Grab Ampere from the play store and test various chargers and cables. I found a couple cables that caused my phone to be charging almost 400mA less than other cables.EDIT: weird... so I had it sitting there charging, it actually DISCHARGED 2% attached to my 1A generic wall USB adapter. I plugged it into the factory charger (1.8A) and it has charged 7% in like 8 minutes. Seems odd... doesn't spell good things for battery life though and this is after I uninstalled everything I could on it and used ATK.
Still using ATK? That's pretty old and hasn't been updated since 2013, so it probably doesn't do it's job very well. I've never been a fan of task killers, as most tasks are going to start right up again. If you want to give something more current a try, give Greenify, or Servicely a look.