Portable Power supply. Design Note 44

Hacked my Laptop battery and charger to make a Portable power supply. My old Laptop was irreparably damaged leaving only a good battery. It is a high performance 12 volt Lithium Ion rechargeable battery that can backup more than 2 hours a laptop and many hours for circuit boards and LED lights. So I made a Portable power supply using the battery. So don’t discard the battery and charger if your Laptop fails permanently. Both battery and the charger costs around Rs. 2000.It can be utilized for making a Power supply or LED lamp. So it is the  good power supply I ever made because it is running on battery and output is fully regulated.

What I did is shown in the images. The power supply gives 4 output voltages with more than 1 Ampere current.

1. 18 Volt – The charger is rated 18.5 volts so one power out was directly from the charger positive going to the battery. A 10 Amps diode is provided here to separate the battery and the 18 volt power supply. So when the charger is on, 18 volt will be available in the socket.

2. 12 Volt – It is tapped directly from the battery.

3. 5 Volt – For this, 7805 Regulator IC is used. The 12 volt from the battery enters into the regulator IC and it gives 5 Volt regulated power. Capacitors are provided on either side of the regulator IC to suppress noise and transients.

4. 3.6 Volt– Two IN 4007 diodes are used to drop the 5 volt DC from the regulator IC to get 3.6 Volts DC. Each diode has a forward voltage drop of 0.7 Volts. So both will drop 1.4 Volts and reduces 5 Volts to 3.6 volts. This low volt is similar to two pen cells so it can be used for Microcontroller or Arduino projects.


Low battery indicator.

The Laptop battery is not necessary to charge frequently because it has great ability to hold charge. So to indicate the low battery voltage, a circuit was provided with a Red LED. The LED lights when the battery voltage drops below 9 volts. The circuit works like this.

When the battery voltage is above 9 volts, Zener conducts and gives base current to T1 and it conducts. When T1 conducts, it pulls the base of T2 to ground. So T2 will be off keeping the LED off. When the battery voltage drops below 9 volts, Zener turns off followed by T1. So T2 gets base current through the 4.7 K resistor and it conducts. This turns on LED. At this time, charger can be connected. As the battery voltage rises above 9 volts, LED turns off. Preset VR can be used to set the Zener break down point.


LED light.

The battery has high power so an LED light can be connected to work in night or as an emergency light. I used a ready made SMD LED light but 6 White LEDs with 100 Ohm resistors will be sufficient. A switch is provided for the LED light for on/off.
You can also make this Portable power supply with a 12 volt 4.5 Ah UPS  battery and suitable charger.


Advantage I found
The Laptop battery pack has an internal short circuit protection circuit so that ,when a short occurs momentarily in the power supply outlets or during charger plug in, the battery shut off the power supply for 10 seconds. If the short circuit is permanent, the battery will be in shut off mode. So this protects the Bread board assembly or during trouble shooting the circuits.