The ten-minute triage: jump start, recharge or replace
A technician who reaches for a new battery before reaching for a tester is guessing with your money. The proper sequence takes about ten minutes. First, resting voltage with everything off: 12.6V is healthy, 12.2V is half-charged, below 12.0V is deeply discharged. Second, a conductance test, which measures the battery's actual cold cranking amps against the figure printed on the label. A battery reading under roughly 70 percent of its rated CCA is on the way out regardless of voltage. Third, a charging check with the engine running: 13.8-14.4V at the terminals means the alternator is doing its job.
Those three numbers produce a clean decision. A healthy battery that went flat because a door was left ajar needs a jump start and a proper 30-40 minute drive, or better, a bench recharge, not a replacement. A battery over two years old that fails the conductance test needs replacing even if it started the car today, because the next failure will not announce itself. And a brand-new battery that keeps dying points at the car, not the battery: a weak alternator, a failing voltage regulator, or a parasitic drain such as a dashcam hard-wired without a cut-off, an aftermarket alarm, or a boot light that never switches off.