[original posting on xboxhacker forum; this post was in response to the question, "any plans on swapping out the capacitor for a battery?".]
Well, that's a good point. Inspection of the board reveals a 32.768 kHz crystal at location 6F. This is the crystal used to keep time in the system: 2^15 = 32768, which is a nice easy number to divide down to get a precisely 1 second pulse with a 15-bit counter. This crystal is hooked up to the MCPX. This, perhaps, answers one question: the backup voltage is 2.5V because the time keeper is in the MCPX, which is a 0.13u part and it is possible that its transistors can't deal with a 3V backup battery. Nonetheless, I disconnected the + terminal of the capacitor and measured the leakage current with the power off: this is 0.14mA. Note that if you totally disconnect the capacitor and the power, the current goes way down, but I presume that's because the clock shuts itself off. Q=CV, so the capacitor holds 2.5 couloumbs of charge when fully charged. At a discharge rate of 0.14 mC/s, the capacitor should last about 5 hours to zero, although the clock probably stops working around 1.3V, so this confirms xboxmagic's 2-3 hour number. I discovered that why my clock sometimes only stays on for about 5 minutes is that it takes a minute or two for the capacitor to fully charge. (There is some series resistor of low value that prevents a *huge* current spike from rushing in if it's discharged). Because I tend to errr...power on my box sporadically for short periods err...I have "odd" gaming habits... ...the capacitor never gets fully charged and thus it looks like it's not keeping time. A 0.14 mA draw is enormous for a battery-backed clock; I looked up the Ibatt for one RTC and it's like 300 nA...about a factor of 500 less. The MCPX probably draws so much current because it's done in a .13u process, and the leakage current at those gate sizes is horrible. So, battery backup? well, even if you used a CR2032 coin lithium battery (220 mAh capacity) with a diode in series to drop the voltage down to 2.5V, it would last probably less than 60 days :/ Plus, in order to drop a battery in, you'd have to disconnect the capacitor charging switch. So in the end, the answer is: MSFT cut cost by not putting a long-life real time clock part on the XBOX, and instead integrated it into the MCPX; because of this, the current draw was high and they were forced to use a supercap which charges while powered on, becuase if they didn't by the time the box was shipped to the customer, the battery would be mostly dead. Eh. back to thesis.