Ni-Cd Battery Charger Circuit

Previously we have built many types of charger circuits, that incudes Solar mobile Charger, Float charger circuit, 12v Battery charger, Power bank circuit etc. Today we are going to build charger circuit for charging Ni-Cd Battery. The process of charging Nickel cadmium batteries can be done in two ways:

1. Fast charging
2. Slow charging

The Fast charging requires a proper shutdown after full charge. Unlike a Lead acid battery or a lithium battery, Ni-cd battery cannot take up a float charge. Also, the shutdown of charger after full charge is not simple. It has to be done based on an algorithm which senses the temperature of the battery and steady decrease in voltage after charge completion. Another part is that the battery has to be fully discharged before fast charging.

Hence, here we are going to make a simple slow charger which can charge a Ni-cd battery at lower and safer currents. This process without auto-shutdown will not damage a cell as much when compared to a fast charger without auto-cut off.

Slow chargers can be used to overcome self-discharge. A Ni-Cd battery self-discharges at a rate of 15-20% per month which is higher when compared to 5-10% per month of a Lithium battery. But, it is lower when compared to a Ni-MH battery whose self-discharge is 20-30% per month.

 

Ni-Cd Battery Charging:

The electrochemical device that supplies energy to the external circuit through an internal chemical reaction is called a Cell. A combination of these cells either in series or parallel connection is called a Battery.

Below is the specification sheet of few Ni-Cd batteries of AA and AAA sizes,

The difference between the standard charging and quick charging is based on the charging voltage, charging current and the cut off method or algorithm.

A fast charging requires precise cut off of power at the end of charge by using the cell temperature or negative change of voltage. The different types in which the Ni-Cd battery can be charged are as below,

The battery used here is as below which is of capacity 600mAh and indicates us to charge the cell at 60mA which is 0.1C slow charging recommended. Hence, we have done constant current slow charge.

 

Components Required:

1. LM317  – 2Nos
2. 1N4007 – 4No.
3. Capacitor, 100uF (Electrolytic) – 1No.
4. AAA or AA battery holder  – 1No
5. POT (100Ὠ) – 1No.
6. LED (Red -1)
7. Resistors (1kὨ -1; 560Ὠ -1; 100Ὠ -2)
8. Perforated Dot board
9. Connecting wires

 

Ni-Cd Slow Charger Circuit Diagram and Explanation:

This is How the circuit looks on Perf Board:

 

1. Step-down Transformer:

A step down AC transformer with the rating of 230V to 15V, 1Amps is used here. Even though the output current capacity of transformer is at 1Amps, the allowable continuous current is only 0.4Amps for safe operation. A transformer with either 230V/0-15V or 230V/15-0-15V can be used.

 

2. Bridge Rectifier:

Full wave bridge rectifier converts AC supply into DC supply through a process called rectification and. A rectifier used here is done using four diodes in bridge configuration.

 

3. Voltage Regulator Circuit:

Here LM317 is used for voltage regulation; it is a three terminal adjustable regulator

 

Thus, the required output voltage is 1.45V max to charge the battery.

V_OUT = 1.25 * {1+ (100/560ohm)}

V_OUT = 1.47v

This LM317 Voltage Calculator can be used if you are looking for some calculator to calculate the resistor or to plan the output voltage.

 

4. Current Limiter Circuit:

Since the charging current for a 600mAh battery will be 60mA. The appropriate Resistor has to be calculated,

I_OUT = 1.25 / R

Therefore, R is set to  21Ὠ to limit the current to 0.06A.

 

Working of Ni-Cd Battery Charger Circuit:

The open circuit voltage without Battery appears as 1.5v which can be seen from the below picture,

As mentioned earlier, the output voltage is 1.49V and the current is limited to 60mA and the Red LED indicates the Charging condition of the battery.