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).
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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).
Step 2: Add a Replacement
...and you can't even recharge it because the charger delivers 9V and might explode the batteries.
Ok so what to do?
Well, replace the missing battery with an LED. An LED always maintain a voltage drop of around 1.2V (depending on what type you use). So you made a battery "prosthesis".
Make sure that your LED is pointed in the right direction when you solder it (LEDs only conduct one way).
Solder a wire on the first battery (red +) that will be the +3.6V of your power supply.
Solder a wire between the 3rd and 4th battery, that will be ground.
Solder a wire where you soldered the positive leg of the LED and that will be the -3.6V supply.