A car battery is a typical lead-acid battery with about 6 cells, each of 2V such that the total battery voltage is around 12V. Typical values of battery ratings range from 20AH to 100AH.  Here we are considering a car battery of rating 40AH such that it’s required charging current would be around 4A. This article aims to describe the principle of operation, design, and working of a simple car battery charger from the AC mains supply and a feedback control section to control the battery charging.

Car Battery Charger Circuit Working Principle:

This is a simple car battery charger with the indication. The battery is charged from a 230V, 50Hz AC mains supply. This AC voltage is rectified and filtered to obtain an unregulated DC voltage used to charge the battery through a relay. This battery voltage is constantly monitored by a feedback circuitry compromised of a potential divider, a diode, and a transistor. The relay and the feedback circuitry are fed by a regulated DC voltage (obtained using a voltage regulator). As battery voltage increases beyond maximum, the feedback circuitry is designed such that the relay gets switched off and battery charging ceases.

Car Battery Charger Circuit Diagram:

Car Battery Charger Circuit Diagram

Car Battery Charger Circuit Diagram

Car Battery Charger Circuit Design:

To design the entire circuit, we first design three different modules- the power supply section, the feedback, and the load section.

Power Supply Design Steps:

  1. Here the desired load is a car battery with a rating of about 40AH. Since the charging current of a battery should be 10% of the battery rating, the required charging current would be around 4A.
  2. Now the required transformer secondary current would be around 1.8*4, i.e. approx 8A current. Since the required load voltage is 12V, we can settle for a transformer with 12V/8A rating. Now the required RMS value of AC voltage is around 12V, the peak voltage would be around 14.4V, i.e.15V.
  3. Since here we are using a bridge rectifier, the PIV for each diode should be more than four times the peak AC voltage, i.e. more than 90V. Here we select diodes 1N4001 with PIV ratings of about 100V.
  4. Since here we are also designing a regulated power supply, the maximum allowable ripple would be equal to the capacitor peak voltage minus the required minimum input voltage for the regulator.  Here we are using a voltage regulator LM7812 to provide a regulated 5V supply to the relay and the 555 Timer. Thereby the ripple would be around 4V (Peak voltage of about 15V and input regulator voltage of around 8V). The filter capacitor value would thus be calculated to be around 10mF.

Feedback and Load Section Design:

Designing of the feedback and load section involves the selection of resistors for the voltage divider section. Since the diode will conduct only when the battery voltage reaches 14.4V, the values of resistors should be such that the positive voltage fed to the diode is at least 3V when the battery voltage is around its maximum.

Keeping that in mind and with necessary calculations we select a 100 Ohm potentiometer and other resistors of 100Ohms and 820 Ohms each.

Car Battery Charger Circuit Operation:

The circuit operation commences once the power supply is available. AC power of 230V RMS is stepped down to a voltage of 15V RMS by the step-down transformer. This low voltage AC voltage is then rectified by the bridge rectifier to produce an unregulated DC voltage with AC ripples. The filter capacitor allows the AC ripples to pass through it, thus producing an unregulated and filtered DC voltage across it. Here two operations take place: – 1. This unregulated DC voltage is fed directly to the DC load (The battery in this case) through a relay. 2. This unregulated DC voltage is also fed to the voltage regulator to produce a regulated 12V DC supply.

Here the relay is a 1C relay and the common point is connected to the normally closed position such that current flows through the relay to the battery and it gets charged. As current passes through the LED, it starts conducting, indicating that the battery is being charged.  A part of the current also flows through the series resistors such that the battery voltage is divided using the potential divider arrangement. Initially, the voltage drop across the potential divider is not enough to bias the diode. This voltage is equal to the battery voltage and thus determines the charging and discharging of the battery. Initially, the potentiometer is adjusted such to its midpoint.  As battery voltage increases gradually, it reaches a point where the voltage across the potential divider is enough to forward bias the diode. As the diode starts conducting the base-emitter junction of the transistor Q2 is driven to saturation and the transistor is switched on.

As the transistor collector is connected to one end of the relay coil, the latter gets energized and the common contact point moves to the normally open position. The power supply thus gets isolated from the battery and charging of the battery stops. After some time as the battery starts discharging and the voltage at the potential divider again comes to a position such that the diode is reverse biased or in off condition, the transistor is forced to cut off and the Timer is now in off position such that there is no output. The common point of the relay moves back to its original position i.e. the normally closed position. Again the battery starts charging and the whole process repeats.

Applications of Car Battery Charger Circuit:

  1. This circuit is portable and can be used at places where AC voltage supply is available.
  2. It can be used to charge toy automobile batteries.

Limitations of this Circuit:

  1. It is a theoretical circuit and may require some practical changes.
  2. Battery charging and discharging may take longer time.


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