I'm not sure this is a realistic use case to try and support. A 10 year old android phone likely has a battery life measured in 10s of minutes, and really isn't something we need to worry about.
While I'm of course an edge case, the fact that Google, Apple and Samsung all provide >5 years OS support for devices now (and battery replacement services) suggests that many people hang onto their old phones for a long time.
That said, I'm not using 7 year old software on my phone. That would be insane. And my browser (fennec) was updated just a few days ago.
For $50 you can buy a whole new phone (refurb that is 4 yrs old from some other country)