This circuit shows how to construct an intelligent lead acid automobile battery charger!
With this charger users do not have to worry about overcharging the battery because it automatically stops charging it when it is full.
This is useful for devices that use automobile batteries such as UPS backup power supplies.
Circuit Working:
Parts List:
| Category | Item | Quantity |
|---|---|---|
| Resistors (All resistors are 1/4 watt unless specified) | ||
| 560Ω | 1 | |
| 220Ω | 1 | |
| 4.7k | 1 | |
| 33k | 1 | |
| 3.3k | 1 | |
| 100Ω (1W) | 1 | |
| 10k | 3 | |
| Preset 25k | 1 | |
| Preset 10k | 1 | |
| Capacitors | ||
| Electrolytic 100µF 16V | 1 | |
| Semiconductors | ||
| Transistor BC547 | 2 | |
| IC 741 | 1 | |
| Diode 1N4001 | 1 | |
| Zener diode 6.2V | 1 | |
| Zener diode 9.1V | 1 | |
| LED 5mm 20mA | 1 | |
| Relay | 1 | |
| Lead acid battery | 1 |
An cheap option for expensive battery chargers is this circuit for an automatic lead acid battery charger.
Getting a cheap battery charger and adding an automatic charge cutoff circuit like the one shown above is the key for saving costs.
A comparator is the main component of this automatic lead acid battery charger.
It compares the lead acid batteries voltage to a reference voltage.
The circuit stops the charging process through the relay when the voltage of the lead acid battery reaches to the set maximum level.
On the other hand the circuit shuts OFF the relay and allows charging to continue when the voltage of the lead acid battery falls below the set minimum level.
The voltage at the non-inverting input rises in line with the battery voltage.
The non-inverting input exceeds the level of the inverting input after reaching a threshold that is been modified using P1 preset.
The comparator is built with hysteresis which is adjusted using P2 preset to stop the circuit from oscillating constantly turning the relay ON or OFF as a result of minor fluctuations in battery voltage.
The lowest battery voltage level at which charging starts is also affected by this.
Adjusting the Circuit for the Automatic Battery Charger:
The best way to calibrate is to replace the lead acid battery with a variable voltage controlled power source.
To get the relay switches to open set P1 and the regulated power supply to 14.5V.
After that adjust P2 and set the regulated power supply to 12.4V until the relay shuts once more.
This process might need to be repeated many times since changes to P1 and P2 have an impact on one another.
Formula:
Building a circuit that constantly monitors the battery voltage and adjusts the charging current is necessary while designing an automatic lead acid battery charger with the IC 741 operational amplifier.
While designing an automatic lead acid battery charger that uses an IC 741 operational amplifier the following formulas and factors are important to follow:
Voltage Regulation and Control:
Reference Voltage Zener Diode Configuration:
Use a zener diode to provide a constant reference voltage Vref.
Normally a voltage such as 6.2V or 9.1V is used across the zener diode VZ.
Vref = VZ
In this example the operational amplifier IC 741 will compare the battery voltage Vbat with the reference value Vref set by the feedback voltage divider.
Lets assume the following:
- IC 741s working amplifier supply voltage is represented by Vcc.
- The indicated reference voltage for the battery charger is Vref.
The following formula is used to calculate the resistor values for the feedback voltage divider R1 and R2:
Equation of the Voltage Divider:
The voltage dividers output voltage Vout is shown below:
Vout = Vcc [R2 / (R1+R2)]
Since R1 = P1 + R8 and R2 = R9 in this circuit design we get the following results when we substitute these numbers in the formula above:
Vout = Vcc [R9 /(P1+R8+R9)]
This formula provides an easy way to calculate the resistor values for the feedback voltage divider in the automatic lead acid battery charger circuit using the IC 741 operational amplifier.
Adjust the resistor values according to the particular voltage requirements ,the features of the power source and the operational amplifier.
How to Build:
Below are the connections steps for Automatic Lead Acid Battery Charger Circuit:
Get the Comparator Circuit ready:
- As advised in the datasheet connect the comparator IC.
- It usually has one output and two inverting and non-inverting inputs.
- To supply a reference voltage to one of the comparator inputs prepare the voltage divider with resistors.
- Connect the lead acid battery that is being charged to the comparators other input.
Include Hysteresis:
- By connecting feedback resistors between the comparators output and one of its inputs hysteresis can be generated.
- To stop oscillations in the circuit use setting P2 to adjust the hysteresis.
Connect the Relay Circuit:
- Connect the comparators output to a transistors base mostly an NPN transistor.
- Connect the transistors emitter to ground and its collector to the relays coil.
- Check that the relay is connected so that when it is active the charging circuit get stopped.
Protection and Power Supply:
- To provide a steady voltage supply to the circuit add a voltage regulator.
- To stop reverse current flow and protect sensitive components add protection diodes.
Adjustment:
- Observe the earlier described calibration process.
- Instead of using a lead acid battery use a regulated power source with variable voltage.
- Relay switches will open at the desired maximum voltage if setting of P1 preset is adjusted.
- Until the relay closes at the required minimum voltage adjust the setting of P2 preset.
- Use a lead acid battery to test the circuit after connecting the charger to a power source.
- When the battery reaches the maximum voltage be sure that the relay stops the charging circuit and when it falls below the minimum voltage it starts with charging.
Completion:
- Once the circuits functioning is positive enough complete the PCB or breadboard connections and connect the components.
Take note:
- When dealing with electrical circuits keep in mind to take the right safety precautions and make sure that the components are arranged correctly to prevent damage.
Conclusion:
The lead acid battery charging circuit automatically monitors the batteries voltage on a regular basis and adjusts the charging current and voltage as needed.
Mass charging, absorption charging and float charging are among the several kinds of charging characteristics that are often used to ensure the highest charging efficiency and batteries health.
An automatic lead acid battery charger circuit that presents a practical and effective way to charge lead acid batteries assures their long life and performance.