Most 'won't start' diagnoses are wrong — and people throw away good batteries or good starters all the time. Here's the 60-second test.
Step 1 — Turn the key with headlights on
Lights stay bright when cranking → battery is fine. Lights dim hard or go out → battery is flat or has internal damage.
Step 2 — Listen carefully
Single loud click + no crank = starter solenoid or main cable. Rapid clicking = battery flat. Strong crank but no start = fuel or immobiliser, NOT starter or battery.
Step 3 — Voltage check
12.6V engine off = healthy battery. 12.2V or less = needs charging. Below 11V = dead cell, replace.
Step 4 — Voltage drop while cranking
Should stay above 9.5V during cranking. Below = battery internally damaged. Above with no crank = starter motor.
Quick rule of thumb
Bright lights + click = starter. Dim lights + slow crank = battery.