In this article, I will be sharing with you a current and voltage constant NiCD battery charger that can be used for any project. This battery has 2.4V, 4.8V, 9.6V. You can also understand how it works and the ways by which you can apply it more.

I personally love to use them. Do you think the same?  In my previous experiments, I have mostly used the alkaline battery because of its high power. Now things have changed and I mostly use NiCD battery. The reason is that it can be recharged many times (+500) by using NiCD battery charger.

UniNiCD NiMH battery charger using LM317T

A general NICD battery has a voltage of 1.2V. it can also be used in the place of a 1.5V battery.

Why?

It is because it has a constant voltage level while it supplies current. When the charge is almost finished, its voltage also drops quickly. This behavior makes it different from other types of batteries (Alkaline). See on the graph.

NiCD battery has 1.2V constant voltage

It must be kept in mind that charging batteries requires caution. A correct method must be used only while working with them.

Basic principles of charging NICD batteries are as follows:

3 types of the battery charger systems to Charge NiCD Battery Charger

There are many ways of charging NICD batteries. For that reason, I have divided them into 3 major categories for your ease.

Constant current

This is the simplest and safest method. The problem is that it takes a very long time to charge like that.

Example: Use of a 500mAh battery

The suitable charging rate for this battery is 0.1 times of the current capacity. Hence, we should charge it with a constant current of 50mA for about 10 hours. For real, it actually takes 14-16 hours. The reason is that when the battery is almost full, the current wont charged completely.

However, if you need to charge faster you can just increase the charging current to 100mA. This will make the charging time faster to 3-5 hours.

I know it sounds great! But you gotta be careful! When the battery is full, you must disconnect it immediately. Otherwise, the chemicals inside it get too hot and will cause the battery damage.

Constant voltage

Typically, NICD battery has a voltage of 1.2V and when it is fully charged its voltage is 1.25V-1.3V

Look at an example:

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

When you do that, at first, there will be very high current. This is because the voltage level between the charger and the battery is very different. This causes overheating so much that the battery is damaged.

Constant current and voltage

Charging like that requires the use of slightly different devices. The search is worthwhile because it is the safest method you will find. The battery voltage however, does not exceed the limit of the battery.

It happens at times when we totally forget to charge the battery for too long. It will still work without any problems because the voltage between the charger and the battery remains the same.

How to Design NiCD battery Charger?

Which method works the best for you? If you ask me, I would pick method no.3.

Some people want to learn the designing of the circuit. This process is quite simple. In this article, we will try to design it together.

First, take a look at the block diagram of this project.

Block Diagram of Universal NiCD NiHM Battery Charger

Here is the step by step process explained for you.

  • 12V unregulated Supply
    Almost all electronic projects that use the AC Main Circuit take help by using this part. It is a simple DC power supply circuit. This type of a circuit is very easy to design.
  • Simple Regulated Supply
    This circuit is a Series Regulator with adjustable output voltage to make a constant voltage above.
  • LM317 constant current
    Is this also an easy circuit? Yes it is! Why? Find out below.
  • Ammeter
    It shows the amount of current flowing into the battery and makes sure that the connection points are normal.
  • 2, 4, 8 Batteries in series

Let’s get started.

12V Unregulated supply at 300mA

See in the circuit below.

12V Unregulated power supply is used.

While using a battery with capacity of about 10V and not more than 200mA, it is advisable to use the DC power supply 12V 300mA. Set as follows…

  • 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.

LM317 constant current

First, you must design a constant current source. There are many ways to do it. Example of this is a transistor circuit. Now, using LM317 is quite simple, too. It is efficient at providing simple variable power supply.

It proves to be very helpful. If I start to explain all about its usage, this post will very long for sure. So, read how LM317 works with its pinout and more.

Now, Let me explain ways to use it to provide a simple constant current.

Look at the circuit below. There is only an IC and Rs. Isnt it easy?

simple LM317 constant current

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

Iout = Vref / Rs

Imagine we are using 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 the 3 types of batteries, you must use different levels of voltage. This is 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

Simple regulated voltage

Here we will observe a simple regulated circuit. It is used to adjust the voltage in all 3 levels given above.

basic Series Feedback Voltage Regulator

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

How do we proceed?

Here is a step by step process to make calculations.

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, the current of RA becomes less 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 will merge all the circuits together to have a figure of complete circuits shown below.

Circuit diagram of constant voltage and current nicd battery charger

We have added an LED1 to show that the circuit power is on. R8 is the limiting current resistor of LED1.

Plus we have added an ammeter which indicates the current being charged into the battery. At first, it shows a high current but when the battery is full, the reading becomes zero.

The voltage to charge the battery by SW1 can be selected.

Parts you might need

0.25W Resistors, tolerance: 5%

  • R1: 1.2K
  • R2: 2K
  • R3: 1.5K
  • R4: 50Ω
  • R5: 1.2K
  • R6: 1K
  • R7: 2.7K
  • R8: 470Ω
  • R9: 27Ω
  • R10: 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

Other parts:

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

How to build?

As you have seen, this project requires quite few parts. Its easier to assemble them on the universal PCB or a perforated board. You should take a look at the diagram before charging the battery.

Update:
Now I have added the copper PCB layout and the components layout also. Although this circuit is very old or you might call it ancient but I find it very useful.

However, if you still find errors, please let me know.

 

The actual-size of Single-sided Copper PCB layout
this is shown at 200 pixel per inches.

component layout of universal Battery Charger

The Component layout of the Universal NiCD Battery Charger

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