Lead Acid Battery Charge Level Indicator

Lead acid battery voltage level monitoring system

Purpose

The motive of this project is to design an efficient and cost effective lead acid battery voltage level monitoring system to display some specific levels of the lead acid storage battery voltage (11-14 volts) as glowing LEDs.

Parts List

LM324 IC (housing 4 op-amps)
Resistors ( of values 680Ω, 1kΩ, 1.2kΩ,10kΩ)
Variable resistor (of 10kΩ)
Zener Diode (6.9V)
LEDs
Power Suply

Circuit Diagram

Description

This a very useful and simple circuit to keep a check on the voltage level of your car battery. The circuit is built around comparators of quad op amp LM324. When the battery is full i.e. it is providing its maximum at 14V all the LEDs will be glowing. When there is decrease in the input supply the output of each op amp goes low accordingly as they are wired as voltage comparators. Hence the corresponding LED goes off.

WORKING

The battery voltage is directly fed to the non inverting input of all the comparators through the voltage divider arrangement of R1 and R7. A battery voltage of 14V will be reduced to 6.90V, 13V will be reduced to 6.40V, 12V will be reduced to 5.91V and 11V will be reduced to 5.41V due to series arrangement of resistors R1 and R7. This reduced voltage is applied at pins 3,5,10 and 12. As these pins are connected with each other the same reduced input voltage will appear on each of the pins.

Now we have to provide some reference voltages to the comparators to compare the reduced battery voltage against them. The 6.9V zener diode ensures that there is a voltage difference of 6.9 volts across the voltage divider network of R3, R4, R5, and R6 in case when the input voltage is between 11 and 14 volts. R2 limits the current through the zener diode.

The reference voltages have been fixed at 6.9V, 6.4V, 5.9V & 5.4V at pins(2,6,9 & 13) by the voltage divider network of resistors R3, R4, R5 & R6 as shown in the table below.

Input voltage	V_reference	pins
11-14 volts	6.90 v	2
	6.40 v	6
	5.91 v	9
	5.41 v	13

After analysis of the circuit the value of the variable resistor R6 was set at 7.35kΩ to get the desired reference voltages.

The reduced battery voltage is being compared against the various reference voltages applied at the inverting inputs of the four different comparators. For example if the battery voltage is 14V the reduced voltage will be 6.90V which will be compared with a reference voltage of 6.90V by the first comparator, 6.40V by the second comparator, 5.91V by the third comparator and 5.41V by the fourth comparator. As 6.90V is equal to or greater than all the reference voltages of all the four comparators so all the four LEDs will glow indicating a healthy battery charge.

If the battery voltage is 13V the reduced voltage will be 6.40V which will be compared with a reference voltage of 6.90V by the 1st comparator. As 6.40V is less than the 6.90V (reference voltage of 1st comparator), its LED will not glow but the LEDs of all the other three comparators will glow because 6.40V is equal to or higher than their respective reference voltages.
When battery voltage, during discharging, drops below the reference voltages of the corresponding comparators their LEDs will go off.

At 12V battery voltage the two lower LEDs will glow while for 11V battery voltage only the lowest LED will glow. No LED will glow if the battery voltage drops below 11 volts showing a deadly discharged battery.

APPLICATIONS

This circuit

1. can be mounted on car dash board to monitor the battery charge level.

2. Can be used with UPS batteries to indicate charge level.

3. Can be interfaced with battery chargers/solar battery chargers/UPS to get over charge/over discharge protection.