You will appreciate this idea if you ever had to clobber together a bunch of AAs or AAAs to get a project working.
This instructable was meant to be part of a larger project, but I decided to submit this part alone to the 9V battery group.
You need to start with a 9V rechargeable battery (actually 8.4V but in a 9V package).
Check out my other Instructables:
MAKE A HIGH VOLTAGE SUPPLY IN 5 MINUTES
Hack The Spy Ear and Learn to Reverse Engineer a Circuit
Super Easy E-mail Encryption Using Gmail, Firefox and Windows
Make a Voltage Controlled Resistor and Use It
Make a Ball Mill in 5 Minutes
SODA CAN HYDROGEN GENERATOR
Step 1: Crack The Case Open
There are seven 1.2V NiMH batteries soldered in series.
To make the power supply even you need to get rid of one or add one.
I ripped off the one near the top. Don't try to unsolder it because these batteries are welded together and the batteries might explode if heated too much (so wear goggles).