Simple Ni-Cd Battery Charger Circuit using Transistors

Nickel Cadmium Ni-Cd batteries are really popular for many uses where you need rechargeable power.

They are known for having a lot of energy and lasting a long time through many charge cycles.

To charge these batteries safely and effectively you need a special charger that is made just for them.

This charger is usually meant for 12V Ni-Cd battery pack.

In this article, we will explain a simple but very effective charger circuit for Ni-Cd batteries.

We will look at how it works the math behind its design and the important details about how to build it.

Circuit Working:

Parts List:

The circuit diagram shown above explains how a charger works for nickel-cadmium Ni-Cd batteries.

This charger keeps a steady current while charging the batteries.

First the transformer lowers the high 230V AC voltage to a safer lower AC voltage.

Next the bridge rectifier D1 changes the alternating current into direct current which is what the charger needs.

To keep the charging current safe for the battery a regulation circuit using transistors is used.

The transistors Q1, Q2 and Q3 work together in this circuit to keep the charging current steady.

Also the transistor Q4 which is a TIP31 model acts as the main current regulator getting rid of any extra power as heat to avoid damage.

The resistors from R1 to R7 are important because they help control the current and provide the right biasing.

Additionally, the fuse F1 which is 500mA protects against too much current.

The charger operates in a constant current mode ensuring that the charging process is both safe and efficient.

Formulas with Calculations:

Below are formulas with calculations for Simple Ni-Cd Battery Charger Circuit using Transistors:

Transformer Selection:

Let us assume the required charging voltage is 12V DC for a Ni-Cd battery pack.

Considering rectifier and transistor voltage drops the secondary voltage of the transformer should be:

Vsecondary ​= Vbattery​ + Vdrop​ = 12V + 3V = 15V

Charging Current Calculation:

The charging current for a Ni-Cd battery is typically 1/10th of its capacity (C/10).

If the battery has a capacity of 1.5Ah then the charging current should be:

Icharge​ = 1.5Ah​ / 10 = 150mA

Resistor R1 Current Sensing Resistor Calculation:

The resistor R1 (0.15Ω, 3W) is used for current sensing and feedback regulation.

The voltage drop across R1 is:

VR1​= Icharge​ × R1 = 0.15A × 0.15Ω = 0.0225V

Power Dissipation in Q4 TIP31:

The power dissipation in the transistor is calculated as:

PQ4 = (Vinput−Vbattery) × Icharge

How to Build:

To build a Simple Ni-Cd Battery Charger Circuit using Transistors follow the below mentioned steps for connections and assembling:

• Gather all the components as shown in the above circuit diagram
• Connect the collector of transistor Q1 to base of transistor Q2.
• Connect the base of transistor Q1 to one end of resistor R2 and the other end of R2 to D1 bridge rectifier.
• Connect the emitter of transistor Q1 to positive supply through resistor R3
• Connect the collector of transistor Q2 to base of transistor Q3.
• Connect the base of transistor Q2 to collector of transistor Q1.
• Connect the emitter of transistor Q2 to GND through resistor R6.
• Connect the collector of transistor Q3 to positive supply.
• Connect the base of transistor Q3 to collector of transistor Q2
• Connect the emitter of transistor Q3 to base of power transistor Q4 through resistor R7.
• Connect the collector of transistor Q4 to one end bridge rectifier through resistor R1.
• Connect the emitter of transistor Q4 to positive of 12V Ni-Cd battery and negative of battery to GND.
• Connect the bridge rectifier D1other pin to one wire of transformer and other wire of transformer to third pin of D1 bridge rectifier.
• Connect the fourth pin of D1 bridge rectifier to GND.
• Connect a fuse to 230V AC for over current protection of the circuit

Conclusion:

This Simple Ni-Cd Battery Charger Circuit using Transistors is a dependable and effective method for charging Ni-Cd batteries by using a transistor to control the current.

It keeps the charging current steady which helps keep the batteries safe and makes them last longer.

It is really important to manage heat well and choose the right parts for the best results.

You can also easily change this design to work with different battery sizes by simply adjusting the resistor values.

References:

Issues with simple NiCd battery charger circuit