One of the major problems in Battery operated devices such as Emergency Lamp is the deep discharge of battery when the load operates for long time. If the Lamp remains on for long time, battery voltage reduces drastically and destroys the fluorescent lamp .The blackening of the ends of fluorescent tube is due to the heating up of filament in low voltage. Deep discharge also reduces the battery life. As a rule 80% current should be present in the battery for normal battery charge / discharge cycles. This add on circuit is meant for battery protection which never permits the battery to go below 4.5 V / 10.5 V and automatically shut off the emergency lamp.
The circuit can be fixed inside the emergency lamp and powered from the battery of the emergency lamp. The circuit is suitable for both 6V 7.5 Ah and 12 V 7.5 Ah batteries. Power to the circuit board of emergency lamp is provided through the Common and NO (Normally Open) contacts of the Relay so that when the relay turns off, power to the emergency lamp will automatically breaks.
When the battery voltage is above 6V / 12 V, Transistor T1 conducts and relay energize. The NO contact makes connection with Common pin and the emergency lamp gets power supply. When the terminal voltage of the battery reduces below 4.5 V / 10.5 V (as set by VR1) base current to T1 reduces and it turns off. The relay then de energize and break the power supply to the emergency lamp. The lamp lights only when the battery voltage goes above 4.5 V / 10.5 V after charging.
Cut off voltage can be set by adjusting the Wiper of VR1. Connect the probes of Multimeter to theBatteryterminals and switch on the lamp. Monitor the battery voltage. When the battery voltage drops below 4.5 volts, slowly turn the wiper of VR1 till LED goes off and Relay de energize. This is the cut off point. The same method is also applicable to 12 volt battery. Allow the battery to charge. When the voltage increases, LED light up.