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 high current battery charger circuit is designed to deliver a significant amount of current to quickly charge a rechargeable battery.
The design of such a circuit depends on the type of battery being charged e.g. lead acid, lithium-ion and the desired charging characteristics
Circuit Components:
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 +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 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 involves a few key formulas:
This is determined by the capacity (Ah rating) of the battery and the suggested charging rate supplied by the battery manufacturer.
Using 0.1 times the Ah rating for the charging current is a safe rule of thumb.
Therefore, a typical charging current for a 100Ah battery would be:
Charging Current (A) = 0.1 * Battery Capacity (Ah)
2. Resistor for Current Limiting (Simple Circuits):
In basic circuits using a regulator IC like the LM317, a resistor controls the charging current.
Resistor (Ω) = 1.25 / Charging Current (A)
3. Charging Time (Estimated):
This provides you with an approximate estimate of the battery charging time.
Recall that this is an estimate and does not take charging inefficiencies into consideration.
Charging Time (hours) = Battery Capacity (Ah) / Charging Current (A)
Circuit Operation:
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 conduction.
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 is typically 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 following explanation provides a step by step guide regarding how to build this high current battery charger circuit.
- Connect the input Vin pin of the IC 7815 to the positive side of the 18V DC power supply.
- Connect the ground GND pin of the IC 7815 to the ground of the power supply.
- Connect the output Vout pin of the IC 7815 to the base B pin of the TIP35.
- Connect the collector C of the TIP35 to the positive side of the 18V DC power supply.
- Connect the emitter E of the TIP35 to the positive terminal of the lead acid battery.
- Ensure that the TIP35 is mounted on a heat sink to dissipate heat during operation.
- Connect the negative terminal of the lead acid battery to the ground of the power supply.
- Depending on the specific requirements of your circuit and stability concerns, you may need to add resistors for biasing or capacitors for filtering.
- Refer to the datasheets of IC 7815 and TIP35 for recommended configurations.
- Power on the 18V DC power supply.
- Use a multimeter to measure the voltage at the output emitter of the TIP35.
- It should be approximately 14.3V.
- Verify that the voltage across the lead acid battery is increasing.
- Monitor the current flow into the battery ensuring it is within the safe charging limits.
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, you may need to incorporate current limiting mechanisms or use a current limiting power supply to protect the circuit and the battery.
Safety Precautions:
- Be cautious with high current circuits, and ensure that connections are secure and insulated.
- Monitor the temperature of the TIP35 transistor during operation to prevent overheating.
Conclusion:
Always refer to the datasheets of the components used IC 7815 and TIP35 for detailed specifications and recommended operating conditions.
Adjustments may be needed based on the specific characteristics of the components you have.
If in doubt, please comment below for an expert advice.