This high current battery charger circuit is a simple yet effective with automatic cut off at a preset voltage.
The TIP35 transistor acts as a voltage follower ensuring that the charging voltage is stable and constant making it suitable for charging high capacity lead acid batteries.
What is a High Current Battery Charger Circuit:
A circuit for a high current battery charger is made to supply an adequate current in order to quickly charge a rechargeable battery.
The type of battery being charged such as lead acid or lithium-ion as well as the desired charging factors decide how such a circuit is designed.
Circuit Components Functions:
Parts List:
| Category | Component | Quantity |
|---|---|---|
| Resistor | 50Ω 2W Resistor | 1 |
| Semiconductors | IC 7815 Voltage Regulator | 1 |
| TIP35 Transistor | 1 | |
| Other | 12V Lead Acid Battery | 1 |
IC 7815 Voltage Regulator:
- The IC 7815 is a positive voltage regulator that outputs a constant current of +15V DC.
- It has three pins: Input Vin, Ground GND and Output Vout.
- In this circuit an 18V DC power source is connected to the input of Vin pin of the IC 7815.
TIP35 NPN Power Transistor:
- The TIP35 is a high power NPN bipolar junction transistor BJT capable of handling high currents (up to 25A).
- It has three pins: Collector C, Base B and Emitter E.
- The collector is connected to the positive side of the 18V DC power source.
- The emitter is connected to the positive terminal of the high Ampere hour (Ah) lead acid battery.
Lead-Acid Battery:
- The high Ampere hour Ah lead acid battery is the main component being charged in this circuit.
- The positive terminal of the battery is connected to the emitter of the TIP35 transistor.
Formulas:
Designing a high current battery charger circuit requires the following formulas:
This is calculated by the capacity (Ah rating) of the battery and the advised charging rate supplied by the battery manufacturer.
It is generally safe to use 0.1 times the charging currents Ah rating.
Thus, a 100Ah batteries normal charging current would be as follows:
Charging Current (A) = 0.1 * Battery Capacity (Ah)
Resistor for Current Limiting:
In basic circuits using a regulator IC like the LM317 a resistor controls the charging current with the formulas below:
Resistor (Ω) = 1.25 / Charging Current (A)
Charging Time:
This provides you with an approximate estimate of the battery charging time.
Keep in mind that this is only a rough figure and does not account for the delays.
Charging Time (hours) = Battery Capacity (Ah) / Charging Current (A)
Circuit Working:
The 18V DC power source is connected to the Vin pin of the IC 7815 voltage regulator.
The IC 7815 regulates this input voltage to a constant +15V DC.
The +15V DC output from the IC 7815 is connected to the base B of the TIP35 transistor.
The IC 7815 provides a stable reference voltage for the TIP35.
The TIP35 transistor is configured as a voltage follower circuit.
The collector C of the TIP35 is connected to the positive side of the 18V DC power source.
The emitter E is connected to the positive terminal of the lead acid battery.
When the IC 7815 provides a constant +15V at its output it biases the TIP35 into working.
The voltage at the emitter of the TIP35 follows the voltage at its base which is the regulated +15V from the IC 7815.
The voltage drop across the TIP35 transistor which is around 0.7V so the output voltage at the emitter is approximately 14.3V 15V to 0.7V.
The 14.3V output from the TIP35 is used to charge the lead acid battery.
Since the voltage is constant the charging current is also constant.
When the battery voltage reaches 14.3V the charging current is automatically cut off as the voltage at the emitter of the TIP35 is no longer higher than the battery voltage.
How to Build:
The steps to construct a circuit for a high current battery charger are as follows.
- Connect the IC 7815s input Vin pin to the 18V DC power supply positive side.
- Wire the IC 7815s GND pin to the power supply of negative side.
- Join the IC 7815s output Vout pin to the TIP35 transistors base pin.
- Connect the TIP35s collector pin to the 18V DC power supply positive side.
- Wire the TIP35 transistors emitter to the lead acid batteries positive terminal.
- To eliminate the heat while operating make sure the TIP35 is mounted on a heat sink.
- Connect the lead acid batteries negative terminal to the power supply of ground terminal
- Add resistors for biasing and capacitors for filtering depending on circuits particular needs and stability concerns.
- Consult the IC 7815 and TIP35 datasheets for the specified designs.
- Turn the 18V DC power source on.
- Measure the output emitter voltage of the TIP35 transistor using a multimeter it should be around 14.3V.
- Check that there is an increase in the voltage across the lead acid battery.
- Make sure the current entering the battery stays within the safe charging ranges.
Notes:
- Ensure that the TIP35 transistor has proper heat sinks to dissipate heat especially when charging high Ampere hour batteries.
- Depending on the specific requirements one need to include current limiting mechanisms or use a current limiting power supply to protect the circuit and the battery.
Safety Precautions:
- When working with high current circuits be careful and make sure all connections are tight and protected.
- To avoid overheating keep an eye on the TIP35 transistors temperature while it is operating.
Conclusion:
Always consult the datasheets of the needed components such as TIP35 and IC 7815 for full details and suitable operating conditions.
Adjustments are probably required depending on the particular features of the parts you are using.
For professional advice, if any doubt kindly leave a comment below.