Getting 5 VDC from "dead" batteries
(Last modified 15 Mar 12)

I'm always needing 5 VDC for running some MCU project, and it isn't always convenient to hook up the bench supply or dig out a USB cable.  This is especially true if I'm in the field, working on an outdoor install.  I also have a collection of battery holders, scavanged from Christmas lights over the years.  This project turns these surplus battery holders into portable 5 VDC sources.

Before (on left) and after

Here is a picture of the original battery holder, with Christmas LEDs attached, on the left.  The battery holder has a switch on the end near the LED cord.  When you turn on the switch, the LEDs blink, thanks to a tiny PCB built into a compartment near the switch (towards the top in this photo).  I reworked the battery holder on the right, replacing the original PCB with a 5 VDC boost converter from SparkFun.

A look inside

I didn't change anything in the battery compartment, just in the switch compartment at the top.  I removed the original LED cable, then wired the switch in series with a wire from the positive terminal of the lower battery.  The wire from the other side of the switch is connected to the input terminal (1-4V IN) on the booster PCB.

Closeup of switch and booster PCB

Here you can see where I've added the SF 5 VDC booster PCB.  I've also wired a small, red LED and 1.5K ohm resistor between the GND and 5V OUT connections on the booster PCB.  The LED lights the inside of the battery holder when the unit is switched on.  I did this so I would remember to turn off the juice when I'm not using the batteries.  Otherwise, the booster PCB will drain the batteries all by itself.  The 1.5K resistor limits current into the LED to about 2 mA, so I used a very bright (but tiny) LED.  You can leave off the LED and resistor if you choose.

You can also see where I've routed the red (5V OUT) and black (GND) wires through an existing hole in the side of the case; I tied a knot in the wires to act as a strain relief.

Another close-up

Here's another view of the reworked switch circuitry, showing how I"ve stuffed the booster PCB into the small compartment.  I stuck a piece of insulating tape over the bare terminal for the negative battery connection, to keep things from shorting out.  The PCB is only held in place by the wiring mess; there was no need or room for mounting hardware.

I originally started this project intending to use a modified Joule Thief circuit, but could not get reliable 5 VDC output and could not fit it into this tiny space.  But the SF booster board works perfectly and only costs $6.

The booster board will work down to 1 VDC input, which means the batteries will be drained completely before you throw them out.  The batteries shown here were pulled from devices that refused to work when the batteries hit about 1.2 VDC.  Given the efficiency of the booster PCB,  I expect a lot more use out of these "dead" batteries before I finally have to dump them.

Note that it's probably not a good idea to use NiMh or other rechargeable batteries in this device.  The boost converter will drain them so low that the batteries could be damaged.

And that's it...
A simple project that I expect will prove valuable in my hobby.  I'm reusing leftover bits from Christmas lights, getting ALL the energy from my AA batteries, and carrying portable power to my MCU boards.  Pretty sweet for an hour of my time...