Automotive Batteries (A Primer)

Let’s start with some basics….

What is a Battery and How Does it Work?

An automotive battery is an electrical storage device and performs a few main tasks including providing sufficient voltage to operate a vehicle's starter motor, ignition, and fuel systems on startup and acting as a voltage stabiliser in the vehicle's electrical system to absorb sudden high voltage.

When a (modern lead-acid) battery is charged it converts the lead oxide paste in the battery plates to lead dioxide at the positive plates and spongy lead at the negative plates. These plates are held apart by the separators to allow electrolyte flow but are linked together at the top of each plate to give continuity. Each of these structures forms an electrical cell, and each cell contributes approximately 2.1 volts.

The most common cause of battery failure is disintegration of the positive plates. The positive plates do all the work in a battery, and as a result they typically begin to crumble after three to four years and the performance of the battery is decreased by the loss of plate material.

Jump Starting

Before we get into various battery problems and testing, we will deal briefly with a safe jump starting procedure. I would like to stress at this point that while it may appear simple, battery jump starting does carry risks, particularly with modern vehicles. The following procedure will ensure the risks are reduced, however if you’re at all uncomfortable or unsure just call your local roadside assist provider (NRMA, RACQ, RAVC, etc).

  • Connect jumper leads to the disabled vehicle first. Positive lead to the positive post of the battery, negative to a good earth on the engine block or failing that to the negative post on the battery.
  • Connect the other ends of the jumper leads to the donor vehicle. Connect the positive lead to positive post first, then negative lead to negative battery post. The donor vehicle should not be running. If possible, use jumper leads equipped with an anti-zap surge protector.
  • Start the disabled vehicle, and leave the jumper leads connected. Leave the disabled vehicle running for five minutes so as to stabilise the voltage, and then turn off the disabled vehicle before disconnecting jumper leads.
  • Attempt to start the disabled vehicle. Note that the five minutes stabilising time is not sufficient to charge the battery, however most batteries will hold enough surface charge to allow you to restart the vehicle if attempted almost immediately. If the vehicle does not start, reconnect the jumper leads and start the vehicle. Leave the disabled car running and turn on the headlights. It is now safe to disconnect the jumper leads as any electrical surge will travel via the headlights rather than the ECU.

How to Test for Common Problems

Loose or Dirty Terminals

Loose or dirty terminals can lead to a low charge condition, as increased resistance means a full charge cannot reach the battery. Also, poor connections can prevent a healthy battery from providing enough charge to the electrical system.

A voltage drop at the battery terminals can be performed by connecting your voltmeter to the battery with one lead on the battery post and one at a point on the battery terminal. An everyday multimeter set to VDC will work for this task.

With your voltmeter in position turn on the headlights (to place an electrical load on the connection) and observe the reading. A maximum of 0.2V is acceptable, however any reading indicates a poor connection. Test positive and negative terminals. If a poor connection is found, disconnect both terminals (negative first) and clean the battery post with a wire brush and the terminal with emery paper or similar. Re-connect terminals (positive first), making sure to push terminals all the way down so as the battery post sits proud, and then tighten. When tight, battery terminals should not be able to be moved by hand. If any terminal cannot be tightened sufficiently then it must be replaced.

Flat Battery

A fully charged battery will have a static voltage of 12.6V or higher. When a heavy load is placed on it (such as when cranking the car) the battery should not drop below 9V. Test the static charge of a battery by placing a volt meter across the terminals.

It is not uncommon for a healthy battery to show up to 12.9V, or even 13-14V if the car has just been running. This is because the battery holds a higher surface charge which will bleed off until the battery stabilises at 12.6-12.9V.

A flat battery will have a static voltage of less than 12.6V, and needs to be recharged. The best way to recharge a battery is on a bench charger, however if this is not available then jump starting the car and driving for approximately 90 minutes may help the battery recover.

If you find that your battery is consistently going flat, but when recharged passes the tests below, you may have a current draw. This occurs when something is drawing current from the battery while the vehicle is idle, and can be tested at your local auto-electrician. You can test for a current draw by connecting a multi-meter in line between the negative terminal and the negative battery post, and observe how much current is being drawn while the vehicle is idle.

