Replacing Batteries (Re)using USB Cables

Nowadays we own a lot of electronic stuff powered by disposable batteries, especially old devices such as radio, toys, tools, and so on.

Often we do not use them anymore to avoid wasting all those expired accumulators, both from economic end environmental point of view. At the same time we have tons of unused USB cables and USB power supplies or battery pack: it is now the moment to convert our old electronic devices from batteries to USB!

Since USB power supply is set to 5 Volt, in this Instructable I will show some example about how to power devices that need a supply close to 5V or less.

• USB cables
• scissor

Optional

• soldering iron and tin
• resistors
• 10 kOhm potentiometer
• insulating tape or shrink wrap
• voltmeter

You can find a huge amount of information about the USB standard on Internet but, in general, if you cut an USB cable, you will find four cables: red and black are used for +5 Volt and ground respectively, the other two (green and white usually) for the transmission signal. Eventually you may have some extra shielding and copper grounding and some reinforcement plastic cord.

For this project, we just need 5 Volt and GND, i.e. red and black wires only.

This is by far the simplest case. First of all be sure that the three batteries are “in series”, i.e. the "+" of the first is in contact with the "-" of the second, and so on. In this case the total voltage is 4.5 Volt, which means that, thanks to the typical tolerances, you can simply power your device with the 5 Volts of the USB cable. The small difference (0.5 Volt) is normally well tolerated by any device. This is the case of the small transparent radio form the 80ies shown in the picture above.

In this case you cannot connect it directly to 5V, otherwise you may blow up your device.

If the device power absorption is quite constant and known, for instance in case of a calculator powered with two batteries of 1.5V (as shown here), it should be possible to connect a resistor in series to drop 5 Volt down to 3 Volt. But which is the suitable resistor value?

In the example shown here, I am using a calculator whose declared consumption is P = 0.00022 Watt, as shown in the second picture here.

Since we know that the supply voltage V = 3 Volt, we can immediately estimate the equivalent resistor of the calculator with the formula P = V²/R, thus:

R = V²/P = (3 Volt)² / 0.00022 Watt = 41 kOhm approximately.

Since we want to increase the power supply from 3 Volt to 5 Volt, and we need to keep the current constant, we need to increase the total resistance accordingly, thus to 41 kOhm * 5 Volt / 3 Volt = 68 kOhm.

The resistance to be added is finally 68 kOhm – 41 kOhm = 27 kOhm.

I just connected a pin of the 27 kOhm to the red cable, then the other pin to the + contact of the calculator, and the - to the black cable, ash shown in the third picture here. The calculator can now be powered with and USB power bank.

These estimation have been done starting from the knowledge of the device consumption, but what about if it is unknown?

Let's consider the first picture in this chapter, where I want to power an LED with the 5 Volt USB cable. In this case I do not know the LED power consumption, but I know that it needs about 2.7 Volt. Instead of a fixed resistor, I replace it with a 10 turns potentiometer of value 0 to 10 kOhm, with two alligator clips. Note that I added two extra socket to connect a Voltmeter in parallel to the load, in this way I can easily measure its voltage. I set the potentiometer to maximum resistance value (10 kOhm), then connect the USB battery bank. After that, I slowly turn the potentiometer to get the desired voltage on the load (here the LED). At this point I disconnect everything and measure the potentiometer resistance: let's suppose it is 3 kOhm. I finally connect a fixed 3 kOhm resistance in series with the load, add some insulation (a shrink wrap) and I finally got the useful LED probe you can see in the last picture.

Replacing AA and AAA batteries in small devices with USB cables is simple and sometimes straightforward, sometimes you need to strive a bit. But in doing that you save money and, even better, it is useful for the environment. If you throw away a broken keyboard or mouse, cut and keep the cables: in this Instuctable you can find some ideas to reuse them.