Possible Duplicate:
Do I really need to install a task manager?
Should I run a task killer on Android or does Android manage applications well enough on its own?
Topic task-management android
Category Android
In my own experience using a task killer can help your battery if you opened too much applications or you don't need them anymore like the facebook app running in background when you turned off wifi or the data connection (3G/EDGE/GPRS/whatever)
Android is very good at managing its memory such that task killers should not be necessary. Processes that are not on the screen will use essentially 0% CPU power until they are either re-activated, or have to perform some background work (such as downloading emails).
If you open so many programs that there is not enough memory left, the Android system will start killing off unused programs on your behalf. It is completely self-managing.
The performance of the phone should not be affected by the number of applications running. That said, I do keep one installed, because every now and again, a process can "go rogue" and start using a lot of CPU power and thus give poor battery life (this is rare though).
More background here: http://geekfor.me/faq/you-shouldnt-be-using-a-task-killer-with-android/
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