# Battery Voltage Monitor. Design Note 36

This is the simplest type of Voltage Monitor you can build in 5 minutes. No need for a PCB and just solder lead to lead and hook up in the Emergency lamp. As long as the battery voltage is above 5 volts, LED lights to indicate a healthy battery. When the battery voltage drops below 4.7 volts, LED turns off to indicate the time for charging. It is a must for emergency lamp since deep discharge  permanently destroys the battery. Let us see what is happening in the emergency lamp battery.

The emergency lamp battery is 6 volt and 4.5 Ah. That means it can deliver 4.5 Ampere current in 1 hour. A new fully charged battery shows 6.2 volts which drops when the lamp is running and needs recharging again.

What usually happens if we keep the battery under charge for long time? Battery over charges and the heat developed inside it will damage the battery permanently. What happens if the battery is not properly charged ?  Deep discharge causes “Memory effect” in the battery and it never attains full charge later. In short, both over charging and deep discharging permanently destroys battery.

So what is the ideal voltage in the battery? It should be between 4.5 volts and 6.2 volts. If the lamp is run on a deep discharged battery, the tubes still light with low light. But the ends of tubes darken and gradually the tube damages. So the lamp should be switched off when it starts dimming.

What this circuit indicates?

The circuit indicates the low voltage level of battery below 4.7 volts. When the battery holds more than 5 volts, LED lights. If the voltage goes below 4.7 volts, LED turns off. So you can charge it again. If properly maintained, the battery will last 3 to 4 years.

How the circuit works?
Simple and nothing complicated. A Zener is doing the trick. Zener is a special kind of diode that conducts only when the voltage into it is above its rated value. For example, the 4.7 volt Zener conducts only if it gets 5 volts or more. So here the 4.7 volt Zener, an LED and a current limiting resistor (R1) are connected in series. Note that, the Zener is connected in the reverse order. This “reverse biasing” is the method of Zener connection. The end of Zener having a black ring is Cathode and it should go to the positive side. When the voltage of the battery is above 5 volts, Zener conducts and LED gets current and it lights. When the battery voltage drops below 5 volts, Zener switches off and LED turns off. That much is the theory in it. The Red LED takes 1.8 volts but its current consumption is low around 16 Milli Amperes.

Red LED has the forward voltage drop of 1.6 volts. So the voltage at the cathode of LED is 1.6 volts. Current limiting resistor is 100 Ohms. So just apply Ohms law
I = V / R = 1.6 / 100 = 0.016 Amps or 0.016 x 1000 = 16 Milli Amps
How to set?

Solder lead to lead and connect the cathode of Zener to the point of lamp switch that goes to lamp circuit board. The wire from it is going to the circuit board. So when the switch is on, both the circuit board and the LED indicator gets power. Drill a 5 mm hole on the case and fix the LED there. That is all. Enjoy its simplicity and use.

See the bread board assembly of the circuit

Prototype powered by a discharged 9 volt battery. Input voltage is 5.36 volts. LED is on

Prototype powered by a discharged 9 volt battery . Input voltage is 3.19 volts – LED off

No need for any adjustments. Simply hook it. It will do its job.