>I have a small battery charger for a portable electronic device that I'm
>trying to wire into the car power system.  Unfortunately, it takes 9V.
>I know I can run it off the 12V if I want to give the battery a short
>life, but how can I bring down the 12V to 9V?  Is it as simple as
>soldering  one of those cheap 3-pin voltage regulators?

Yes, as simple as a 3-terminal regulator...  (7809)
Don't forget to solder 2 small capacitors, one from the input pin
to ground and one from the output pin to ground. These will keep
the 7809 from oscillating or becoming unstable. Keep the caps
as physically close to the regulator as practical. Cap values are
rather non critical.  0.1uF @ 50volts or more is fine. Ceramic
disc types are cheapest and work just fine.

Errr.. forgot to ask... how much current does your 9V device draw?
7809 is rated @ 1 amp max. Better to keep current below 0.75 A.
If you need more, there are other ways...
You may also need a small heatsink on the 7809 to get rid of heat
unless your current draw is very low.

Need more? Ask!

Steve