Charge the battery for 24 hours, let it rest for 24 hours and then load test it with a resistive tester. harbor Freight sells them cheap. If you have a bad/weak cell, it will show up under a load test, but yu have to let the battery sit for 24 hours so that you are not just measuring a surface charge.
2015 Seaark 1872 MV CC Etec-90
Helix 12 CHIRP SI, HB 1199, MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner