Car Battery Guide

Testing the right way

Before you test

Switch the engine off and let the car rest at least an hour, ideally overnight. Right after driving or charging, the battery has a surface charge that makes it look better than it is. In a hurry? Switch the headlights on for a minute, off again, and wait five minutes.

Clamp on correctly

For the most consistent results, connect the tester directly to clean battery terminals when they are accessible and the vehicle manufacturer allows it. If your car has designated remote test or jump-start points, follow the vehicle’s and the tester’s instructions. Always use the same connection points when you compare results over time.

Give the tester the right rating

Choose the battery type and the rating from its label: the CCA value and its standard (EN, SAE …). A wrong rating gives a wrong health figure.

Tip: Take a photo of the battery label the first time you test it. Next time, you’ll have the right CCA rating at hand – and Batteryism can scan the label and keep the rating with the battery.

Test the same way every time

Same tester, same settings, ideally similar conditions. That makes it easier to spot real changes over time.

How often to test

Before winter, whenever the car starts slowly, and regularly while a car is in storage. If you follow a battery over time, comparable tests a few months apart are a good routine.

Understanding the numbers

Battery types

Flooded (conventional) batteries are the classic type. EFB (enhanced flooded) batteries are made for cars with start-stop. AGM batteries hold the acid in glass mats and are used in cars with demanding start-stop systems and many electrical loads. The type is printed on the label – tell your tester which one it is.

Voltage

Resting voltage shows charge, not health. Approximate values for a rested, conventional 12-volt lead-acid battery at room temperature:

AGM and EFB batteries read slightly higher when full. These values don’t apply to lithium batteries.

Cold cranking amps (CCA)

How much current a battery is rated to deliver at −18 °C. Compare the measured CCA with the rating on the battery label. A falling value can indicate ageing and reduced starting power, especially in cold weather. Always consider the tester’s verdict and the battery’s state of charge too.

State of health (SOH)

The tester’s estimate of your battery’s condition compared with a new one. A low reading can indicate wear, but the percentage isn’t an exact measure of remaining life, and each tester calculates it in its own way.

State of charge

How full the battery is right now. A healthy battery that is only half charged can test badly.

Internal resistance

Internal resistance generally increases as a battery ages. A consistent upward trend can indicate deterioration, even when other test results still look good.

The tester’s verdict

The exact wording varies between testers.

Example: reading a test

What does this mean? At 12.48 volts the battery is probably not fully charged, and its measured starting power is about three-quarters of its rating. Charge it and test again before drawing conclusions.

Illustrative values to show how a result can be read – not a diagnosis.

What if the test looks bad?

A poor result doesn’t always mean a new battery. Before you buy one:

1. Check that the correct battery type and CCA rating were entered.
2. Check that the tester clamps have a clean, secure connection.
3. If the battery is undercharged and safe to charge, fully charge it and test again.
4. Compare with earlier measurements to see whether this is a trend or a one-off.
5. If the result remains poor, or the tester reports a bad cell, have the battery checked by a professional. Replacement may be necessary.

Why battery history matters

A single battery test is a snapshot. A series of tests tells a story.

Readings change with temperature, charge level and how the test is done – so one result doesn’t always tell you everything. Testing regularly and comparing results over time can reveal changes that are otherwise easy to miss:

• a gradual decline in starting power
• rising internal resistance
• changes in battery health
• clues that help distinguish temporary low charge from long-term wear

Consistency matters more than frequency: use the same tester, the same settings and similar conditions whenever possible. Readings from different tester brands or models may not be directly comparable.

Batteryism brings all your readings together, so you can follow your battery’s story.

Charging

When to charge

When the charge is low, after many short trips, and before winter. For a car that stands unused, check the charge regularly – or keep a suitable maintenance charger connected during long storage.

Use a smart charger

A modern smart charger controls the charging automatically and can switch to maintenance mode when the battery is full. Choose the mode the charger’s manual gives for your battery type – many have an AGM mode, and some have settings for EFB.

Connect safely

Follow the charger’s and the car’s instructions. Usually: red to the plus terminal first, then black to a bare metal point on the car body – or to the minus terminal if the manual says so. Plug in the charger last, and disconnect in reverse order.

Caring for the terminals

Signs of trouble

A white or blue-green crust on the terminals, a clamp that can be turned by hand, or slow cranking and clicking even though the battery tests well.

Cleaning

Follow the car’s instructions before disconnecting the battery – some cars need specific steps. Disconnect the minus cable first, then the plus. Wear safety glasses and gloves. Brush off deposits with a wire brush and a little baking soda in water, keeping the cleaning liquid out of the battery. Rinse with clean water and dry. Reconnect plus first, then minus, and tighten until the clamps can’t be turned by hand.

Protect them with grease

After tightening, coat the outside of the terminals and clamps with a thin layer of battery terminal grease or protective spray – petroleum jelly works too. Never put grease between the post and the clamp: metal should touch metal.

Winter and storage

Why winter is hard

Cold slows the battery’s chemistry so it delivers less, while a cold engine needs more power to turn. A battery that was borderline in autumn often fails on the first frosty morning – test it before winter.

A charged battery copes with frost

A well-charged battery withstands severe cold far better than a discharged one, which can freeze in quite mild frost and be damaged. Keep it charged in winter.

Storing a car

For a car or motorcycle in storage, check the charge regularly or keep a suitable maintenance charger connected. For long storage you can disconnect the minus terminal, if the car’s manual allows it.

Replacing a battery

Signs it’s time

A Replace or Bad cell verdict that remains after charging and retesting, slow cranking even when fully charged, or a steady decline in your readings over time.

Choosing a new one

Start from the vehicle manufacturer’s battery specification – the owner’s manual, a parts catalogue or your workshop will have it. It sets the size, terminal layout, capacity, cold cranking amps and technology. Cars with start-stop need the technology they were built with, usually EFB or AGM.

Register the new battery

Some cars need a new battery registered or coded with a diagnostic tool, so the car charges it correctly – many do not. Check the car’s documentation, or ask your workshop if you’re unsure.

Recycle the old one

Car batteries contain lead and acid, and used ones must be recycled. Take yours to an authorised battery collection point or ask your battery retailer about take-back options.

Safety

Acid and gas

Batteries contain sulphuric acid and can give off explosive hydrogen gas when charging. Wear safety glasses, keep sparks and flames away, and charge in a ventilated place.

Tools and jewellery

A spanner across both terminals causes a dangerous short circuit. Take off rings and watches, and loosen the minus cable first when removing a battery.

Never charge a frozen battery

Never charge or jump-start a frozen, cracked, swollen or leaking battery. A frozen battery may have internal damage. Don’t try to charge it until it has thawed safely and been inspected – if you suspect damage, have it checked by a professional.