I observed this on my HP Laptop dv8t 1100: Sometimes I inadvertently had unplugged the power cord to the laptop. For some unknown reasons, if the power cord is unplugged (battery not being charged) suddenly the CPU usage would go up to the roof eventhough the computer is not really doing anything. Latest instance: I noticed the laptop slow-down - only a few webpages were opened and nothing else... nothing out of the ordinary. The laptop felt as if it was freezing up. I checked the Task Manager, Performance tab: the overall CPU usage ws 65%! All 4 cores and all 4 hyperthreads were at 60-70% use (never seen it like this even during the busiest trading moment). Once I realized that the power cord fell off and re-plugged that in... voila... usage back down to about 14% instantly. The screenshot was captured moments after I plugged back in the power cord. You can clearly see the difference. It seems as if the laptop is fighting for something, or in a thrashing mode when the power is unplugged. I can't figure out what it is. These laptops are designed to work with or without the power plugged in for a few hours. As it is I can't use this laptop when I travel. Appreciate it if anybody can offer some possible explanations what caused it.
Although I never observed that exact problem, I did have a similar one, whereby the backlight would constantly dim whenever I unplugged the laptop. I found there was a way to correct for this somewhere in the control panel (power mode?). So you might want to snoop around that area, at least to see if you can toy with some of the power settings to troubleshoot. One other approach would be to monitor BIOS to see if Voltages drop substantially when unplugged. They should be within regulation limits in both cases. If not, it might indicate battery and/or regulation issue.
Try changing settings in control panel so that they are the same in power settings for when plugged in and unplugged. Sometimes windows will try to slow down certain apps to "conserve power", and they won't work so well so will increase overall CPU in total. Other than that, it's really weird and you could easily have a virus. Try downloading "process explorer" and then when you unplug it, go to the tree of that app and see exactly which process is eating up the CPU. It could simply be a faulty driver, or even out of date java plugin trying to work with your browser, etc etc.
Click on the "Resource Monitor" button in the lower right and go through all processes that take CPU, DISK, Networking etc. The Resource monitor is a fantastic tool on Win 7. It's the only way of finding it. Edit: you can also look through the Event Viewer and see if anything seems to be causing a lot of errors.
Look at the Processes tab in the Task Manager. Click on the CPU column to sort by processes which are consuming the most CPU. Which process is the CPU hog?
Check your power profile. Your maximum processor state while on battery is lower than while plugged in. This means that your laptop will not run at highest frequency (speed) while on batteries to extend runtime. It could also be that your power settings are set for performance while plugged in- meaning that speedstep is not enabled, where while on-batteries speestep is lowering the frequency to conserve batteries. You can change this under advanced power settings. Your HP might have also come preloaded with custom power management app to manage this behavior as well.
Although all the posters have good ideas the ones suggesting you check your power profile are most likely on the money. Your CPU could be at 100% but only running at 20% of its maximum speed. To investigate google cpu-z and download it ... it will show you both your processors maximum and what its actually doing at present. Look at both core speed and multiplier ... for example my cpu is currently at core speed 2000Mhz and multiplier x6. At full speed it's 3200 and x9. Your power profile is probably too restrictive on battery.
dtrader98, braincell, vikana, Trader13, Glavez, kiwi_trader: Thank you all for your comments and suggestions! I will experiment with some of those and post my findings!