Task managers actually decrease battery life because most apps that are running are running for a reason. When the task killer "kills" the app it will generally try to restart which uses extra cpu cycles which use extra battery power.
The best way to increase battery life is to download an app that tells you which apps or processes are using the most battery and uninstalling the apps if the aren't used very often. My personal favorite is gsam battery monitor, it is free from the play store.
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