This is just a review as i thought it was worth to mention how long battery has come in Smartphone industry, particularly from Xiaomi which has made chips to regulate charging. 

Model: Xiaomi 13T Pro, has ( Surge P1 fast charging chip, Surge G1 battery management chip) 

Country: Iraq ( so really hot weather)
Scenario variable: Hot weather and most of time using SIM 4G network data.

After continuous and heavy use for more than 2 years,  with 981 charging cycles reported from AIDA64 the battery retains ≈90% of its health. 

Compared with my experience with Xiaomi MI9, 13T Pro leaves it in dust. With Xiaomi MI 9 it lost in 2 years 800 mah of the 3000mah so i had to change battery every 2 years before i had enough and shifted to 13T Pro, which at this rate consider to change it in around 2 years ( Causal once in 4-5 year upgrade)

Why this matter:
I think it matters most when deciding to buy a second hand smartphone.
 
Personally, now i’m seriously considering buying a 1–2 year old secondhand flagship for my next upgrade. Previously, I was reluctant to buy a phone with a used battery and less update support, so I always opted for 'flagship killers' or high-end mid-rangers. Looking back, that same money could have bought me a secondhand 1-2 year old flagship.