# Universal nicd and nimh battery charger using LM317T

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

### 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.

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

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.

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Leon

I am a big fan of your site, but that last time there are more and more errors (mistakes) in your circuitry. these charger circuits take the cake on that….. LM317T Contant current and NiCd charger circuits; A lot of errors damn! circuit 2 ; + therminal of C1 bufferelco should be connected to the both cathodes of D1 and D2. Then the center tap CT should be connected to GND.. circuit 3 ; R6 and R7 should be connected to GND and not to the +….current flows out of the base of the PNP transistor Q2 circuit 3 with… Read more »

Jack

Re: Nicad Battery Charger by IC LM317T. Nicad/NiMh batteries require approx 1.65V/cell for charging. So for 2 cells you require close to 3.3V at the battery. Then you need about 1.5V for the LM317 to work as per spec. So the input voltage to the LM317 should be 3.3+1.5 = 4.8V. Then remove R7 which messes up the whole circuit. The Vref in the LM317 is 1.25V. So if you want 50mA then 1.25V/.05A = 25 Ohms. 24.9 Ohms for R6 is close to that. With 27 Ohms you get near enough 46mA. Next is the “4.8V” which I am… Read more »

Jack

By the way, I think 27 Ohms is correct because the LM317 also supplies some current which I did not take into consideration. The reference voltage made up of the ZD1 and the Vbe of Q2 is good enough for a battery charger. However, if you put the voltage feedback lines from S2 instead of some internal voltage then you have better control of the battery voltages, and you do not need S1. So your feedback resistors will be 96 Ohms instead of R3; and the other two resistors will be nearest value to 1694 Ohms and the one for… Read more »

J.Richert

Is the schematic now updated with the corrections?

zala

is this circuit now updated to corrected one ?