Step 1: The Materials and Tools
1) Three 9 volt batteries (I know 9 x 3 = 27, not 28, but electronics don't care!)
2) Wire - I used 22 gauge because it's what was laying around, but 20 would work just as well.
3) Electrical tape
4) Heat shrink tubing - preferably black and red to color code + and -
5) Electrical connector pins - find them inside any male electrical connector (If they already have wire attached to them, you are ahead of schedule!, just make sure the wire is of sufficient gauge to handle your power requirements.)
6) Solder and flux
1) Wire strippers
2) Wire cutters
3) Soldering iron
4) Needle nose pliers
5) Crimpers - for crimping the electrical pins onto your wire. Not necessary if your pins are already attached to wire. You can also use a pair of vise grips.
6) Heat gun - not necessary if you are using colored permanent markers instead of heat shrink tubing.
Step 2: Prepare Your Wire
After your wire is cut, strip about 1/2 inch and tin it.
*to tin wire, cover the stripped part in flux. Heat your soldering iron up and melt a glob of solder on the end. Touch the melted solder to the exposed wire. You may have to run the tip of the soldering iron along the length of the exposed wire. This covers all the individual strands of the wire in solder, making them stay together, and is much easier to work with.
Slide a small piece of red shrink tubing on one of your wires, and a piece of black on the other, and shrink it down using your heat gun. Radiant heat from your soldering iron will work also, just be careful not to burn through the heat shrink. This will allow you to easily tell positive from negative when using this power supply. If you don't have colored shrink tubing, you could use black and red permanent markers.
If your wire already has pins attached, you are done preparing the wire. If not, strip some of the insulation from your wires and crimp the pins on.
Step 3: Attach Your Wire to Your Batteries
Using the needle nose pliers, twist the tinned end into a small circle. This small circle must be small enough to fit into the negative terminal of one of your batteries.
***Avoid touching the soldering iron to the battery for more than 2 or 3 seconds---if you heat it up, it could damage the battery or explode!*** YOU HAVE BEEN WARNED! Proceed at your own risk! Safety glasses or a face shield may be a wise choice at this point...I always wear them when soldering anyway.
Fit the twisted end of the wire into the negative terminal of one of your batteries. Put a tiny amount of flux in there. Get a big glob of solder on your soldering iron, and VERY QUICKLY solder the wire to the terminal. Hold it in place a few seconds until it cools down.
After the solder cools, give a little tug on the wire to see if it is soldered in there properly. You shouldn't see any movement. If you do, pull it out, clean up the wire, and try again.
Repeat this for the positive side, using the positive terminal on a different battery.
Step 4: Assemble It!
Take the two batteries that have wires soldered to them and place them next to each other. Make sure the wires are on the outsides.
Take the third battery, turn it upside down, and plug it into the other two batteries.
Use the electrical tape and tape it up. I made sure the wires were taped up along the side of the upside down battery so they have some support. This way there won't be a lot of strain on the solder joint.
That's it! Congratulations, you have just made a simple, portable, cheap, pocket sized 28 VDC power supply!