In this post we will discuss 3 useful 12V battery charger circuits which can be used to charge all types of lead acid batteries.
What is a Battery Charger?
A battery charger is an electronic device which is used to charge batteries with controlled charging, ensuring the battery is never overcharged.
A battery charge is designed to charge batteries with constant voltage and constant current so that the battery is able to charge without any stress and retains a long life.
The first 12V battery charger circuit explained below is a simple battery charger without any sophisticated electronic circuit.
It must be switched OFF as soon as it is fully charged.
12V Battery Charger Circuit using a DC Power Supply and an Ammeter
Here are the components and connections in the circuit:
Parts List:
| Component | Description | Quantity |
|---|---|---|
| Resistor | 1k / 0.5W | 1 |
| Capacitor | 100µF / 35V | 1 |
| Transformer | 14V / 5A / 230V | 1 |
| Ammeter | 0 – 20A | 1 |
| Diodes | 6A4 | 4 |
| Red LED | 5mm, 20mA | 1 |
This is the input side of the transformer.
It’s connected to the mains power supply to step down the voltage for the charger.
The transformers secondary output is connected to a bridge rectifier.
A bridge rectifier converts the alternating current AC from the transformer into direct current DC for charging the battery.
The output of the bridge rectifier is connected to a filter capacitor.
The filter capacitor helps smooth out the DC voltage reducing ripples in the output.
The positive output of the bridge rectifier is connected to the positive terminal of a 20A ammeter.
An ammeter measures the current flowing through the circuit.
The negative terminal of the 20A ammeter is connected to the positive terminal of a lead acid battery which you want to charge.
The negative terminal of the bridge rectifier is connected to the negative terminal of the lead acid battery.
This provides a complete circuit for the current to flow from the rectifier to the battery.
Now lets discuss the Ah formulas for battery charging:
First you need to determine the capacity of the lead acid battery in Ah.
Lets say your battery has a capacity of B Ah.
The charging current (I) in amperes can be measured using the 20A ammeter.
The charging time (t) in hours is the duration for which you intend to charge the battery.
The formula to calculate the total charge (in Ah) delivered to the battery is:
Ah = I * t
where:
- Ah is the current capacity of the battery, I is the charging or the discharging current and t is the time.
For example, if your ammeter shows a constant current of 5A for 5 hours, you are delivering a total of 5A * 5 hours = 25 Ah of charge to the battery.
Make sure that the charging current does not exceed the battery manufacturers recommended charging current to avoid overcharging or damaging the battery.
Also monitor the battery voltage and condition during the charging process to ensure safe and effective charging.
Simple 12V Gel Battery Charger Circuit using LM338 IC
The second battery charger explained below uses a LM338 IC to deliver a constant current and a constant voltage to the battery to ensure a very efficient and a healthy charging for the battery.
Parts List:
| Component | Description | Quantity |
|---|---|---|
| Resistors (all 1/4 W CFR) | ||
| 1k | 1 | |
| 1Ω / 2W | 1 | |
| 470Ω | 1 | |
| 10k | 1 | |
| 2.2k | 1 | |
| Capacitors | ||
| 0.1µF | 1 | |
| Voltage Regulator | LM338 | 1 |
| Transistor | BC547 | 1 |
| DC Source | 15V | 1 |
| LEDs | ||
| Red LED | 1 | |
| Green LED | 1 |
Components Functions
This starts with input voltage source.
The LM338 is a voltage regulator that is configured to provide a stable output voltage.
In your case, you want to adjust the output voltage to 14V for charging a 12V battery.
The BC547 transistor is used to control the adjust pin of the LM338 IC.
By varying the voltage at the ADJ pin, you can adjust the output voltage of the LM338.
An LED is connected between the collector of the BC547 transistor and the ADJ pin of the LM338.
It serves as an indicator to show the charging status.
When the LED is on it indicates that the battery is charging and when it turns off it signals that the battery is fully charged.
There is a resistor between the base and emitter of the BC547 transistor.
This resistor is used for current sensing and controlling the charging current.
Working Principles:
The LM338 voltage regulator is configured to output a voltage of 14V which is suitable for charging a 12V battery.
