This simple Battery Monitor lights an LED when the battery voltage drops below 9 volts. It is an ideal add on circuit to monitor the charge level in 12 volt miniature batteries used in Portable devices or Alarm systems. In the standby state, LED remains off.
Working of the circuit is based on the base biasing of transistor T1. When the battery voltage is above 9 volts, base-emitter voltage will be same. This keeps both T1 and LED off. When the battery voltage reduces below 9 volts due to consumption, base voltage of T1 drops while its emitter voltage remains same since capacitor C1 is fully charged. At this stage, base of T1 becomes positive and T1 turns on. Capacitor C1 discharges through the LED and it lights.