I have 11 vehicles licensed and insured, 6 modern 5 antique, plus a couple of farm vehicles. Most of them sit for long periods of time, some with battery maintainers plugged in, some without. When I go to start one and it doesn't crank, the first thing I go to , to check the battery, is my Harbor Freight battery load tester. There are other brands that do the same thing, but it was 20.00 about 10 years ago and works great, even though it doesn't say Snap On on it. A voltage tester tells little about the state of the battery, a load test shows whether it has enough power to start the engine. It is the first tool I use to determine a possible battery problem. If the battery checks out OK then on to battery cables etc etc. if not a charge and then another load test, if it fails after a correct charge, then a new one is called for. I know that many on this forum do not have a auto mechanic background, but a battery load tester is easy to use and inexpensive, and worth the small investment.