I am an AFOL (Adult Fan Of LEGO) that sometimes uses powered elements in the shows that I attend to display my MOCs (My Own Creation). It's not always practical to run standard LEGO battery boxes as the batteries tend to run down after a few hours and depending on how I've built my MOC it's not always possible to hide a battery box on board. I wanted to make a fairly small self contained unit I could plug in when needed.

For this project you'll need

a 9VDC battery pack

a LEGO Power Functions wire

some small heat shrink tubing

a soldering iron

solder

Step 1: Locate a Power Pack That Supplies 9VDC

I found this one online cheap and it has a 1000mA rating. According
to info I found online the Power Functions M motor I’ll be using it with only
draws 65mA normal load and 850 mA stalled so it should have more than enough
power to run it without worrying about burning out if the motor locks up.

Check out Philo's website here. There's a lot of helpful information and ratings on motor comparisons for all of LEGO's motors

http://www.philohome.com/motors/motorcomp.htm

Step 2: Cut Off the Cord Cap

Cut off the end of the power cord and strip back the
insulation to expose the wires. Plug the power pack into the wall and make note
of the polarity. This one happened to be labeled red as positive.

Step 3: Locate Your Power Functions Cord

Cut the Power Functions wire in half. Then strip the ends of
the wires and twist them together in pairs left and right. I twist them
together because of the way the Power Functions system works. It uses two wires
for constant power for items like the IR receiver. Other devices use the other
two points to reverse directions of motors. By tying them together you can use
a Power Functions switch to reverse direction or just use it as is to have the
motor turn in a single direction.

Step 4: Check Polarity

Use a multimeter continuity setting to test the wires point to point and make note of which is
connected to 9V.

Step 5: Soldering

Slip a piece of heat shrink insulation over the power cord. Solder the positive wire to the Power Functions wire making sure to put a piece of heat shrink insulation on before you solder.Repeat for the negative wire.

Step 6: Check for Power

Plug the power pack in and make sure everything is working properly being careful not to short any wires together.

Step 7: Heat Shrink

If everything checks out apply the heat shrink insulation over the splice. Slide the heat shrink you placed on the cord earlier over the connections and apply it.

Step 8: That's It. You're Done!

If all worked well you now have a power functions compatible cord you can use for powering your MOCs! Back in step 3 you may have noticed I used a Power Functions switch to get the connector end. You can use the switch end rather than the connector if you’d like, just make sure to pay attention to the polarity when doing your splicing.