Why won't my 2009 Mazda CX-9 start after sitting for three months?

You say the battery is okay, and it’s possible for it to show 12 volts on a multi-meter, yet still be ready to be replaced. The trick is to load test the battery. That is, measure the voltage across the terminals while the battery is trying to crank the engine. Sometimes, a battery that seems okay with no load will turn to jelly once you apply the starter-motor load.

Cars (and batteries) don’t like sitting around doing nothing. The car’s alarm and even things like the clock in the dashboard will all continue to run, slowly draining the battery. A drained battery, if sat long enough, will often not recover even if you try to recharge it. Beyond that, an auto electrician will be your best first stop.