Lexus CT200h battery warranty issue

I own a 2016 Lexus CT200h with just around 40,000 miles, and I was noticing that I was averaging 23mpg when I normally average around 40mpg so I assumed that the hybrid system was not working properly. Knowing that I had gotten a warranty with it when I bought it pre-owned over a year ago, I took it back to the dealership that bought it from to ask them if they could check to see if my hybrid battery was working how it was supposed to be. They came back and told me that the battery is holding a charge, but is just draining faster than normal. I asked if this would be covered under my warranty to get a new one but they said that it is not because the battery is technically still working, just not necessarily perfectly, and the warranty only covers things that are completely broken, and not things that happen to the car overtime from basic ownership even though I don't think that this should've happened from "basic ownership". I went to a local Lexus dealership and got quoted over 4,000 dollars to replace my current battery with a new one. Now I'm left with a cars whose whole purpose is to be gas saving but now can't hold a charge. Not to mention all the weird sounds the cars makes from constantly switching from the hybrid system to the gas engine with the battery dying so frequently. Not sure if this is a common warranty issues, or my dealership just being jerks but make sure to get the details about what your warranty covers!