S5 battery drain - of a different kind?

[JP123]

So, I've had my Galaxy S5 for a long while and it also updated the OS to version 6.01 on 2016/08/02.
All was fine until two or three days ago when I noticed that the battery would drain much faster than previously.

To give the story:
Last evening (10 pm) I see the battery is at around 30%; plug into the charger; full charge obtained; put the phone on my desk overnight; (phone is on "idle" - not turned off, as usual); battery show 96% (that would be approximately 8 hours).
Great ! Battery is holding .... NOT !
I take the phone with me at the office (there is WiFi there but I am not using the S5 the entire day. Check at noon: 75%; check at 5pm: 61%; check at 9:45 pm: 40%.
Note that I have NOT sent messages nor made a call all day .. just received one single message from my daughter.

The battery used to be good for at least 2 full days with lots of calls and messages. Not anymore.

Here's the weird thing:
If I go to Settings>Battery>Battery usage> I get to the graph and at the bottom of the screen several items such as Android OS, Android system, cell standby ... etc., the Android OS is using 46% (and decreasing as I am presently watching the S5 while it's on).
Android system: 4%
Cell standby: 3%
Bluetooth, WiFi, Device idle and screen all at 1%.

I have searched here for answers (lots of posts on that but not to what applies here - maybe?)

So, is someone aware of a similar problem where the Android OS is gobbling up the battery for the past two-three days only?

Hoping to get some replies/solutions/

Thank you !

 

[JP123]

I forgot this:
I received a "Fast Charger" (Samsung brand) at Christmas and I have been using that instead of the original one since then: no problems until 2-3 days ago.

Edit/update: I have just turn the phone ON after a full recharge and, without even using it at all, after 10 minutes it has dropped from 100% to 88%. !! ??