In this DIY Project, I will show you how to build a simple Lead Acid Battery Charger Circuit using easily available components. This circuit can be used to charge Rechargeable 12V Lead Acid Batteries with a rating in the range of 1Ah to 7Ah. 

Introduction

Lead Acid Batteries are one of the oldest rechargeable batteries available today. Due to their low cost (for the capacity) compared to newer battery technologies and the ability to provide high surge currents (an important factor in automobiles), Lead Acid Batteries are still the preferred choice of batteries in almost all vehicles.

The main concern with any battery is it discharges over time and must be recharged so that it can provide the necessary voltage and current.

Lead Acid Battery Charger Circuit Image 1

Different batteries have different strategies of charging and in this project, I will show you how to recharge a lead acid battery using a simple Lead Acid Battery Charger Circuit.

Warning: Before proceeding further, I want you to know that this circuit is tested in specific test conditions and we do not guarantee that it will be 100% successful. Try this circuit at your own risk. Take all the necessary precautions as you might be handling Mains Voltage and high potential DC.

How to Recharge a Lead Acid Battery?

To charge a battery from AC we need a step-down transformer, a rectifier, filtering circuit, regulator to maintain the constant voltage. Then we can give the regulated voltage to the battery to charge it. Think if you have only DC voltage and charge the lead acid battery, we can do it by giving that DC voltage to a DC-DC voltage regulator and some extra circuitry before giving to the lead-acid battery. A car battery is also a lead-acid battery.

SIMPLE LEAD ACID BATTERY CHARGER - Block Diagram

As seen in the above block diagram, a DC voltage is given to the DC voltage regulator. The voltage regulator used here is 7815, which is a 15V regulator. The regulated DC out voltage is given to battery. There is also a trickle charge mode circuitry which will help to reduce the current when the battery is fully charged.

Circuit Diagram

The circuit diagram of the Lead Acid Battery Charger is given below.

Lead Acid Battery Charger Circuit Circuit Diagram

Components of Lead Acid Battery Charger Circuit

  • 7815
  • Bridge Rectifier
  • Resistors – 1Ω (5W), 1KΩ x 2, 1.2KΩ, 1.5KΩ x 2, 10KΩ
  • Diodes – 1N4007, x 3, 1N4732A (Zener)
  • 2SD882 NPN Transistor
  • LEDs x 4
  • 50KΩ Potentiometer
  • 12V Relay

Component Description

7815

The 7815 is a part of the 78XX series of linear voltage regulators. You might have used 7805 and 7812 which produce a regulated voltage of 5V and 12V respectively. Similarly, the 7815 Voltage regulator produces a constant regulated voltage of 15V.

Lead Acid Battery

Lead Acid  Battery is a rechargeable battery developed in 1859 by Gaston Plante. The main advantages of the Lead battery are it will dissipate very little energy (if energy dissipation is less it can work for a long time with high efficiency), it can deliver high surge currents and available at a very low cost.

Calibrate the Circuit

Before seeing the working, let me show you how to calibrate the circuit. For calibrating the circuit, you need a variable DC Power Supply (a bench power supply). Set the voltage in your bench power supply to 14.5V and connect it to CB+ and CB- of the Circuit.

Lead Acid Battery Charger Circuit Calibrating

Initially, set the jumper between positions 2 and 3 for calibrating. Now slowly turn the 50KΩ Potentiometer until the “Charged” LED turns ON. Now disconnect the power supply and connect the jumper between 1 and 2. Your circuit is ready as all you need is a DC (or AC) supply of 18V.

NOTE

  • The 14.5V we set in the calibration is called the Tipping Point. When the Tripping point is set to 14.5V, the battery will charge for about 75% of its capacity.
  • If you want to charge 100%, then set the tripping point to ≈16V by removing the 7815 regulators and directly supplying 18V DC, but this is not recommended.

Circuit Explanation

  • The circuit mainly consists of a Bridge rectifier (if you are using AC supply stepped down to 18V), 7815 Regulator, Zener Diode, 12V Relay, and a few resistors and diodes. 
  • The DC voltage is connected to the Vin of the 7815 and starts charging the battery through the relay and the 1Ω (5W) resistor.
  • When the charging voltage of the battery reaches the tripping point i.e. 14.5V, the Zener diode starts conducting and provides enough base voltage to the transistor.
  • As a result, the transistor is active and its output becomes HIGH. This high signal will activate the relay and the battery is disconnected from the supply.

Lead Acid Battery Charger Circuit Image 2

NOTE:

  • The battery should be charged with 1/10th it’s charging current.so the voltage regulator must generate 1/10th of the charging current produced by the battery
  • The heat sink should be attached to the 7815 Regulator to get better efficiency.

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