When testing a battery a great tool is a hydrometer. Inexpensive but actually does a better job than most " battery testers " at many auto parts stores. It performs specific gravity test. Which is high falutin talk for how much acid is in the battery fluid. Any reading in the green zone indicates a good battery. Each cell must be measured as one "dead " cell will cause problems. Dead cells give a yellow or red zone reading. This test can be performed with battery still installed in boat. Just ensure that nothing is running off the battery.

Uncharged batteries will reflect " bad readings ". So best time to test with hydrometer is right after full charge cycle has been performed on battery in question.