Automatic Battery Charging Circuit

The significant usage of batteries is present everywhere, whether it is in your mobile phone, emergency light, or automobile. Chargeable batteries are also commonly used in inverters, where their DC voltage is converted to mains AC voltage and utilised to power domestic equipment during power outages.

The value of a battery stems from the fact that it stores power. Furthermore, when the power in a battery runs out, it can be replaced, topped up, or charged (obviously only with chargeable batteries), making it an extremely efficient and cost-effective power source.

A battery charger circuit may be relatively basic in construction, but batteries typically do not appreciate crude charging voltages, hence it is always advised that excellent quality, constant voltage kind of chargers be used to maintain the battery in good shape and consistent.

 Most of us are not engineers, yet we want to be able to solve and avoid battery problems in a straightforward manner. We utilise the battery charger to fix such problems. It is safe to use for all users. 

This page discusses the Automatic Battery Charger, its operating concept, and many other topics.

Fundamental Charging Parameters

While charging the battery securely, three key elements must be followed.

  1. Constant Current (CC)
  2. Constant Voltage (CV) and
  3. Auto Cut-off

Constant Current – The quantity of battery charging current is fixed in this case. The voltage is varied to keep this current flowing.This is a simple form of charging batteries, with the current level set at approximately 10% of the maximum battery rating.  This method is suitable for Ni-MH type of batteries.  The battery must be disconnected, or a timer function used once charged.

Constant Voltage – The current will be changed as required to charge the battery while the voltage remains constant.allows the full current of the charger to flow into the battery until the power supply reaches its pre-set voltage.  The current will then taper down to a minimum value once that voltage level is reached.  The battery can be left connected to the charger until ready for use and will remain at that “float voltage”, trickle charging to compensate for normal battery self-discharge.

Auto Cut-off – It continually checks battery charging voltage and switches off the charging voltage when the battery reaches full charge.

These are the three main fundamentals that must be met in order to effectively charge the battery without reducing battery life. Let’s take a quick look at the essential characteristics listed above.

Constant Voltage Circuit

Constant voltage permits the charger’s full current to flow into the battery until the power supply reaches its predetermined voltage. Once that voltage threshold is attained, the current will taper down to a minimum. The battery may be left attached to the charger until ready to use, and it will continue to charge at that “float voltage,” trickle charging to compensate for typical battery self-discharge.

CV charging is a method of charging batteries by regulating a preset constant voltage. Its primary advantage is that it avoids overvoltages and irreversible side reactions, hence extending battery life. Due to the constant voltage, the charging current reduces as the battery charges. At a nearly stage of the charging process, a large current value is necessary to ensure a steady terminal voltage. Fast charging is provided by a high charging current ranging from 15% to 80% SOC; nevertheless, the high current strains the battery and can cause lattice collapse and pole breakage.

As previously indicated, we examine the CV mode of a lithium battery charger in which we must manage the battery voltage from 6.4V to 8.4V. The voltage regulator IC LM317 can accomplish this with only two resistors. The circuit below depicts a battery charger circuit in constant voltage mode.

To calculate the output voltage for the LM317 Regulator,

  • Vout= 1.25*(1= (R2/R1)) where, 1.25 is the reference voltage.

The output voltage (Vout) should be 8.4V in this case. The value of R1 in this circuit should be less than 1000 ohms, thus we choose a 560 Ohms Resistor. We can compute the value of R2 using the preceding algorithm.

  • 8.4V= 1.25*(1+(R2/560ohm)
  • R2= 3.3KOhm.

Alternatively, any resistor value combination that produces an output voltage of 8.4V can be used.

Constant Current Circuit

CC charging* is a basic approach that charges the battery with a little constant current throughout the charging process. When a specified value is achieved, CC charging ceases. This method is commonly used for charging NiCd, NiMH, and Li-ion batteries. The charging current rate is the most critical aspect, because it has a big impact on how the battery behaves. As a result, the fundamental problem of CC charging is determining an appropriate charging current value that meets both charging time and capacity utilisation. A high charging current gives a speedy charge but also has a considerable impact on the ageing process of the battery. A low charging current allows for good capacity utilisation but results in a relatively sluggish charge, which is undesirable for EV applications.

The LM317 IC may be used as a current regulator by using a single resistor. The battery charger circuit for this current regulator is depicted in the diagram below.

As stated above, we are treating 1000 mA as a Constant Charging current.

To calculate the resistor value for the required current is (provided in the battery data sheet) as,

Resistor (Ohms) = 1.25 / Current (Amps)

  • R= 1.25/1A= 1.25 ohm.

To make this circuit, we’ll need a 1.25 Ohm resistor. We don’t have a resistor with a resistance of 1.25 ohm, therefore we use the next closest number, 1.5 Ohm, as shown in the circuit diagram.

Auto Cut-off Circuit

The auto cut off feature is the most critical aspect of battery charging. The auto cut-off circuit is now used in the majority of batteries. The circuit schematic below depicts a battery charger circuit with an auto shut-off feature. It is implemented using the LM317 adjustable voltage regulator.

This circuit provides variable DC supply output voltage and charges the battery. The LM317 is a monolithic integrated circuit that comes in three distinct packages. This adjustable voltage regulator has a load current of 1.5 amps and an output voltage range of 1.2 to 37 volts.

Auto cutoff battery-charging-circuit1

It primarily employs fundamental power supply components such as a transformer, rectifier, filter, and regulator. The AC power source is stepped down by the Stepdown Transformer (230V to 15V). The rectifier then employs four 1N4007 diodes to convert step-down AC to DC.

C1 and C2 capacitors are used to power the filter. We utilised the C1 LM317 IC to manage the voltage. It also functions as a current controller. In this case, the variable resistor VR1 modifies the supply to the voltage regulator’s ADJ (Adjust) pin, resulting in a change in output voltage.

We’ve showed the green and red LEDs here. The green LED indicates that the battery is charging, while the red LED indicates that the battery is fully charged.

When the battery is fully charged, the Zener diode (12V) provides a reverse voltage that flows to the base of the BD139 transistor, turning it ON. Because of this conduction in a transistor, the voltage regulator’s ADJ pin connects to ground, cutting off the regulator’s output voltage. To avoid the thermal Runway during this continuous process, utilise a heat sink with a voltage regulator.

The IC LM317 has a changeable output voltage. This voltage may be adjusted via an ADJ pin, resulting in a higher overall output voltage.

  • Vout = Vref (1 + R2/R1) + IADJ R2

Where Vout is the output voltage.

According to the resistor position, the formula will be,

  • Vout = VREF (1 + VR1 / R1) + I ADJ VR1

It is critical to choose the charging current carefully in order to preserve battery life. This charging current is determined by the battery’s capacity (ampere-hour rating). Every battery has an ampere-hour rating. This is the battery’s charge storage.

The optimal charging time for batteries is 2-3 hours. This charging current rate varies depending on the kind of battery, therefore you may adjust the charging current based on the battery capacity and type.


I hope this post has helped you comprehend the entire circuit of an automated battery charger. Battery chargers are used for a variety of purposes, including mobile phone chargers, electric vehicle battery chargers, and charging stations. We can construct the battery charger circuit utilising SCR, Op-Amp, various regulator ICs, and other components based on the battery specifications.

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