Battery Charger

Cigarette-lighter Battery Charger Schematic Circuit Diagram

Enjoying Hi-Tech Gadgets Outdoors

On a sunny Summer day, you step outside, eager to enjoy the fresh air while indulging in your love for modern gadgets. These include your son’s radio-controlled car, your daughter’s MP3 player (borrowed after some negotiation), and your cherished portable DVD player. All of these devices rely on rechargeable battery, which is convenient when you’re near a power source with their respective chargers.

Overcoming Battery Woes in the Countryside

However, venturing into the countryside complicates matters, often leading to the unfortunate scenario where your gadgets’ batteries run out. This situation is where Murphy’s Law kicks in. Luckily, there’s a simple, cost-effective solution at hand, especially if your car is nearby. This solution requires basic electronic components that most enthusiasts likely already possess. Even if you need to purchase the items, the entire project shouldn’t exceed £10. Notably, this project harks back to traditional electronics, reminiscent of a bygone era, as it doesn’t involve microcontrollers or specialized integrated circuits, as illustrated in Figure 1.

Cigarette-lighter Battery Charger Schematic Circuit Diagram 1

Cigarette-lighter Battery Charger Schematic Circuit Diagram 2

Cigarette-lighter Battery Charger Schematic Circuit Diagram Components List

Efficient Battery Care with Constant Current Charging

Taking care of your batteries is crucial, especially if you are mindful of charging times. Whether you’re dealing with the now rare and problematic nickel-cadmium (NiCd) batteries or the ubiquitous nickel-metal-hydride (NiMH) batteries, a constant current charging method is essential. For a normal or slow charge, the charging current should be 10% of the battery’s rated capacity, as indicated on the label. If you opt for a fast charge, it can go up to a maximum of 100% of the capacity. To recharge these batteries from a car battery, you can construct a constant current generator, which forms the core of this solution.

Building the Constant Current Generator

Constructing the constant current generator involves two standard transistors, T2 and T3. T3’s conductivity is regulated by R3 and T2. Due to the inherent nature of transistors, the voltage difference between the base and emitter of T2 remains around 0.6 V. This voltage, determined by the current passing through one of the resistors (R4–R7) and the battery being charged, establishes the charging current (Ich), calculated as Ich = 0.6 / R. As T2 turns on during charging, T1 saturates, and if the current drops significantly (indicating a faulty battery or poor connection), the LED indicates a problem. Additionally, diode D1 safeguards the circuit from potential reverse polarity issues with the battery.

Practical Implementation and Customization

To facilitate ease of use, a dedicated PCB (printed circuit board) has been designed for this project, accommodating a rotary switch (Lorlin part no. PT6422/BMH) that simplifies the circuit’s operation. Various charging currents (400, 130, 60, and 10 mA) are available through different switch positions (1–4). The maximum battery voltage permissible for charging is 9.6 V, considering the voltage drop across T2. If you need different charging currents, you can adjust the circuit by replacing specific resistors (R4–R7) using the formula R = 0.6 / Ich and ensuring the power dissipation (P = 0.36 / R) is within limits.

Safety Precautions and Conclusion

While the circuit inherently protects against short circuits, it’s crucial not to exceed the maximum power dissipation in T3 (65 W) and the heat sink’s capacity on the PCB. A reasonable maximum charging current, such as 500 mA, should suffice for most NiMH and NiCd batteries if allowed a few hours for charging. Considering it was a sunny day, these precautions should ensure a worry-free and efficient battery charging experience.


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