Laptop Charger To Recharge Vehicle Battery

Laptop chargers can be used to recharge car and motorcycle batteries by adjusting the charger output voltage. So that the battery will not over voltage or overcharged and the charger is not damaged.

The procedure is useful during covid-19 lockdown, when vehicle is rarely used so the battery needs to be charged. This procedure is quite easy and can be done in your spare time, on Sunday or holidays. Also read the detailed explanation of the process to recharge the battery.

The following video shows when the circuit is tested on a car battery with a capacity of 36 amperes hour), usually it is called 40AH.

CAUTION: Before charging the battery, check the battery water level first. And keep the battery cap loose in it's hole for better gas ventilation. Overcharging can blow up the battery. The maximum charging voltage for dry cell battery is around 13.8 volts, while for wet cell battery is around 14.4 volts, please refer to your battery specifications to prevent overcharging.

The maximum current needed by the battery when charging is about 10 percent of the battery capacity. So for a 36 amperes hour battery, the maximum charging current is 3.6 amperes.

The laptop charger has an output voltage of 19 volts and a maximum current of 3.95 amperes, as the following photo shows. So it has just enough current to charge a 36 ampere hour battery.

This simple circuit regulates the laptop charger voltage to match the battery charging voltage and current. The output of this voltage regulator can be adjusted by connecting some diodes in series.

Components list:
Tr = 2N3055 power transistor with heatsink
IC = LM7815 voltage regulator
C1 = C2 = condenser 22 mf 25 v
D1 = D2 = diode 1N4007
R = 22 kilo ohms resistor

The above circuit will reduce the voltage of 19 volts from the laptop charger to 15 volts with the help of IC LM7815. Furthermore, the IC supplies current to the base of the transistor 2N3055 through several series diodes.

Theoretically, a single diode will drop voltage by 0.6 volts. So for two diodes, the 15 volts IC output will be reduced to around 13.8 volts, before entering the transistor base. Those diodes can be jumpered to adjust the output voltage.

In the transistor, voltage will drop again about 0.6 volts. So the transistor output will be around 13.2 volts.

Condenser and resistor function as voltage stabilizer. Without this component a digital voltmeter will show a higher no-load voltage, which can be confusing.

Measured no-load voltages are:
two diodes = 13.6 volts
one diode = 14.03 volts
no diode = 14.50 volts

The photo below shows the arrangement of the circuit components on a matrix board.

The video shows that the battery condition is still quite good, it looks blue in the battery indicator. As shown by the yellow arrow in the following photo.

But the battery voltage is quite low at 12.55 volts. By using a jumper cable, the charging current can be adjusted by selecting the number of diodes connected. The test results on the charging current fot that car battery are:
two diodes = 0.4 amperes
one diode = 1.7 amperes
no diode = 2.6 amperes

I prefer to use the lowest output voltage, because I don't want the laptop charger to be overloaded. The charger is still being used to charge the laptop battery. And the 2N3055 power transistor can overheat, if it produces a higher output voltage. Even if a larger charging current is needed, a circuit with one diode is usually enough for a 36 amperes hour car battery.

Based on the 2N3055 transistor datasheet, it is capable of carrying collector currents (Ic) up to 15 amperes continuous, and a maximum power of 115 watts. From the test result it can be concluded that the transistor is still within safe limits. If you really need to make a car battery charger from a laptop adapter with a larger current, then the heatsink must be bigger.

If it is applied as a simple motorbike battery charger, the circuit as seen in the video is sufficient, easy and inexpensive. IC LM7815 is theoretically capable for carrying current up to 1 ampere. So to charge a small battery such as a 5AH motorbike battery, where the charging current is around 0.5 amperes, there is no need for a power transistor. But it needs to be ensured the IC capacity, because the specification can differ depending on the manufacturer. If this IC is hot, it needs to be cooled by heatsink.

Diodes arranged in series can also reduce the voltage to match the battery voltage, without the need for an IC and power transistor. But the diode has a small current limit, less than 1 ampere. If you use a lot of large current diodes arranged in series, usually the cost is more expensive than using an IC as a voltage regulator and power transistor.

The charging current will decrease as the battery voltage rises. If the battery voltage is the same as the charging voltage, the charging current will be very small. So to prevent overloading on the circuit, charging can be done in stages. The initial stage uses two diodes, then the number of diodes is reduced gradually until the battery reaches its maximum voltage.

There is another simple and popular circuit that uses incandescent lamp around 5 to 21 watts 12 volts, the size of the lamp is depended to the size of the battery. The lamp is connected in series to battery to limit the charging current, when using a laptop battery charger as a vehicle battery charger. But on this circuit, charging will not stop even though the battery is full. Because the charging voltage is much higher. So that the battery can be overcharged, and the charger can also be damaged.