Battery charger circuit with indicator, over current & overcharge protection

LM317 battery charger circuit

The circuit is a 6V LM317 voltage and current control battery charger circuit which generates a regulated 6V DC output.

The transformer T1 steps down the input 230V/50HZ AC supply to a 6V AC. Then it converted to a 6V DC, by a bridge rectifier circuit. The capacitor C1 filters the rectified output.

The circuit uses an LM317 regulator, which is an adjustable linear positive voltage regulator which can operate in an input voltage range of 3-40V.

battery charger, 6v lead acid battery charger circuit, lm317 charger circuit

The value of resistor R1 and R2 determines the output voltage value of LM317.

Equation for LM317 output voltage, Vout = 1.25 * (1+ R2/R1)

In the given circuit, the resistor combination (R1 & R2) will obtain a maximum output voltage of 6.125 volts.

The LM317 has a maximum operating current of 1.5 Amperes, with an internal current limiting and thermal overload protection. But, the circuit is already designed with an additional overcurrent protection. Current limiting arrangement regulates the output voltage of the LM317 to limit the current flow above a fixed value. The input voltage to the battery will be adjusted itself, with respect to the charging current. The output voltage of the circuit varies from 1.25V to 6.125 V. When the current flow through the sensing resistor R3 increases, the base current of the Q1 also increases. So it will reduce the resistance across R2 and thereby the value of Vout.

This circuit is designed for charging 6V 4.5AH lead acid batteries. But the output voltage and current limit of the circuit can be varied to use with other batteries. The charging voltage and current depends on the value of resistance R2 and R3 respectively. Thus, by replacing the resistance R2 and R3 with a potentiometer.  We can constantly adjust the output voltage and current of the circuit.

But, while using the circuit with other batteries, the charging rate and other parameters should be considered.

Components required

LM317 – Voltage Regulator IC

Resistor – R1, R4  – 1k, R2  – 3.9k, R3 – 2 ohms

Capacitor – C1  – 2200uf

Diode – D1-D5 -1N4007

Transistor – Q1- BC547

Transformer – T1- 230V/6V,1A

Automatic battery charger with LED indicator and Overcharge protection circuit

Here the circuit for a 6V automatic battery charger circuit with overcharge protection, LED charging indicator and current limiting feature.

The circuit regulates the charging of the battery by taking feedback of the voltage across the battery terminals. The circuit charges the battery as far as it has a voltage below the threshold limit. And when reaching a value equal to the threshold, the circuit automatically cut OFF the supply from the battery.

The LED D1 and D2 indicate the status, whether the battery is charging or not. The red light (D2 LED) indicates the battery is charging and the green light (D1 LED) indicates that the battery has fully charged.

The battery charger circuit can work with a wide range of input DC supply voltages. The circuit can operate in the range of voltages approximately above 6.2V and up to a maximum 18V (Maximum operating voltage of IC7555).

Circuit Operationautomatic battery charger, battery charger with LED indicator

Normally a 555 IC has a threshold and trigger voltage of 2/3 and 1/3 of the supply voltages respectively.

Here the zener diode 1N4735 of 6.2V has connected to the control voltage terminal (pin 5) as shown in the circuit. The trigger input (pin2) has connected through a voltage divider network and threshold input (pin6) has connected directly from the battery. This adjusts the threshold voltage and trigger voltage to a fixed value of 6.2V and 3.1V, for any input voltage values above 6.2V.

The output (pin 3) of the 7555 is connected to the base of the transistor Q1, which controls the charging current of the battery. When the voltage of the battery goes below 6.2V, the trigger input at pin2 becomes 3.1V. Then the output switches to the high state and ON the transistor Q1. Similarly, when the voltage reaches the threshold value 6.2V the output turns to the low state and OFF the Q1.

The arrangement with transistor Q1 and Q2 works as a current limiter circuit. When the current flow to the battery increases, it proportionally increases the voltage drop across the current sense resistor R6 also. Thereby, the base current of transistor Q1 pulled down by the transistor Q2 and reduces the collector current through the Q1. So, the circuit can able to limit any chance of overcurrent.

Components required

Integrated circuit -IC1 – 7555

Resistor – R1,R2,R3,R7,R8 – 1K, R4, R5 – 100K

Diode – D1 – Green LED, D2 – Red LED, D3 – Zener 1N4735

Transistor – Q1 – 2N2222, Q2 – BC547

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6 Responses

  1. Ashley says:

    how much voltage is of C1 capacitor?

  2. Addy says:

    what is the range of current & voltage around R2 & R3 respectively during at 6v 4.5 Ah battery charging in lm317 circuit?

    • admin says:

      The instantaneous charging current and output voltage value varies with respect to the voltage level of the battery. The maximum charging current is around 350mA. R2 voltage range 0 to 4.87V, R3 current 0 to 350mA.

  3. olatunji says:

    the diagram is not showing on my browser

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