Battery Charger with Indicators by Laptop Charger

Laptop chargers can be used to recharge car and motorcycle batteries by adjusting the voltage. Typically, laptop chargers are around 19 volts. Therefore, this voltage needs to be lowered to prevent overcharging and damage to the charger.

This procedure is quite easy, and the circuit is simple yet reliable, with readily available components and easy assembly.

The top photo shows this simple circuit that can be made using a matrix board, with recycled components.

There are two LED indicator lights for easy operation.

Also read the detailed explanation of the battery recharging process.

Also read about charging a battery with a laptop charger that uses a voltage regulator IC.

CAUTION: Before charging the battery, check the battery's water level. Keep the battery caps loose while charging to allow for proper gas ventilation. Overcharging can occur. The maximum charging voltage for dry-cell batteries is around 13.8 volts, while for wet-cell batteries it is around 14.4 volts. Please refer to your battery's specifications to prevent overcharging.

This simple circuit lowers the laptop charger voltage to match the battery's voltage and charging current.

The components used are:

D1 - D8 = BY127 diodes, average current 1 to 1.5 amperes

L1 = 5 mm green LED

L2 = 1 watt white LED, surface mount device (SMD)

Dz = 12 volt 1 watt zener diode

R1 = 2.7 kilo-ohm, 0.5 watt resistor

R2 = 6.8 kilo-ohm, 0.5 watt resistor

R3 = 120 ohm, 1 watt resistor

S = 1 to 3 amperes fuse, depending on the load size, in this case, the battery.

The maximum current required by the battery during charging is approximately 10 percent of its capacity. So, for a 36 ampere-hour battery, the maximum charging current is 3.6 amperes, if the battery is very discharged or has very low voltage.

The laptop charger used in this article has a voltage of 19 volts and a maximum current of 3.95 amperes, as shown in the following photo.

This makes it powerful enough to charge a 36 amperes-hour battery.

The circuit above reduces the 19-volt voltage from the laptop charger to around 13 to 14 volts by passing through several diodes.

The LEDs serve as indicators.

The white LED indicates that the system is working properly, and it is ready for use. The white LED lights up when the circuit is unloaded. This LED is intentionally made large to reduce the voltage so that the voltmeter reads more normally when unloaded.

The green LED lights up when a load draws current, such as when charging the battery. When the circuit is loaded, the white LED goes out.

Theoretically, if current flows through one diode, the output voltage will drop by 0.6 volts. This is because diodes require forward voltage.

When connected to a 19-volt laptop charger, the no-load output voltage measured through eight diodes is 15.4 volts.

It's important to note that, although this voltage may seem too high for charging a battery, the voltage will drop if the circuit is loaded. This is due to the nature of diodes, where the forward voltage increases as the current increases, so the output voltage will decrease as the output current increases.

When the circuit is unloaded, the white LED lights up brightly, indicating that the circuit is ready for use. Meanwhile, the green LED lights up very dimly.

If the circuit is loaded with a 220-ohms load, the voltage drops to 14.3 volts, with an output current of approximately 0.065 amperes. At this point, the white LED will dimly lit, while the green LED lights up brightly.

The voltage becomes 12.9 volts when the circuit is loaded with a 24-ohms load, with a current of approximately 0.5 amperes. At this point, the white LED will go out, while the green LED remains lit brightly.

The voltage becomes 12.5 volts when the circuit is loaded with a 12-ohms load, with a current of approximately 1 ampere. The green LED remains lit brightly. The white LED goes out because the voltage is insufficient to allow current to flow through the white LED and zener diode.

The voltage above is considered normal for battery charging. If the voltage is deemed too low, the number of diodes can be reduced to achieve a higher output voltage. Diodes are reduced starting from the output end (D8), for example: the output is taken from D7, or even from D6. The current supply connection to the white LED can be changed if the diode is reduced, or it may not be changed.

If the required continuous current is less than 0.5 amperes, for example for charging a motorcycle battery, a smaller diode such as the 1N4007 can be used. Because this diode is very popular, many manufacturers produce it, but sometimes the quality is less good.

See also the video on YouTube:

The charging current will decrease as the battery voltage increases. If the battery voltage is equal to the charger voltage, the charging current will be very small.

When the battery voltage reaches approximately 14 volts, the green LED will dim. If the conditions and settings are correct, as is typically the case when charging a small battery, the white LED will begin to light up again. This indicates that the battery is about to fully charged.

Resistor 1 (R1) serves as a safety measure if this power supply circuit is connected to several circuits with ICs that cannot withstand the high voltage leakage caused by induction from the transformer. Therefore, R1 can be omitted if conditions are deemed safe.

The advantage of this circuit is its high reliability, as it lacks active components such as transistors or ICs (Integrated Circuits) that are prone to damage. Its simple operation makes it easy to understand, the components are widely available on the market, and it is also easy to assemble. Even if damaged, it is easy to repair.

The disadvantage is the lack of a voltage stabilizer. The voltage can drop when the load increases, and the voltage rises again when the load decreases. This is because diodes require a forward voltage to conduct current.

However, this voltage fluctuation is used to activate the indicator lights. And because the circuit is supplied with a laptop charger with a stable voltage, the output voltage can be considered stable enough for applications such as battery charging.

Theoretically, power efficiency is low due to voltage loss in the diode and its conversion to heat. In reality, the temperature increase in the diode is insignificant.