Well, i maintain some 300 generators, and i dont trust just checking voltage vs a load testing . I have seen batteries pass a voltage tester but fail under load testing. ive done side by side testing of carbon pyle testers vs the midtronics tester, and i rely on these tests, so as not to have units fail in dead of winter with utility power out.
i know, a voltage tester while turning over engine will also give a pretty accurate test, but if done in warm weather the engine will turn over alot easier than during winter. The midtronics can calculate available cold cranking amps remaining.
a carbon pule tester can vary amp load on battery to simulate winter starting. I generally use a 50% of listed CCA as passing score.