. The circuit's small size, low power consumption, and low cost are ideal for battery-powered systems.īatteries can be permanently damaged by frequent low-battery indications or by overdischarge. Supply current drawn by the MAX9030 is typically 35♚ when active and only 0.05♚ while in shutdown. The Figure 1 circuit accommodates any type of LBO output (active high or active low). The standard component values corresponding to this performance are: Assuming a 2.5% duty cycle and assuming that LBO trips the comparator on when battery voltage equals 3V, then the resulting trip levels are 1V for low and 2V for high. The typical V OUT for a comparator is V DD. The comparator's high and low trip levels are found using Kirchoff's current laws: Off-time is derived from the equation for time-varying voltage across a discharging capacitor: On-time is derived from the equation for time-varying voltage across a charging capacitor: The MAX9030 then turns on, causing the LED to flash in accordance with the following analysis: Note that LBO (active high) is usable as shown, but an active-low warning (active-low LBO) requires the optional circuitry shown in the box. Operating this low-battery-warning LED at low frequency and low duty cycle saves power and extends battery life. The device remains in shutdown while the battery voltage is at normal operating levels it asserts LBO when battery voltage falls below a preset threshold.įigure 1. The MAX9030 is a small, inexpensive comparator with shutdown capability in a 6-pin SC70 package. You can greatly reduce the LED's power consumption by operating it at a low frequency and a low duty cycle.Īn existing low-battery output (LBO) like that found on DC-DC converters offers a convenient way to light the LED (Figure 1). By drawing current, however, the LED exacerbates the low-battery condition. The usual method for implementing the low-battery warning featured in most battery-operated equipment is to illuminate an LED. Equations and circuit analysis are included for determining duty cycle and comparator trip points. This circuit also helps conserve battery current in the off cycle by placing the comparator in shutdown. The circuit accomplishes this without draining excessive battery current that can lead to permanent battery damage and, ultimately, hazardous waste disposal. This LBO (low-battery-indicator) is achieved by pulsing the LED at a low frequency and low duty cycle. This application note presents a circuit that uses a low-power CMOS comparator to provide an LED visual indication of a low-battery condition.
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