7805 Constant Current Batter Charger circuit

I am going to show the working of a constant current battery charger circuits.

They use a few components, 7805 and some transistors.

We often use 7805 in DC regulator, 5V 1A. A lot of digital circuits use them as a source. It can keep a stable voltage.

In the datasheet, 7805 have many helpful. One thing is we can use them as constant current regulation.

It is so easiest and convenient than use transistor circuits.

Because of a one-man show with a few parts. So, become easily the battery charger circuit.

So what!

Imagine you use a digital multimeter, right? Because it is highly accurate and using easy.

It uses the 9-volts battery as the main energy.

When we often apply it use very consumption. It runs out of energy.

You need to change a new one.

What can you do it?

7805 Constant Current Batter Charger circuit

Use a rechargeable battery of 9 volts (8.4V). It is rechargeable of over 200 times so worth it.

But we need the right charging way.

Let me show you a nice 9V Ni-MH battery charger.

Of course, when you finished reading. You can apply it to charge any battery like 7.2V, 12V battery, and more. As your ideas.

Idea circuit

Firstly, should know about this battery before, as Figure 1. More detail as follows:

#1. It is a Ni-MH type, has a lot of features:

  • No memory effect. You do not need to run out of the capacity. Before charging.
  • High current.
  • Cheap!
  • And more popular for now.
Ni-MH rechargeable battery of 9V

Figure 1: Example Ni-MH rechargeable battery of 9V.
If you are looking for it on Amazon. Click here

#2. Enough Energy—An output voltage around 8.4V and provided of 200mAh (applied current of 200mA within 1 hour.)

Even, just voltage only 8.4V. But can replace a common 9-volt battery.

Because it can supply a constant current to enough to use for a long time.

#3. Current Charging—The label on it, charging is divided into 2 types:

  • Standard model—with a current of 20 mA for 10 to 15 hours.
  • Fast model—with the current of the 50 mA for 5 hours.

At all models use a voltage of about 20% to 50% or 12V to 18V DC voltage only.

When we know enough. Let’s see the current constant circuit. Or the current regulator using IC-7805 better.

basic current constant using LM7805

Figure 2: Basic current constant or current regulator using 7805

In datasheet will see that a resistor-R1 pass current from a pin output of IC to load. Then, it gets the current output to pin ground, too.

The circuit inside 7805 can keep the output current is solid status. Even we change any input voltage. But do not forget it run well over 5V input.

What is more? Want to know how to find R1?

If you say yes, read below…

How to find R1-value

You can find the current constant with a simple formula:

IO = 5V / R1


IO: the current constant output
5V: Voltage of IC-7805
R1: Resistance-R1

In this case, I want to supply a constant current of 50 mA or 0.05A.

So try some new, to find out the R1 as follows:

R1 = 5V/0.05A
= 100 ohms

Is it easy?

Figure 2 basic current constant of IC-7805

Practical applications circuit

When getting the concept circuit. Then, test the circuit as this idea.

Appear good results.

And, should have an LED to indicate the connection terminal of a battery.

Sure, the power to the battery, should is DC voltage.

So, newly designed as Figure 3.

When we test the circuit. It works well, use charges time for about 5 hours.

Then, measure the charged current around 70mA. It rises too many currents.

Because some current flows through LED1 and R2. Which battery don’t hot.

current constant battery charger using 7805

Figure 3 the real circuit of current constant battery charger by LM7805

For other devices, You will well familiar F1, T1, D1-D4, C1, and C2, include is a DC power supply set. The voltage across C1 of 16V and supply it approximately 0.5A. Because of happened to I has 0.5 A transformer fitted.

Have you ever wondered why the LED is connected to the battery.

But disconnect the battery out. LED turns off.
– When trying to measure the voltage drop across R1, on without charging, will none voltage at all.

But when connects the battery charging appears voltage drop across R1 will always have the 5V, even if we use any rechargeable battery type or short-Circuit.

___this feature, you also understand it like me, when have the voltage constant and resistance not change, so the current flowing through R1 also must stable. And current flow the output also same them.

Look at 7805 pinout

How it builds and application

For this current constant battery charger, you make the PCB as copper-layout in Figure 4 and solder all parts on components-layout as Figure 5

7805 current constant PCB layout

Figure 4 the Actual-size, Single-sided PCB layout

components layout  7805 current constant circuit

Figure 5 the components-layout

__R1, Should be calculated to fit the battery and if current over than 0.3A should hold heatsink with IC1.

Figure 6 is the current constant battery charger using LM7805
The prototypes I use the universal PCB. Because of little devices and easy to build.

Application current constant using LM7805 projects

Figure 6 Application this projects

However, This circuit have disadvantage is without timer when finished for 5 hour then cut off power from battery, to protected battery to overheated.

Components lists

IC1: LM7805
D1-D5: 1N4007, 1A 1000V Diode
LED1: LED as you need
0.5W Resistors, tolerance: 5%
R1: 100 ohms
R2: 180 ohms

Electrolytic capacitor
C1: 1,000uF 25V
C3: 100uF 25V
C2,C4: 0.01uF 100V, Mylar Capacitor
T1: 12V 0.5A transformer
Other parts, fuse, PCB, Wires, etc.

NiCad and Ni-HM Constand Current Battery Charger

This is a Nicad battery charger circuit that old, but very effective. Because of the principle of charging with Constant current And controlled by constant voltage. The high efficiency. Reduce anxiety on rechargeable batteries for a long time. We use a simple IC-7805 is that everyone must know. To control the constant voltage. We maintain constant current circuits with transistors. And reliability than using a single resistor.

Like this circuit. Simple Ni-cad battery charger with little parts

Constant current battery charger circuit

Normally ni-cad battery often applies current be the stable charger. This circuit was designed to go up for use to do battery-package aforementioned a kind cell 1.25V x 4 amounts are group model AA that 50mA or cell 1.25V model C that 250mA by building serial. And can fine decorate other values get easy.

Follow the circuit resistor R1 and R2 use fine output voltage. While having no-load appraise 8V current output railings will flow through resistor R6 and R7 anyone. And when there is the size increases transistor Tr1 as a result quietly work, make at the dot Y have tall go up. Be TR2 work and dot Z be to add less.

It makes output voltage decrease lower and trend reduction until the equilibrium point value filtration of resistor R6 and R7 that. Must live the experiment thus then must use amp meter just rightly.

But this circuit is still not as good as performance. Nicad Battery Charger using LM317T
and for basic: Simple Ni-cad battery charger with little parts

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  1. Ygsh

    Hmmmm good……How to know that battery is fully charged now, so that to use micrcontroller for over charging protection………..(Very basic question in mind::: How 5v can charge 9v battery when others using 12v or more for it???)

  2. m3vuv

    this doesent seem to work, put 8v into 7805 and get the same out,current is 1.5 amps!

  3. Trevor

    Your first diagram is wrong. GND from the 7805 should connect to the right hand side of the 100ohm resistor…

    • Hello Trevor,
      Thank you for your kindness. I just updated that circuit.

      I hope you will help me more.

      Have a good day for you.

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