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.
Insist on seeing the numbers before agreeing to a swap. Any competent mobile technician carries a conductance tester and will show you the printout or screen without being asked. If the verdict is a drain rather than the battery, a parasitic draw test with a multimeter in series takes another 20-30 minutes and saves you replacing a good battery twice.