Foxwell battery tester, the pro version, deep deep discount to almost half price.
https://www.amazon.com/dp/B01ANF1D7U/
“Limited-time-deal” of 42% off, plus a 15buk-off clicky brings it down to 95bux.
If Mr Project Farm uses it for his tests, it’s good enough for me. 
Plus, I got 2 Foxwell OBD scanners and like 'em a lot, so I’m familiar with the brand. This doodad tests the battery, charging system, everything. Same kind of tool that Mr AA uses when you can’t start your car.