Soon after the HTC EVO 4G was released in June 2010, we learned that the reason the battery level takes a significant dive soon after being unplugged from its charger (especially after being charged overnight) is that it doesn't trickle charge. So once the battery shows a 100% full charge, the charger turns off and your EVO drains while being plugged in. A popular workaround, usually after a one-time only calibration, has been to unplug the EVO after it's been sitting at 100% for a while, plug it back in until it reaches 100% again, and then immediately unplug it. While this method isn't difficult to do, it's inconvenient and something that we shouldn't have to deal with at all.
And you don't have to if your EVO is rooted.
One of the many benefits of rooting is the ability to flash a new kernel. And wouldn't you know it, ms79723's HTC kernel #15 v7 (based on netarchy's 4.2.1 kernel) prevents your charger from turning off, enables trickle charging via AC and USB power, and is available for flashing (tutorial here) right now. It allows standard and extended capacity batteries to be charged to their maximum capacities without damage, and there are separate versions for use with either Sense or AOSP ROMs. See important update below.
I've been using the latest version of the Sense kernel on SteelROM RevI.2 for about a week now and am consistently able to unplug my EVO and keep the charge at 100% for several hours of idling, which is something I could never do before. Last night, I unplugged it before I went to bed, leaving 3G and WiFi on like I always do, and when I woke up 8 hours later, the battery was at 99%!
Find the version of this amazing kernel for your ROM type (Sense or AOSP) at the link below.
[xda-developers] Thanks, raul & Mitchell!
Update: As several commenters have pointed out below, there is a good deal of controversy surrounding trickle charging. Some have had great success with it (like me), while others wouldn't touch this kind of kernel with a 10-foot-pole due to reports of smoking and damaged batteries. The topic is even more polarizing than task killers! One of G&E's guest contributors will be posting a pros and cons piece on the matter of trickle charging soon. In the meantime, please refer to the links in the comments to read about the potential dangers of using this and similar kernels.