That light may've been a very bad judgement call, but I think this is only an issue that'll show up if you marathon games or regularly leave the controller unplugged, and with standby charging in some ways it's less of an issue than it was on PS3 since the system NEEDED to be on for charging.
The single die LED in the controller uses a reflector lens that projects and amplifies the light to the back of the controller. Tearing down the controller will reveal the design and after doing so myself, my assumption based on my engineering degrees say that the DS4 light bar is no more of a detriment to its battery life then DS3 (4) single die red LED that designated what player the controller was linked to.
1 single die LED light draws an absolute maximum of 20mA of power, probably significantly less in the DS4 (I would guess between 5-10mA), and cannot drain a 800mA battery in to a charge state in 5-6 hours which is typical for most DS4 controllers.
I would venture a guess and say that the reason why the DS4s controller battery charge is noticeably less then a late model DS3 Controller (they increased battery size throughout its life) is that full audio is probably being streamed to the controller regardless of whether you actually use it or not. The constant stream of audio data is probably the biggest culprit in draining the battery while playing anything.
I don't know how difficult it would be for Sony to disable the sending of audio data to the controller when it is turned off. I would venture a guess that a toggling system would cause too many unknown variables and stability issues and the easiest solution for both the consumer and the programmer was to just make it standard data stream to the controller regardless of use.
I also noticed that watching Netflix/VOD tends to drain the controller battery just as much. I usually suspend the controller or leave in plugged in when using these features.
The only real difference between the DS3/DS4 is the touch pad and audio streaming. The Light bar is just a redesign of the DS3 controller select LEDs (LED technology is the same). I do not think the touch pad is the culprit but it very well may be.
TLDR: New Features have caused the DS4 battery life to decline like persons health on an all fast food diet.
#StopLightBarHate