Battery full charge alarm circuit

This is Battery full charge alarm circuit for a general battery charger. It is a simplified circuit and cheap, use one transistor BC557 and display with 2 LED.

The battery that uses with a car, motorcycle or lamps lighting of fishermen. They always are a Lead-acid battery. Which each cell in the battery will have the voltage per cell is about 2.3V.

How many currents for charging

The duration of charging depends on a charging current. If the current is high, it will complete fast. But the battery is too hot.  So usually, the current is lower about 10% of battery current per an hour.

Battery capacity for charging rate
Battery capacity for charging rate

For example, I have a 6V battery. It has the battery capacity about 5Ah. So I use a constant current source about 0.5A. And it takes time about 12 hours for fully charging.

There are much 6V battery charger circuit HERE

Notice: During charging, the current will reduce continuously until the lowest is about 10mA. Which battery is near fully charged. But we cannot know that battery full or not. Because of no indicator to alarm us.

So this circuit may help us.

Two LED indicator
It has 2 LED indicator to display two status as follows.

  • Green LED—show while a battery is charging.  Then, it will go out if the battery is full.
  • Red LED—It will light up if you put a battery in backward. Important! the battery must charge with proper polarity only.

How it works

In the circuit below, is a full battery charger alarm circuit.  While it is charging a battery.  There are a current flows through a battery and circuit normally.

Also, a green LED lights up. And it will go out when fully charged.  Because a transistors-Q1 works.  And, both resistors-R1 and R2 are set a bias current to the transistor.

The Diode-D1 limits the voltage exceeds 0.7 volts.

Alarm light circuit for battery charger
LED alarm circuit for battery charger

When the battery is fully charged the green LED1 goes off. Because the currents stop flowing or may flow less than 10 mA.

But the green LED1 may go out. Because the battery is a wrong polarity. The output is a short circuit. The red LED2 will glow when the wrong battery polarity.

The maximum current that this circuit can check, in between 1A to 3A. If a charging current is less than 1A. The D1-diode should be 1N4001.   If the current is more than 1A but less than 3A, D1 should be 1N5401 instead.

Select battery voltage 6V or 12V—The resistor-R3 of 470 ohms will be used with the 6V Rechargeable battery.  If change R3 is 680 ohms to use for 12V battery.

Recommended: Recycle Free Li-ion battery from E-waste

How to builds

This project has a few components. So can be assembled on a universal PCB board as Figure 

In the assembly circuit, Start with lowest parts first, such as a diode and then resistors and sort of high continuously.

For the devices have various polarity should be careful in the assembly circuit. Before placing these components should set the polarity on the board and match together correctly.  Because If you put backward, may cause the parts or circuit damages. How to check polarity and input device.

component layout of full charger alarm circuit
Component layout of full charger alarm circuit

The soldering iron is less than 40 watts. And Use of lead solder containing lead and tin in the ratio of 60/40. Including the need also to have a flux inside the lead.

After that, put the components and completely solder. To check correctness again. But if you enter the wrong position. Should use a desoldering pump or a Desoldering remover. To prevent damage to PCB.

The components List
Resistors size ¼ watts +5%
R1: 56 ohms
R2: 100 ohms
R3: 470 ohm (680 ohms)
D1: 1N4001 (1N5401)
D2, D3: 1N4148,  75V 150mA Diodes
LED1: 5mm Green LED
LED2: 5mm Red LED
Q1: BC557B, 45V 100mA PNP Transistor
Others components.
The universal PCB board.

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15 thoughts on “Battery full charge alarm circuit”

  1. Dude I try this at home. but when I change the battery polarity the green led glow and when I use right polarity of the battery to the circuit its same,the green led is glow the red led in not glowing please help me

  2. Dude I try this at home. but when I change the battery polarity the green led glow and when I use right polarity of the battery to the circuit its same,the green led is glow the red led in not glowing please help me

  3. I have same problem red led is not glowing i check all parts is connect same to same but wrong poltery red led is not glowing

  4. i tried this at home and correctly finish this project… the question is how long it takes to full charge 12V 5AH Lead Acid Battery in 1Amp 12V transformer? why the Green LED don’t turn off after 5 hours of charging may cause my battery to heat to much.

    i need some explanation from the expert…. thanks for the kindness reply.

  5. Bro I am making a phone charger using transformer of 12v 1amp as input voltage , I used ic7805 to get output 5v to charge my phone but I have an problem 🙁 to charge my phone I need 5v and 1amp as output, I am getting 5v as output But I can’t get 1amp current I’m getting lower than 1amp in output . so my phone is not charging how can I get 1amp as a output pls tell me I really frustrated pls help mee sir. I need 5v 1amp output plss tell me a solution !!

  6. If I connect a relay collector point of 557 & the positive o/p taken from relay . Then Works or not. If it works then the circuit is fully automatic.

  7. 56K resistor caught on fire when the battery was fully charged and the polarity was reversed. I threw in another diode so that the battery can’t be reverse biased, and the problem was fixed.

  8. Hi, I have a doubt. If i want to charge a 3v (2 aaa battery for example), what value of resistance should I would put?


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