Certainly makes sense (get it?), doesn't it. Nearly every EVO owner I know has, at some point or another, complained about the fact that their battery life drops about 5-10% as soon as they unplug it. Well, it turns out that's because of there's an overcharge protection chip inside Li-Ion batteries.
XDA user willy900wonka decided to tear his extended EVO battery apart, and lo and behold:
What I found was that a 8205 chip is used to provide protection and prevent over charging and over-discharging. Here is a quote from a google search.
S-8205A/B Series Applications-Optimized-Battery-Protection Circuits.
All contained voltage detectors are of high precision between 15mV and 100mV depending on the function. Overcharge detection voltage is 3.55V..4.4V with 25mV accuracy. Overdischarge detection voltage is 2.0V..3.2V with 80mV accuracy. Discharge overcurrent detection voltage is 0.05V..0.3V with 15mV accuracy and charge overcurrent detection voltage is -0.05V..-0.3V with 30mV accuracy. The detection voltage in short circuit case is 0.5V..1.0V with 100mV accuracy.
So there you have it. Despite what some have said, custom kernels can't really defeat the protection... buzzkill. Better than burning out your battery though, I suppose.