Q: macbook pro 2015 disastrous battery

Hello ,

I purchased my 2015 mac retina  in may 2015 (the most powerful and expensive version), it came with Yosemite and everything was great  , around 9 sometimes 10 hours of battery , worked like a charm.

In November or October this terrible ElCapitan upgrade cam up and i did it , battery instantly goes down to 4-4.5 hours.

I did not restore at the time to the old version coz i did not know how to and I'm a university student and have office from university license so i was afraid not to be able to download office again since it is a one use only ,,, anyway it does not matter.

Now i had time and i decided that after 7 months of extremely terrible battery under ElCapitan i want to downgrade back to Yosemite.

I deleted the drive and now on Yosemite the battery is 4 hours!!!!!! SMC reset did nothing. I wonder really wonder what is wrong. could it be a hardware issue that was somehow triggered with that first ElCapitan upgrade?

I purchased the most powerful mac with radeon and 500 GB SSD (my first apple product) for 2700$ , but  it proved to be the biggest waste of money