There is a general misunderstanding of what most consumes battery time and, conversely, what most saves it. If you go to you go into your settings and look under Battery, you'll almost certainly see that far and away the largest power consumer is your screen. Your phone, WiFi, GPS, and the OS itself will likely come after that. Starting with Android 2.2, app killers in many/most instances actually consume MORE power than they save because...well...I'll let this article explain it:
http://www.tested.com/tech/android/923- ... -be-doing/
What this means is that if you use your phone a lot...and your screen is on a lot...then there is no app on the planet that is going to seriously extend your battery. There's no getting around this. But what if you DON'T have your screen on all the time, then what? Well, then you're in luck....but it doesn't come from task killers.
One of the best apps I've found to extend battery time is the weirdly named "Battery Saver (1.5 to 4 days)". This app does something pretty simple but effective -- whenever your screen is off and you aren't charging, it cycles your WiFi and/or 3/4G on and off, at a cycle of your choosing. So let's say you set it to cycle on and off every 10 minutes. This means that your email and such will be checked 6 times per hour -- pretty reasonable -- but your WiFi and/or 3/4G will only be cranking for 6 minutes out of the hour instead of 60. Is this going to wildly extend your battery? My experience is no. But it WILL give you a real boost.
There are also other things you can do yourself to extend battery time, like turning off GPS and turning on screen auto-brightness (potentially very useful for extending battery with AMOLED screens, but ineffective I believe with LCD since I think in LCD screens the back lighting never changes, just the amount of light the LCDs let through...but I'm not sure).
Another thing you can do is restrict apps from using background data so they only only reach out to the world when you fire them up, but if you use Battery Saver 1.5 to 4 Days, they won't be able to activate the phone's radio transmitter even if you don't do this:
http://www.makeuseof.com/tag/why-you-do ... elly-bean/
So there are a lot of things you can do to extend your battery....but app killers themselves do nothing at best and actually consume more power at worst.
