Battery voltage monitor and protector using transistor

Irrespective of their applications, a good power supply is always needed to propagate electronic circuits. Hence, it becomes a priority to keep record of power supply, especially when the circuits and their corresponding gadgets are battery powered. Systems such as household power supply inverters, emergency lighting units are placed in this category where dependence on battery is always there. So, the user always needs to keep an eye on the battery levels so that these systems do not run out of power.

Fig. 1: Image Showing Battery Indicator

Most of the Emergency lamps do not have low battery indicator which is essential to indicate the deep discharge of battery. So if the Emergency lamp is switched on continuously, battery will discharge deeply and causes 2 problems. 1. Ends of the fluorescent tube will turns black and fails. 2. If the battery is discharged fully, it may develop memory effect and will not accept full charge. To avoid these problems, the battery should not be allowed to discharge below its 50% efficiency. This simple add on circuit will give a visual indication when the battery enters in to the deep discharge stage.

6v battery

Fig. 2: Image of a Battery

A few components are sufficient to make this battery level indicator. It can be connected along with the existing circuit board.

When the power is switched on and if the battery is holding 6 volts, both Red and Green LEDs light. Red LED indicates that the emergency lamp is on. Green LED indicates that the battery voltage is above 4.5 volts (Preset level). When the battery voltage drops below 4.5 volts, Green LED turns off indicating that the battery is entering into the deep discharge stage.

When the power is switched on, Red LED lights and current passes through VR and the wiper of VR is set so as to forward bias T1 at 4.5 volts. So till the voltage drops to 4.5 volts, T1 conducts and Green LED lights.

How to set

1. Fully charge the emergency lamp battery. The terminal voltage should be 6.8 volts in the fully charged state.

2. Switch on the emergency lamp and monitor the battery voltage using a multimeter.

3. Fix the low voltage level. Say 4 or 4.5 volts. Keep the lamp switched on till the terminal voltage of the battery drops to the desired low voltage.

4. Switch off the lamp and check the voltage. If it is the desired low voltage level, say 4.5 volts, slowly adjust VR till Green LED turns off.

5. Charge the battery again. When the voltage level rises, Green LED turns on.