I still do NOT agree with the above.
I use a manual task killer and can easily show that it does work, and works very well.
The S5 when left alone will use the battery in the course of a day, or the 2nd day.
Manually killing WIFI and DATA and closing all tasks will easily get as much as 4 or 5 days from a single charge.
The above is talking about Automatic Task Killers, and I agree that they WILL create a viscious cycle of apps being killed off and then restarting.
But, if you are not a Notification Nut, and don't care about the Social Apps, turn everything OFF and when you need to read email, turn on WIFI or DATA which ever you need.
For me, Battery Doctor is excellent in this manual use. I do not allow it to run Automatically. It is set for Manual Custom Mode.
I have used Battery Doctor for over 2 years on my Samsung S which only has about 285 mB of free RAM.
On that phone, I had to charge the battery 3 or 4 times a day.
After installing Battery Doctor, that phone would give me 2 or 3 days of use between charges.
You have to use some intelligence to decide how the phone is going to work "for you".
YMMV with task killers, but if used properly, they can and will improve battery life.
There are no social apps on my phone, period. Email is not allowed to sync, ever. When I want to read email, I turn on data and download the day's load in about 3 seconds. That is fast enough for me.