Battery Charger

Simple NiCd Charger Schematic Circuit Diagram

Building a Simple NiCd Battery Charger with Basic Components

Constructing a NiCd charger becomes remarkably straightforward using commonly available components and a low-cost LM317 or 78xx voltage regulator. This setup incorporates a current limiter, employing R3 and a transistor, allowing the charging of numerous cells until a specific ‘fully charged’ voltage, determined by the voltage regulator, is achieved. Moreover, it provides clear indications of whether it’s actively charging or has attained the fully charged status. If the storage capacitor (C1) is excluded, the charging process turns pulsed. This mode allows for a higher charging current, retaining all control characteristics while ensuring effective charging.

Charging Operation:

The charger operates seamlessly. When the cells aren’t fully charged, a controlled charging current flows from the voltage regulator. This current, however, is regulated by resistor R3 and transistor T1. These components work together to manage the charging process, ensuring efficient and safe charging for the NiCd cells. The circuit provides a reliable and cost-effective solution for charging NiCd batteries without the need for sophisticated or expensive components.

Simple NiCd Charger Schematic Circuit Diagram

Setting the Current Limit and Charging Indicator:

To establish the current limit (Imax), the formula Imax ≈ (0.6 V) ÷ R3 is utilized. For a desired Imax of 200 mA, R3 is calculated as 3 Ω. The LED serves as an indicator, illuminating when current limiting is active. Indicating that the cells have not yet reached full charge. Due to the voltage across the LED, the potential on the reference lead of the voltage regulator rises by about 2.9 V.

Consequently, a specific minimum number of cells is necessary. For an LM317 regulator, at least three cells must be charged (3 × 1.45 V > 2.9 V + 1.25 V) because the voltage between the reference lead and the output is 1.25 V. If a 78xx regulator with a voltage drop of approximately 3 V (plus 2.9 V) is used, the minimum number increases to four cells. As the cells approach full charge, the current decreases gradually, causing the current limiter to deactivate and the LED to turn off.

Voltage Regulation for Fully Charged Cells:

In the fully charged state, the voltage on the reference lead of the regulator is determined by the voltage divider R1/R2. For a 7805 regulator, the value of R2 is selected so that the current passing through it is 6 mA. Considering the current through the regulator (around 4 mA), this results in a total current of about 10 mA flowing through R1. If the voltage across R1 is 4 V (9 V – 5 V), a resistance value of 390 Ω is obtained.

Consequently, the end-of-charge voltage can be adjusted to approximately 8.9 V. Since the current through the regulator is contingent on the device manufacturer and the load, the value of R1 must be fine-tuned accordingly. Additionally, the storage capacitor’s value must align with the chosen charging current. It can also be omitted for pulse charging, as previously mentioned. This configuration ensures efficient and controlled charging of the NiCd cells, promoting their longevity and performance.


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