Replacing The Super Capacititor

[xman]

Been wanting to do this test for a very long time but I haven't had much Xbox time of late. I found the time today and it required soldering a couple of wires to the cap's solder pads on the underside of the board and run the wires outside the Xbox so I could connect my meter to them and watch what the voltages did. This machine hadn't had power to it for over 18 months so we can assume the cap was fully discharged.

These are the voltages I got-

Initial bootup - 2.36v
After 15 minutes - 2.40v
After 30 minutes - 2.42v
After 45 minutes - 2.43v
After 60 minutes - 2.44v

Now I turned off the machine but left it plugged in-

Straight away - 2.15v
After 15 minutes - 2.15v
After 30 minutes - 2.15v
After 60 minutes - 2.15v

Now I unplugged the machine from the wall-
Straight away - 1.48v
After 15 minutes - 1.32v
After 30 minutes - 1.20v
After 45 minutes - 1.13v
After 60 minutes - 1.03v

Interesting set of figures I think. Now I need to work out a safe battery voltage to replace the cap with that will allow a diode inline so the Xbox circuit doesn't try to recharge the Un- rechargeable battery supply allowing for the .6 of a volt the diode will drop. Any thoughts on this subject?.

[professor_jonny]

if you uses a 3.3v coin use and two diodes in series it would over a forward voltage drop of .6 across each reducing the voltage to 2.1

you could also try a 2.5v zeener as a shunt regulator I guess with a rather high value of series resistor to limit the battery drain?