I bought a battery tester a year ago and have been happy with it:
https://www.amazon.com/dp/B07Z67MMGC...d_asin_title_8
I knew from testing the battery in my daughter's 2020 X1 last September that it would likely need to be replaced this year, but when I checked it this weekend while doing some other maintenance (bleeding brakes), it said the health was down to just 16% and should be replaced. So I got a new battery from Autozone, and that battery tested at 100% right off the shelf.
Before I got the battery tester, I'd relied on the free testing they do at the auto parts store, but with five cars in the family fleet - it's nice just to be able to grab my tester and check the battery health while I'm doing an oil change.