Automatic Battery Charging Circuit – Complete Guide

Do you believe that battery chargers have become an important part of our everyday lives, in both a private and professional manner?

The fact that we want to use portable electronic equipment that needs a battery to operate Similarly, in the market, there are varieties of battery-operated electronic equipment available like Mobiles, Electric Bicycles, laptops, etc.

Most of us are not engineers but want to be able to fix and prevent battery issues simply. To solve such issues, we use the battery charger. It is safe for all users to operate. Also, it is safe to travel from one place to another (via road), so anyone can use it with flexibility.

The geeky people are always curious about the functioning of the battery chargers. In this blog, we are going to discuss the Automatic Battery Charging Circuit and its parameters.

Fundamental Charging Parameters

Three fundamental parameters need to be considered while charging the battery safely:

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

Constant Current: Here, the amount of battery charging current is fixed. This current is maintained by varying the voltage.

Constant Voltage: Here, the current will be varied as per the battery charging requirement while keeping the voltage constant.

Auto-Cut-Off: It continuously detects the battery charging voltage and, once the battery reaches the full charge level, cuts off the charging voltage.

These three are the basic, fundamental things that are needed to charge the battery successfully without affecting battery life.

In lithium-ion batteries, apart from these parameters, thermal management and step charging are also important to maintain the battery voltage and its life. Li-ion batteries use the BMS (Battery Management System) to maintain these parameters.

Let's figure out the above fundamental parameters in brief.

Why is CC and CV important?

The charging current rate is the most important factor that significantly influences the behavior of the battery. It is a simple method that uses a small constant current to charge the battery during the complete charging process. When the battery reaches its predefined value, CC charging stops.

Mostly, this method is used for charging NiCd, NiMH, and Li-ion batteries. A high charging current charges the battery fast but significantly affects the battery life. Therefore, a low charging current provides high capacity utilization but charges the battery slowly, which is inconvenient for EV applications.

For example, in the 2S lithium-ion battery pack, two 18650 cells of 3.7V each are connected in series, so the total voltage is 7.4V. This battery pack should be charged when the voltage goes down to 6.4V (3.2V per cell) and charging should be completed up to 8.4V (4.2V per cell). Hence, the 6.4V and 8.4V values are already fixed for this battery pack.

Another method is constant voltage charging, which maintains the predefined voltage to charge the battery. If the voltage is constant, the charging current decreases as the battery charges.

The battery charging requires a higher current value to provide a constant voltage at an early stage. A high charging current of 15% to 80% provides fast charging, but it stresses the battery and can affect its life.

In CC mode, we decide the charging current. This current depends on the C rating of the battery or cell (mentioned in the datasheet of the battery) and on the Ah (ampere-hour) rating of the battery.

Suppose we have decided on a value of 1000 mA as the constant charging current. So initially, when the battery charging starts, the charger should get into CC mode and push 1000 mA into the battery by varying the charging voltage. Due to this, the battery will charge, and the voltage will start to increase slowly.

Constant Voltage Circuit

Here, we consider the CV mode of the lithium battery charger, in which we have to regulate the battery voltage from 6.4V to 8.4V, as discussed earlier. The voltage regulator IC LM317 can do this by using just two resistors. The below circuit describes the 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.

Here, the output voltage (Vout) should be 8.4V. To build this circuitry, the value of R1 should be less than 1000 ohms, so we use a 560 ohm resistor. With the help of the above formula, we can calculate the value of R2.

• 8.4V= 1.25*(1+(R2/560ohm)

• R2= 3.3KOhm.

Alternatively, you can use any resistor value combination that provides an output voltage of 8.4 volts. For this combination, you can use the online LM317 Calculator to make your work easier.

Constant Current Circuit

Using a single resistor, the LM317 IC can be a current regulator. The below diagram shows the battery charger circuit for this current regulator.

As per the above explanation, we are considering 1000 mA as a constant charging current.

