Simple NiMH Battery Charger Circuit

This circuit is like a special watering can for NiMH batteries, the kind you find in cameras, toys and other gadgets.

Regular chargers might not be perfect for these batteries, but this NiMH charger circuit is designed to give them exactly the right amount of power to keep them going strong for a long time.

Circuit Working:

Parts List:

Here is a basic battery charger designed for nickel metal hydride batteries that require regulated current during charging.

This charger delivers a current of 140 mA enabling rapid charging.

The power supply section comprises a 0 to 18V AC 1 ampere step down transformer a full wave bridge rectifier D1 to D4 and smoothing capacitor C1.

Current regulation is achieved through R1, R2 and the epitaxial darlington PNP transistor TIP127.

R1 limits the charging current to 140 milliamperes.

The LED and R2 are crucial for controlling the base current of T1 and consequently its output.

The LED creates a voltage drop of approximately 2.6V at the base of T1 while the emitter base junction of T1 drops around 1.2V.

Therefore, the current passing through R1 is calculated as 2.6 to 1.2V 10 ohms = 0.14 Amps or 140 Milli amps.

The LED serves as a charging status indicator illuminating only when the battery is connected to the circuits output and the input voltage is normal.

Formulas:

When creating a circuit for a NiMH battery charger, keep the following formula and factors in mind:

Calculating Charging Current I_charge:

Known as the C rate, the charging current should normally be a percentage of the battery capacity C.

A typical charging rate for NiMH batteries is C/10 to C/20.

One method for calculating the charging current I_charge is to use:

I_charge​ = C / 10​ to C​ / 20

where:

• C is the battery capacity in ampere hours (Ah).

Finding the Resistor Value for Current Limiting R1:

A resistor R1 can be connected in series with the battery to restrict the charging current.

Ohms Law may be used to compute the value of R1:

R1 = V_supply​−V_{battery​​} / I_charge​

where:

• V_supply​ is the DC voltage supplied to the battery (after rectification and filtering).
• V_battery​ is the nominal voltage of the NiMH battery.
• I_charge​ is the desired charging current.

Choosing the Charging Voltage:

For NiMH batteries, the charging voltage is often between 1.4 and 1.5V per cell.

The total charging voltage V_charge for a NiMH battery pack linked in series may be computed as follows:

V_charge ​= N × V_cell​

where:

• N is the number of cells in series in the battery pack.
• V_cell is the desired voltage per cell during charging (e.g.1.4V to 1.5V).

Notes:

Calculating Charging Current (Icharge):

Known as the C rate, the charging current should normally be a percentage of the battery capacity C.

A typical charging rate for NiMH batteries is C/10 to C/20.

One method for calculating the charging current I_charge is to use:

I_charge​ = C / 10​ to C​ / 20

where:

• C is the battery capacity in ampere hours (Ah).

Finding the Resistor Value for Current Limiting R1:

A resistor R1 can be connected in series with the battery to restrict the charging current.

Ohms Law may be used to compute the value of R1:

R1 = V_supply​−V_{battery​​} / I_charge​

where:

• V_supply​ is the DC voltage supplied to the battery (after rectification and filtering).
• V_battery​ is the nominal voltage of the NiMH battery.
• I_charge​ is the desired charging current.

Choosing the Charging Voltage:

For NiMH batteries, the charging voltage is often between 1.4 and 1.5V per cell.

The total charging voltage V_charge for a NiMH battery pack linked in series may be computed as follows:

V_charge ​= N × V_cell​

where:

• N is the number of cells in series in the battery pack.
• V_cell is the desired voltage per cell during charging (e.g.1.4V to 1.5V).

Notes:

These formulas offer a fundamental structure for creating a circuit for a NiMH battery charger.

Depending on the particulars of the battery, the intended charging settings, and the circuit components used, adjustments can be required.

To prevent overcharging or the battery overheating, always take precautions and keep an eye on the charging process.

How to Build:

To build a Simple NiMH Battery Charger Circuit follow the below mentioned connections steps:

Transformer and Rectifier:

• Connect the transformer to the input side of the bridge rectifier.
• The transformer should have a center tapped output, with the center tap connected to the negative side of the bridge rectifier.
• The positive and negative outputs of the bridge rectifier connect to the positive and negative terminals of the smoothing capacitor C1 respectively.

Current Regulation:

• Connect resistor R1 in series with the positive output of the rectifier.
• This resistor limits the current to 140mA.
• Connect resistor R2 and the LED in series from the base of the transistor TIP127 to the positive supply.
• R2 helps control the base current of T1.

Transistor and Charging Circuit:

• Connect the emitter of the transistor to the positive of bridge rectifier through resistor R1 and via through C1 and ground.
• The collector of the transistor is connected to the positive terminal of the capacitor C2 and also to the positive terminal of the battery connector.

Charging Status Indicator:

• The LED serves as a charging status indicator.
• It should light up when the battery is connected to the output of the circuit and the input voltage is normal.

Final Assembly:

• Mount the components on a circuit board and solder them together.
• Ensure proper insulation and spacing, especially around high voltage components.
• Use a heat sink for the transistor if it tends to get hot during operation.

Testing:

• Before connecting a battery double check all connections and component values.
• Connect a battery to the output and verify that the LED lights up indicating that the circuit is working correctly.

Note:

• Please note that working with electricity can be dangerous.
• Ensure you have the necessary skills and take proper safety precautions when building this circuit.
• If you are unsure, it is best to seek help from someone experienced in electronics.

Conclusion:

A NiMH battery charger circuit is designed to safely and efficiently recharge nickel metal hydride batteries.

It typically includes a power supply section, a current regulation section and a charging status indicator.

The circuit ensures that the battery is charged at the correct rate preventing overcharging and damage to the battery.

References:

Datasheet TIP127