How to test a car battery

How to test a car battery

If your car's instrument panel includes an ammeter, it will tell you how well the charging system is working - the difference between the charge going into the battery and the power being used from it.

A battery-condition indicator shows only that the generator is charging, by the rise in the voltage. It does not tell you how high or low the charging rate is - though normally any rise means that the charge is adequate.

Many cars have only an ignition warning light, a red warning signal that should go off after the engine starts.

This tells you that the generator is producing electricity - not whether it is producing enough to keep the battery charged. But any abnormal behaviour of the light means that something is wrong somewhere.

Before making checks on the charging system, check that the battery is free of any defects which could produce symptoms similar to those of a faulty generator.

How to test a car battery

Check the electrolyte in one cell at a time. Put it back in the cell that it came from.

If the engine will not turn over, check for loose or broken starter motor, solenoid or earth connections.

Inspect the battery for loose, dirty or corroded terminals. Clean corroded terminals and leads with very hot water. Protect them with a little petroleum jelly, not grease, and refit the leads tightly.

Remember that battery acid is highly corrosive and poisonous. Avoid getting it on your clothes. Wash off immediately if it contacts your skin.

When carrying out any tests on the engine while it is running or turning over, keep hair and loose clothing away from belts and pulleys.

Hydrometer check

How to test a car battery

A battery- controlled indicator calibrated in volts, and with a red-green-red scale.

Check the battery's state of charge with a hydrometer , which measures the strength of the acid in the electrolyte, or battery fluid.

This gives no clue, however, to the battery's capacity - its ability to sustain a charge well enough to perform its tasks.

Battery capacity depends on the size and number of the plates in each cell. If any plates are damaged, that cell's capacity is reduced. The electrolyte in a sealed-for-life battery cannot be checked readily.

Battery-condition indicator

Some cars are fitted with a battery-condition indicator, which is a form of voltmeter. It may be calibrated in volts, by a sliding coloured scale, or by three bands of red-green-red.

How to test a car battery

The ammeter shows the amount of current flow to or from the battery.

When you switch on the ignition, the indicator shows the battery voltage, just over 12 volts for a 12volt battery or about the red-green division.

A lower reading means that the battery is not fully charged.

If the reading is well down while all the circuits and lights are switched off - the battery is not holding its charge, or is 'flat'.

When you start the engine, the indicator shows the generator output. It should move slowly to around the 14 volt mark, or midway into the green sector.

It should stay steady at all engine speeds if the car has an alternator, or at speeds higher than idling if there is a dynamo.

If the indicator drops to 12 volts or lower, check the fan belt (SHEET loo) or the generator output.

The ammeter

Some cars still have ammeters fitted on the instrument panel. An ammeter tells you how well the charging system is working, and gives more immediate information than a voltmeter.

The ammeter shows the amount of current going into or out of the battery, or the difference between the two. Thus it tells you at a glance whether the battery is being charged by the generator or discharged by a heavy load.

In practice, if the charging system is in good condition the reading should always be strong.

If the ammeter shows a very low or negative reading, you know immediately that something is wrong, whereas a voltmeter gives less information and is much slower to respond to a problem.

The only disadvantage of an ammeter is that it is connected in series with the battery and the generator. It requires a heavier cable, and if the ammeter circuit develops a fault, there is more danger of damage to an alternator.

Get our PDF

Did you find this helpful?

You can now get the whole How A Car Works site as a beautiful PDF ebook.

  • Over 250 illustrated pages
  • Print out the pages you need and take them to the garage.
  • Support the site

It's just $5 to buy - exceptional value for money.

Download the PDF

The PDF is 80mb in size - it's high resolution!

Related

Using a car battery charger

in Electrical systems

A high charging rate If the charging rate is much more that 4 amps, take off the cell caps or...

Checking the batteries

in Electrical systems

Keep the area around the battery filler caps clean, to prevent dirt falling in when they are removed. Batteries...

Testing an alternator and checking output

in Electrical systems

Testing drive-belt tension Check drive-belt tension by pushing or pulling the belt inwards at the centre of its longest straight...

Checking battery leads and connections

in Electrical systems

Use solder wire to fill out the screw hole in a battery post if the hole is too large....

Renewing alternator brushes

in Electrical systems

Removing the brushes and cleaning the strips Unfasten the recessed bolts to remove the moulded cover. Note where the leads...

Tags: battery, alternator, electrical,


comments powered by Disqus