Most Power Functions pieces will run in classic 9V mode. The IR receiver can't, but motors and lights can.
A hacky solution:
Many people have done this by cutting up an extension wire, and connecting the two outer cables to their power source of choice.
- GND – ground wire – Always connected (0V regardless of battery pack direction)
- C1 – control wire 1 – 9V when battery pack is forward, 0V when in reverse
- C2 – control wire 2 – 0V when battery pack is forward, 9V when in reverse
- 9V – power wire – On when battery pack is on (9V regardless of pack direction)
A purist solution:
If you prefer a purist solution, you could get a classic 9V battery box that takes a single regular 9V battery.
They have the classic 9V wire connectors on the top (a little hard to see in the image, but there are 2x6 studs with metal connectors). Just pair that with a Power Functions extension wire (which has a 9V connector adapter) to connect your Power Functions elements.
That will give you a purist solution which doesn't require chopping up your wire.