I used to access the 'memory' batteries in my Maxxum 7000 and replace them. Blast from the past. Those were pretty easy though; one board located under the battery grip and a soldering iron did the trick. As near as I can tell it takes a special battery that we can't lay hands on anyway. Here's a link to get the process rolling if you want to replace it:  Product Repair - Electronics Support | Sony USA

Your battery is shot and no amount of charging will rejuvenate it. Had my internal die on mine. Set time at startup and would have to reset again whenever the battery got swapped*. Dead to the point it wouldn't stick no matter how fast the battery got changed. Instead of resetting each swap it was quicker to take pic of clock or slate then batch adjust all file attributes later on. *I put up with the time/date dance until the camera went in for servicing.

As far as I know, all commercially available cameras with color sensors use interpolated pixels. That is, each pixel records either Red, Green or Blue and for each pixel the missing colors are interpolated. Have a look here for some explanation: https://en.m.wikipedia.org/wiki/Bayer_filter The only way to get 'real' pixels (that is, with full color information for each pixel) is to use a function called 'pixel shift multi shooting'. A rather cumbersome and specialized feature that the A6500 doesn't have.