Three practical 12V battery charger circuits that are used to charge any kind of lead acid battery is the topic explained in this post.
What is a Battery Charger?
An electronic device called a battery charger is used to charge batteries in a controlled way confirming that the battery is never overcharged.
In order for batteries to charge stress free and maintain a long lifespan, a battery charger is made to charge batteries with constant voltage and current.
Without any difficult electronic circuits the first 12V battery charger circuit shown here is a simple battery charger.
As soon as it is fully charged it needs to be turned OFF.
12V Battery Charger Circuit using a DC Power Supply and an Ammeter:
Below are the circuit parts and connections details mentioned:
Parts List:
| Component | Description | Quantity |
|---|---|---|
| Resistor | 1k 0.5W | 1 |
| Capacitor | 100µF 35V | 1 |
| Semiconductors | ||
| Transformer | 14V / 5A / 230V | 1 |
| Ammeter | 0 – 20A | 1 |
| Diodes | 6A4 | 4 |
| Red LED | 5mm 20mA | 1 |
This is the transformers input side.
In order to reduce the voltage for the charger it is connected to the main power source.
A bridge rectifier is connected to the secondary output of the transformer.
To charge the battery a bridge rectifier transforms the transformers alternating current (AC) into direct current (DC).
A filter capacitor is connected to the bridge rectifiers output.
By smoothing out the DC voltage the filter capacitor lessens output ripples.
A 20A ammeters positive terminal is connected to the bridge rectifiers positive output.
The current passing across the circuit is measured by an ammeter.
To charge a lead acid battery connect the 20A ammeters negative connection to the batteries positive terminal.
The lead acid batteries negative terminal is connected to the bridge rectifiers negative terminal.
This creates a complete circuit that allows current to move from the battery to the rectifier.
Let us now discuss about the Ah formulas for charging batteries:
The lead acid batteries Ah capacity should be calculated first.
Assume that your battery has a B Ah capacity.
The 20A ammeter can be used to measure the charging current (I) in amperes.
The charging time (t) measured in hours is how long one plans to charge the battery.
Use the following formulas to get the batteries total charge (in Ah):
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 the ammeter displays a steady 5A current for 5 hours than the battery gives a total of 5A * 5 hours = 25 Ah of charge.
To stop battery overcharging or damage one should see the charging current does not go above the charging current specified by the battery supplier.
During the charging process keep a close watch on batteries voltage and condition to guarantee safe and efficient 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 | Quantity |
|---|---|
| Resistors (all 1/4 watt unless specified) | |
| 1k | 1 |
| 1Ω 2W | 1 |
| 470Ω | 1 |
| 10k | 1 |
| 2.2k | 1 |
| Capacitors | |
| Ceramic 0.1µF | 2 |
| Semiconductors | |
| Voltage Regulator IC LM338 | 1 |
| Transistor BC547 | 1 |
| DC Source 15V | 1 |
| Red LED 5mm 20mA | 1 |
| Green LED 5mm 20mA | 1 |
Circuit Working:
This starts with input voltage source.
The LM338 is a voltage regulator that is configured to provide a stable output voltage.
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 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.
Formulas:
The LM338 regulator output voltage (Vout) is calculated using the LM338 datasheet formula:
Vout = Vref × (1 + R2/R1) + Iadj × R2
where:
- Vref is the internal reference voltage which is around 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 very small in the order of microamperes.
To set Vout to 14V rearrange the formula and solve for R2:
R2 = R1 × (Vout / Vref – 1)
One 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).
To keep the charging current for the battery at a safe level the user may need to choose the right resistor value for the application.
Simple 12V Battery Charger Circuit with Auto Cut Off:
The third 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 | Quantity |
|---|---|
| Resistors (all 1/4 watt unless specified)) | |
| 240Ω | 1 |
| 0.5Ω 5W | 1 |
| 1k | 2 |
| Potentiometer 10k | 1 |
| Semiconductors | |
| Voltage Regulator IC LM338 | 1 |
| Transistor BC547 | 1 |
| Transformer 0 – 12V 5A | 1 |
| Diodes 6A4 | 5 |
| Zener Diode 14V 1W | 1 |
| Red LED 5mm 20mA | 1 |
| Green LED 5mm 20mA | 1 |
| Capacitors | |
| Electrolytic 1000µF 25V | 1 |
| Ceramic 0.1µF | 1 |
The circuit starts with a 15V DC power source 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.
This circuit is used to provide a stable and adjustable output voltage.
By connecting the input voltage to the LM338 set the output voltage to a desired level.
The output of the LM338 is connected to the lead acid battery for charging.
The circuits 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 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 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 it acts as an auto cut off mechanism to prevent overcharging.
Conclusion
We have seen how different designs from simple capacitive chargers to more effective transistor and regulator based models offer various levels of control and protection in the context of 3 Simple 12V Battery Charger Circuits
These circuits offer a strong basis for secure and efficient 12V battery maintenance no matter whether a beginner looking for an easy solution or a hobbyist hoping for better charging performance.