Corroded terminals: the cheapest cause of the most expensive symptoms

White or greenish deposit on a battery terminal looks like a cosmetic detail. By symptoms it is indistinguishable from a dead alternator, a dead starter, or a finished battery — which is exactly why it is the place to start.

What corrosion does

The deposit creates resistance in the joint. Resistance in a power circuit means a voltage drop exactly where maximum current is needed.

The symptoms:

• The starter turns sluggishly or clicks instead of spinning.
• The car will not start after standing, although the battery is charged.
• Voltage sags when heavy consumers switch on.
• Intermittent faults across systems — modules see the sag and log their own codes; mechanics in the ghost codes article.
• The alternator does not fully charge — the current cannot pass.

The key point: all of those look like expensive failures. Checking the terminals takes two minutes.

Why it appears

Electrolyte vapour escaping through the case — typical of maintenance-free batteries too, just less so.

Overcharging — a faulty regulator heats the battery and accelerates vapour loss; covered in the alternator article.

A loose terminal — micro-arcing at the contact.

Dissimilar metals in the joint — electrochemical corrosion.

Moisture — the engine bay after washes and rain.

A clue from the side: deposit on the positive terminal more often means overcharging, on the negative undercharging. Not a diagnosis, but a direction.

Cleaning it properly

1. Switch off, remove the key, and let the modules sleep.
2. Remove the negative first — so a tool cannot short the positive to the body.
3. Neutralise the deposit with a baking soda solution — it chemically neutralises acid residue. Keep it out of the cells.
4. Clean terminal and post with a brush back to bare metal.
5. Dry it.
6. Reassemble: positive first, negative last.
7. Protect with a dedicated terminal product or petroleum jelly — on top, not between the contact faces.

An important detail: the grease goes on after tightening. Grease between terminal and post is exactly the resistance you just removed.

What to check afterwards

Terminal voltage with the engine running — roughly 13.8–14.6 V.

Codes — after a power interruption they often appear on their own; what resets is covered in the battery replacement article.

Quiescent current — to rule out the drain that caused the deep discharges; covered in the parasitic drain article.

The body earth strap — it corrodes the same way and produces the same symptoms.

Prevention

• Inspect the terminals once a season — five minutes under the bonnet.
• Check tightness — a loose terminal arcs and corrodes faster.
• Watch charging voltage — overcharging is the root cause.
• Do not pressure wash the engine directly at the terminals.

In short

• Corrosion produces symptoms indistinguishable from a dead alternator or starter.
• Checking the terminals takes two minutes and comes first in any electrical problem.
• Negative off first, on last.
• Grease goes on after tightening, never between the contact faces.
• Deposit on the positive suggests overcharging; on the negative, undercharging.

Download the app — system voltage and codes are read with an ELM327 adapter over Bluetooth SPP on Android.