I am gonna show you a current and voltage constant  NiCD battery charger project. For 2.4V,4.8V,9.6V Also, if you understand how it works you can apply it more.

I love them. Are you the same as me?  In the past, I used the alkaline battery. Because it is high power.  But now I mostly use NiCD battery. Because it can be recharged many times(+500).

A general Nicad battery has a voltage of  1.2V. But we can use it instead of 1.5V battery.

Why?

It has a constant voltage level while supplying current. And when the charge is almost empty Its voltage will drop quickly. Which is different from other types of batteries (Alkaline). See on graph.

But charging batteries requires caution. And use the correct method only.

Basic principles of charging NiCD batteries.

3 types of the battery charger systems

There are many ways to charge NiCD batteries.  I would like to divide into 3 major categories as follows

Constant current

This method is easy and safe. But it takes a very long time. 

Example of a 500mAh battery.

The suitable charging rate is 0.1 times the current capacity. So, we should charge with a constant current of 50mA for 10 hours. But actually takes 14-16 hours. Because when the battery is almost full, the current will be charged less.

If needing to charge faster. We can increase the charging current to 100mA. And the charging time will be faster to 3-5 hours.

Good sound? But…be careful!

When the battery is full, have to disconnect it immediately. Otherwise, the chemicals inside it get too hot. Until causing damage

Constant voltage

Normally, NiCD battery has a voltage of 1.2V and when fully charged is 1.25V-1.3V

For example

If you want to charge 4 batteries, set the constant voltage to 1.2Vx4 = 4.8V.

But at first, there will be very high currents. Because of the voltage level between the charger and the battery is very different. It may cause overheating until the battery is damaged.

Constant current and voltage

This type of charging uses more slightly devices. But it is worthwhile because it is the safest method. And the battery voltage does not exceed the limit of the battery.

Sometimes we may forget to charge it for too long. Without any problems Because the voltage between the charger and the battery is the same.

How to design

Which is better for you. In this, I will pick no.3.

Some want to learn how to design the circuit is simple. In this project, we will try to design it together.

First, see the block diagram of this project.

Here is a step by step process.

• 12V unregulated Supply
  Almost all electronic projects that use the AC Main Circuit must use this part. It is a simple DC power supply circuit. That is easy to design.
• Simple Regulated Supply
  It is Series Regulator with adjustable output voltage to make a constant voltage above.
• LM317 constant current
  It is an easy circuit. Why? read below.
• Ammeter
  Shows the amount of current flowing into the battery and know that the connection points are normal.
• 2, 4, 8 Batteries in series

Let’s getting started.

12V Unregulated supply at 300mA

See in the circuit below.

When we use a battery with a size of about 10V and not more than 200mA. It is advisable to use the DC power supply 12V 300mA. Set …

• The voltage of the secondary winding is 9V at 300mA.
• Capacitor-220uF 25V is the filter to smooth voltage. We use 220uF for this current.

Recommended: Unregulated power supply Working

LM317 constant current

First, design a constant current. We have many ways to do it. Example, transistor circuit. Now, using LM317 is simple, too. It is good at the simple variable power supply.

It has a lot of helpful. If I would explain to you all about it. This post will very very long surely. So, read how LM317 works pinout and more. Now, Let me explain to you how to use it is a simple constant current.

Look at the circuit below. Yes! There only is IC and Rs. Is it easy?

In short, we can find the output current (Iout) easily, too.

Iout = Vref / Rs

Imagine we use Rs of 10 ohms.
Vref = 1.25V
So, Iout = 1.25V / 10 ohms = 0.125A

If we want Iout = 50mA = 0.05A

Rs = 1.25V / 0.05A = 25 ohms

You may use 24 ohms to 27 ohms.

Important: The input voltage must always be greater than the output voltage at 0.7V.

When charging all 3 types of batteries. So, use different levels of voltage must be specified as follows:

• 2 Batteries
  Vin = 3.1V; Vout = 2.4V
• 4 Batteries
  Vin = 5.5V; Vout = 4.8V
• 8 batteries
  Vin = 10.3V; Vout = 9.6V

Related circuits about battery chargers

• Constant current battery charger circuit
• Simple Ni-cad battery charger with little parts

Simple regulated voltage

Then, we look at a simple regulated circuit. To adjust the voltage in all 3 levels above

It is a Series Feedback Voltage Regulator circuit that uses a transistor and a Zener diode.

Learn more: Series Regulator with adjustable output voltage

So what?

Here is a step by step to calculate this circuit.

RB?

When Vout = 10.3V
Get VB = 10.3V + 0.6V = 10.9V
Find RB = (12V – 10.9V)/1mA = 1.1K
or
Use RB = 1.2K

RA?

Some current of Vout is a bias current of Zener diode of 5mA. So, the current flows RA of 4mA.

VZD = 2.4V

It is a way to find RA = (10.3V – 2.4V)/ 4mA
Thus RA is about 2K

We will see that the bias current of Zener Diode is still of 5mA. Though Vout is lower than 10.3V.

Because… When Vout reduces. And the current of RA is lower, too. But the current of RB is more. So, the bias current Zener diode is 5mA.

Do you understand?

RE?

Then, set the current of RE is 2mA.
So, we can find RE = (2.4V + 0.6V)/2mA
Use RE is 1.5K

RV?

Vout = 3.1V
RV = (3.1V – 3V)/2mA = 50 ohms
Use 50 ohms

Vout = 5.5V
RV = (5.5V – 3V)/2mA = 1.25K
Use 1.2K

Vout = 10.3V
RV = (10.3V – 3V)/2mA = 3.65K
Use 2.7K and 1K in series

Then, we merge all parts together to become complete circuits below.

We add LED1 to show circuit power on. And R8 is limiting the current resistor of LED1.

Plus we add an ammeter to indicate current charging into the battery. In first it read high current. But when the battery is full we read it is zero.

We can select the voltage to charge the battery by SW1.

Parts you will need

0.25W Resistors, tolerance: 5%

• R1: 1.8K
• R2: 2K
• R3.1: 50Ω
• R3.2: 1.2K
• R3.3: 2.7K
• R3.4: 1K
• R4: 1.5K
• R5: 470Ω
• R6: 27Ω
• R7: 5Ω

Electrolytic Capacitors

C1: 220µF  25V

Semiconductors:

• D1,D2: 1N4001, 50V 1A Diodes
• D3: Zener Diode 2.4V 0.5W
• LED1: LED
• Q1,Q2: 2SC1815, 45V 100mA NPN Transistors
• IC1: LM317 voltage regulator IC

Others

• T1: Transformer 9-0-9, 300mA
• Meter
• Switch 3 selector
• Wires, AC main, battery holder, and more

How to build

Since this project has quite fewer parts. So, we may assemble them on the universal PCB or perforated board. You should check diagram well before charges the battery.

Update:
Now I add The copper PCB layout and the components layout.
Though this circuit very old or very ancient. But still useful.

However, if this error please tell me.

See: LM317K pinout

The actual-size of Single-sided Copper PCB layout
At 200 pixel per inchs.

Not only that you may like these projects

• How to discharge Ni-Cd battery
• Automatic NiMH battery charger circuit
• Simple NiMH NiCd Battery Charger circuit

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