Corrosion: A Hidden Cause of Starting Failures
You turn the key, the battery seems fine, but the engine barely cranks or won't start at all. Before assuming the worst, check the battery terminals. A white or blue-green powder build-up on the terminals creates resistance that can rob the starter motor of the current it needs — even with a perfectly healthy battery underneath.
Terminal corrosion is common in New Zealand because of coastal humidity, temperature fluctuations, and older vehicles with leaking battery cases.
What Does Terminal Corrosion Look Like?
- White or grey powder on the terminal clamps (lead sulphate)
- Blue-green fluffy crust on the clamps (copper sulphate from copper contacts reacting with battery acid)
- Visible white crust at the base of the battery post itself
- Clamps feel loose or crumbly when you press them
Even a thin film of corrosion can prevent adequate current flow to the starter motor.
How to Clean Corroded Terminals Safely
You'll need: baking soda, water, an old toothbrush or wire brush, petroleum jelly or terminal grease, and protective gloves and eye protection.
- Turn the ignition off and remove the keys.
- Disconnect the negative (black) cable first, then the positive (red). This prevents accidental shorts.
- Mix one tablespoon of baking soda with a cup of warm water. Apply to the corroded terminals with the toothbrush. The fizzing reaction neutralises the battery acid causing the corrosion.
- Scrub firmly, rinse with a little clean water, and dry thoroughly with a rag.
- Inspect the terminal clamps — if they're cracked, heavily pitted, or won't clamp firmly, replace them. Clamps cost $5–$20 NZD from any auto parts store.
- Apply a thin coat of petroleum jelly, dedicated terminal grease, or even Vaseline to the clean posts before reconnecting.
- Reconnect positive first, then negative.
Try starting the car. In many cases this is all that's needed.
Diagnosing vs. a Dead Battery
Corrosion and a flat battery can look identical from the driver's seat. The key difference:
- If you can move or wiggle the terminal clamp and the dash lights flicker, corrosion (or a loose clamp) is the issue.
- If the terminals are clean and tight but the battery reads below 12.0 V on a multimeter, the battery itself is the problem.
Preventing Recurrence
- Apply terminal grease or an anti-corrosion felt washer (available at Repco, Supercheap Auto) each time you reconnect the battery.
- If your battery case is cracked or leaking, replace the battery — leaking acid accelerates corrosion dramatically.
- Check terminals every six months as part of your regular service.
When to Call a Mechanic
If cleaning the terminals doesn't resolve the no-start and your battery tests healthy, the fault has moved elsewhere — starter motor, alternator output, or wiring. A mechanic with a battery/charging system tester can run a full diagnosis in under 30 minutes.
Repeated heavy corrosion that keeps coming back can indicate an overcharging alternator (voltage above 14.8 V), which is damaging the battery. Worth having checked.
Cost to Expect
Terminal cleaning as part of a service is typically included at no extra charge or billed at a minimal labour fee ($20–$40 NZD). New terminal clamps are $5–$20 NZD per pair. A full battery and charging system test is usually $30–$60 NZD at most workshops.
WoF Note
WoF inspectors check that the battery is secure and the terminals are in acceptable condition. Badly corroded or loose terminals can result in an advisory or, in severe cases, a fail on electrical safety grounds.