Car battery charger with Temperature Protection

This is a simple car battery charger circuit with 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 the maximum, the feedback circuitry is designed such that the relay gets switched off and battery charging ceases. Figure below shows the drawing

Figure 1 - Drawing of battery charger

Shutdown relay when charging the battery is complete

 In detail, we note that the relay is controlled by the transistor T1, brought into conduction via resistor R1. At the base of T1, there are also two diodes (D6 and D7) which come from the outputs of the operational amplifiers Amp1 and Amp2. These two operational amplifiers are part of the LM358 integrated circuit, each with its own function.

Amp1

Amp1, controls the temperature of the transformer, via the NTC thermistor, mounted in close contact with the transformer itself (very important, mandatory). If the temperature becomes excessive, the NTC value decreases, and with it, the voltage on the (+) input also decreases; the output therefore switches to a low level, blocking the conduction of transistor T1 with the consequence that relay will de-energized (opens the Na contact), disconnecting the appliance from the mains. The NTC should have a value of no less than 5 kohm; the value of Trim1 must be chosen based on the NTC, and its value must be one and a half times that of the NTC (for example, if the NTC is 10 kohm, a trimmer of approximately 15 kohm will be used)

 Amp2

AMP2 reads the voltage on the battery; when the value of the battery charge is considered complete, the operational switch switches to a low level and, as in the previous case, disconnects the device from the mains.

It is therefore a rather versatile battery charger, which everyone can calibrate as they wish, with a little patience.
The first calibration must be carried out by adjusting the Trim2, which will allow you to define the battery voltage value which the appliance must disconnect from the mains.
The second calibration is carried out by adjusting the Trim1 trimmer, to set the temperature at which the battery charger must disconnect from the mains; you can proceed experimentally by trying to heat the NTC thermistor with any heat source and checking if and when the relay de-energizes.
Both the Amp1 and Amp2 operational amplifiers use the 5V voltage stabilized via the L78L05 regulator as the reference voltage for the non-inverting input.
LM358, requires its own power supply, which is obtained by connecting pin 8 to the positive pole of the power supply and pin 4 to the negative pole (as indicated in the diagram).

List of components

• Transformer: the choice of the transformer depends on the type of charging you want: with a 230/12 V, with a power not less than 30 VA, you obtain a slow charge, with a limited current; this current can be increased by adding the capacitor CX.  A faster charge is achieved with a 230/15 V transformer with 45 VA.
• D1, D2, D3, D4, and D5: are five diodes with 4 A; they should be Schottky type, as they have a lower voltage drop and heat less
• D6, D7, D8: they are common diodes such as 1N4001 series etc.
• Relay: equipped with a 12 V coil and a Na contact (normally open and at least 1A) which closes when the relay is energized
• R1: Base resistor of T1, 150 kohm, 1/4 W
• NTC: NTC resistor (thermistor); it must have a value between 5 and 12 kΩ at environment temperature
• TRIM1: trimmer; it must have a value approximately 1.5 times more compared to the NTC
• TRIM2: 10 kΩ trimmer
• L78L05: integrated voltage regulator
• C1: 0.1 µF capacitor
• T1: NPT transistor type BC517 or equivalent
• Amp1 and Amp2: use the two operational amplifiers of the LM358
• CX: optional electrolytic capacitor, value from 2200 to 4700 µF
• Fuse: 500 mA fuse, to be inserted on the primary of the transformer
• PIC18F97J60 (optional)
• Display LCD 16x2 (4-bit or 8-bit MPU Interface) (Optional)
• Manual winding and automatic winding selector (Optional)
• SCR Diode 2N6397 (optional)

Project cost

The cost of the project without the automatic charger battery would be around 40$, while the automatic charger would be around 50$. For any questions contact our expert.