Battery Testing & Replacement
Most car batteries last 4-6 years. A failing battery causes slow cranking, dim headlights and warning lights. Test it with a multimeter before replacing — sometimes the alternator is the actual culprit. Swapping a battery is a 15-minute job once you have the right size.
29-minute deep-dive — multimeter testing, alternator check, full replacement.
Healthy battery reads 12.6V engine off, 13.7-14.7V engine running. Below 12.4V engine off = needs charging or replacing.
Always disconnect the NEGATIVE (black) terminal first when removing, and connect it LAST when refitting. Stops sparks if a spanner touches the chassis.
Modern cars need a memory-saver to keep radio codes, ECU settings and clock when the battery is disconnected. £10 from Amazon.
Buy a battery with the matching CCA (cold-cranking amps) rating to your handbook — too low and it won't start in winter.
TEST FIRST: Set a multimeter to DC volts. Probe red to + terminal, black to -
Engine off: should read 12.6V or higher (12.4-12.6V is borderline, below 12.4V is failing)
Start the engine and re-test: should read 13.7-14.7V (alternator is charging)
If engine-off voltage is low but charge voltage is normal, the battery is failing
REPLACEMENT: Locate the battery (under bonnet, in boot, or under a seat — handbook will say)
Plug in a memory-saver via the OBD or cigarette socket if you want to keep ECU/radio settings
Loosen the negative (-) terminal clamp first. Wrap it in cloth and lift away from the post
Loosen the positive (+) terminal clamp. Lift away
Undo the battery hold-down clamp. Lift the old battery out (heavy — 15-25kg)
Clean the terminals and tray with a wire brush if corroded
Fit the new battery. Reattach POSITIVE first, then negative
Refit the hold-down clamp. Check tightness of both terminals
Start the engine and confirm it runs normally
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