To calculate the resistor value for the required current, which is provided in the battery data sheet,

Resistor (Ohms) = 1.25 / Current (Amps)

• R = 1.25/1A = 1.25 ohm.

So we need to use a 1.25-ohm resistor to build this circuit. We don’t have a resistor with a resistance of 1.25 ohm so we chose the closest value of 1.5 Ohm which is mentioned in the circuit diagram.

Auto Cut-off Circuit

The auto cut-off is the most important parameter in battery charging. Nowadays, most batteries use the auto cut-off circuit. The below circuit diagram shows the battery charger circuit with the auto cut-off feature. It is implemented by using the adjustable voltage regulator LM317.

This circuit will give an adjustable DC supply output voltage and charge the battery. The LM317 is a monolithic integrated circuit that is available in three different packages. This adjustable voltage regulator delivers a 1.5A load current and provides an output voltage range from 1.2 to 37 V.

Working of Auto Cut-off Circuit

It uses basic power supply components like a transformer, rectifier, filter, and regulator. The step-down transformer (230V to 15V) steps down the AC power supply. Next, the rectifier uses four 1N4007 diodes, which convert step-down AC into DC.

Capacitors C1 and C2 are used for filter operation. To regulate the voltage, we have used C1 LM3used aC. It also works as a current-control device.

Here, the variable resistor VR1 changes the supply to the ADJ (adjust) pin of the voltage regulator, and hence it changes the output voltage.

Here, we have shown the green and red LEDs. The green LED shows the charging condition of the battery, and the red LED represents the complete charge of the battery.

When the battery is completely charged, the Zener diode (12V) generates the reverse voltage, which flows to the base of the BD139 transistor and turns it on. Due to this conduction in a transistor, the ADJ pin of the voltage regulator will connect to the ground, which cuts off the output voltage from the regulator. During this continuous process, to avoid the thermal runway, use a heat sink with a voltage regulator.

IC LM317 provides a variable output voltage. This voltage can be varied with the help of an ADJ pin so the total output voltage is as,

• 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

Supplying Current depending upon Battery Rating

It is very important to decide the charging current to maintain the battery's life. This charging current depends on the capacity of the battery (ampere-hour rating). Every battery has a specified ampere-hour rating. This is the charge storage for the battery.

Kindly refer to the below sample calculations for charging time. The below calculations are approximate. The charging current is not the same all the time. As the battery reaches near full charge, the charging current decreases.

For example, we have a battery with a capacity of 50 Ah:

First, we will calculate the charging current. As per the standard, the charging current should be 10% of the battery capacity.

Therefore, charging current for 50A Battery = 50 Ah x (10/100) = 5 Amperes.

But due to some losses, we may take 5-8 Amperes for battery charging purposes.

Suppose we considered 8 Amp for charging purposes,

Then, the charging time for 50Ah battery = 50 / 8 = 6.25 Hrs.

But this is an ideal case, practically, it has been noted that 40% of losses occur in the case of battery charging.

• 50 x (40 / 100) = 20 …..(120Ah x 40% of losses)

Therefore,  50 + 20 = 70 Ah ( 50 Ah + Losses)

Battery charging time = Ah / Charging Current

1. 70 / 8 = 8.75 Hrs ( in real case)

Therefore, a 50 Ah battery would take around 9 Hrs to fully charge in case of the required 8A charging current.

If your battery has a capacity of 50 Ampere-hour, then you should not use a charger with a charge current of 5A. If so, then it will take around 10 hours to charge the battery and definitely, you won’t like this.

An ideal charging time for batteries should be 2-3 hours. This charging current rate can vary with the types of batteries so you can set the charging current as per the battery capacity and its type.

Final Words

I hope this article helps you to understand the complete guide to an automatic battery charger circuit. The battery chargers are varied with applications like mobile phone chargers, electric vehicle battery chargers, and charge stations. As per the battery specification, we can design the battery charger circuit using SCR, Op-Amp, different regulator ICs, etc.

Tags : Battery charging circuit , Battery charging with auto cut-off , Constant voltage , what is CV and CC