The BC547 transistor controls the voltage at the ADJ pin of the LM338.
By varying the base current to the transistor, you can control the output voltage of the LM338.
This setup allows you to adjust the charging voltage.
The LED is connected in parallel with the ADJ pin of the LM338.
When the BC547 transistor is turned on the LED is illuminated indicating that the battery is in the charging mode.
As the battery charges and its voltage approaches the set 14V level the LM338 output voltage decreases reducing the current through the BC547 transistor, causing the LED to turn off when the battery is fully charged.
The current sensing resistor between the base and emitter of the BC547 transistor limits the base current and, in turn the charging current to a safe level.
Formulas:
The LM338 regulator output voltage (Vout) can be calculated using the LM338 datasheet formula:
Vout = Vref × (1 + R2/R1) + Iadj × R2
where:
- Vref is the internal reference voltage (approximately 1.25V for LM338).
- R1 is the resistance between the ADJ and Vout pins.
- R2 is the resistance between the ADJ and the ground.
- Iadj is the current flowing into the ADJ pin, which is typically very small (in the order of microamperes).
To set Vout to 14V, you can rearrange the formula and solve for R2:
R2 = R1 × (Vout / Vref – 1)
You can adjust R2 using the potentiometer to achieve the desired 14V output voltage for battery charging.
The current through the BC547 transistor and, consequently the charging current can be controlled by the current sensing resistor between the base and emitter of the BC547.
The charging current is related to the base current and the transistors gain (hfe).
You may need to select an appropriate resistor value for your specific application to limit the charging current to a safe level for your battery.
Simple 12V Battery Charger Circuit with Auto Cut Off
The next circuit is a 12V battery charger with an auto cut off feature designed to prevent overcharging of the lead acid battery.
It is also a fast charger circuit which ensures a fast charging for the battery.
Parts List:
| Component | Description | Quantity |
|---|---|---|
| Resistors (all 1/4 W CFR) | ||
| 240Ω | 1 | |
| 0.5Ω | 5W | 1 |
| 1k | 2 | |
| Potentiometer | 10k | 1 |
| Voltage Regulator | LM338 | 1 |
| Transistor | BC547 | 1 |
| Transformer | 0 – 12V 5A | 1 |
| Diodes | 6A4 | 5 |
| Zener Diode | 14V 1W | 1 |
| Capacitors | ||
| 1000µF | 25V | 1 |
| 0.1µF | 1 | |
| LEDs | ||
| Red LED | 5mm, 20mA | 1 |
| Green LED | 5mm, 20mA | 1 |
The circuit starts with a 15V DC power source, which is connected to the input of an LM338 voltage regulator circuit.
This LM338 voltage regulator is responsible for providing a controlled output voltage to charge the battery.
The LM338 is an adjustable 3 terminal voltage regulator.
In this circuit, it is used to provide a stable and adjustable output voltage.
By connecting the input voltage to the LM338, you can set the output voltage to a desired level.
The output of the LM338 is connected to the lead acid battery for charging.
In this circuit, a negative zener diode is used for voltage sensing and feedback.
The zener diode is rated at 14V.
When the battery is being charged and its voltage reaches or exceeds 14V, the zener diode breaks down and conducts.
The collector of the BC547 transistor is connected to the ADJ (adjust) pin of the LM338 IC.
The ADJ pin on the LM338 is used to set the output voltage.
By changing the voltage at this pin, you can adjust the LM338 output voltage.
When the battery voltage reaches or exceeds 14V the zener diode conducts and the BC547 transistor is switched on.
When the transistor is turned on, it effectively connects the ADJ pin of the LM338 to ground.
This causes the LM338 to regulate its output voltage down to a very low level effectively shutting off the charging current to the battery.
The battery is no longer being charged, and this acts as an auto cut off mechanism to prevent overcharging.
Conclusion
To conclude, when the battery voltage reaches 14V, the zener diode triggers the BC547 transistor to turn on which in turn, grounds the ADJ pin of the LM338 causing the LM338 to reduce the output voltage to a minimum effectively cutting off the charging current.
This mechanism prevents overcharging and helps protect the lead acid battery from damage due to excessive voltage.
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