Causes of Battery Failure and How To Test for Them

Sulphated Battery

A sulphated battery refers to the condition of the plates, and means that the battery will not accept and hold a charge. A sulphated battery will typically recharge to 12.6V but may not maintain charge over a 24 hour period.

A voltage drop test will help determine the condition of a battery that is showing 12.6V but still will not start the vehicle. To do this, connect your volt meter as pictured above, and have someone attempt to start the car. Before attempting to start the car, make sure it is in park/neutral, handbrake on, and the person starting the car is in the driver’s seat with their foot on the brake. Watch what voltage the battery drops to when attempting to start the car, anything lower than 9v indicates a failed battery.

A sulphated battery can also be tested by measuring how much current a battery is accepting while charging. Due to the high current load this procedure requires a clamp type ammeter and will note be covered in this article.

Dead Cell Battery

A dead cell indicates that one or more plate has shorted or buckled. Elecrolyte may bubble in the failed cell/s, and smoke may be emitted when attempting to start the vehicle.

When testing static voltage, a dead cell battery will typically show 2, 4, 6, 8, or 10V depending on how many cells have failed. Remembering that each cell contributes roughly 2V to the battery, a dead cell battery will not hold more than 2V for each functional cell and so voltage will not increase with charging. To test for a bad cell, jump start the vehicle as above and run for roughly 30 minutes. If charge increases to 12.6V perform a voltage drop test as above. If voltage does not increase then you have confirmed the battery has a dead cell and requires replacing.

Open Circuit Battery

An open circuit battery occurs where one of the links joining the cells has separated, therefore severing the path between positive and negative terminal. A battery will normally go open circuit when a large load is placed on it, although it can occur with physical damage caused by vibration or impact. [b]An open circuit battery poses a risk to all of the electrical components in the vehicle and can cause the vehicle to stop when running. An open circuit battery should never be jump-started.[/b]

To test for an open circuit battery, connect your volt meter as shown above. If the battery is showing 12.6V, turn on the headlights and observe voltage drop. If the voltage drops to 0V (or close to) it is open circuit and requires replacing.

If the battery is showing less than 12.6V, turn on headlights and observe voltage drop. Then connect jumper leads as per jump starting procedure above, but instead of starting the disabled vehicle, start the donor vehicle. Leave the jumper leads connected for roughly five minutes, before switching off the donor vehicle and disconnecting jumper leads. Perform voltage drop test again, and if the under-load voltage has not increased you have confirmed an open circuit battery.

Battery Specifications

Some key terms are listed below

  • CCA: This is a measure of how much current the battery can deliver at -18 degrees celsius over 30 seconds, while maintaining an average 1.2V per cell or higher. Put in practical terms, this is the starting power of the battery The higher the CCA, the better the battery and the longer it should last.
  • RC: This is the reserve capacity and represents the number of minutes at 26.7 degrees celsius that a battery can supply a load of 25amps and maintain 10.5v (for a car battery).

When replacing a battery always remove the negative terminal first, and replace it last. Always clean battery posts and terminals with a wire brush, regardless of how clean they appear.

Alternator Testing

No batteries tech article would be complete without a brief discussion on alternators and alternator testing should be carried out after jump-starting and/or changing a battery.

Your alternator provides power to keep the vehicle running, and any excess current is used to recharge the vehicle’s battery. An alternator test is carried out by connecting a volt meter to the battery as before, and measuring the voltage across the battery. The alternator should always be tested with lights on high beams, and if possible with the engine speed at 1500-2000rpm.

An acceptable charging rate is 13.5V-15V, although most cars rarely charge higher than 14.5V. Undercharging will lead to a flat battery and can cause the vehicle to stop as the limited amount of voltage in the battery is depleted.

An overcharging alternator can cause problems such as

  • High evaporation rate of electrolyte
  • Distorted battery case
  • Separation of lead oxide from positive battery plates

OK I think that covers everything… If after reading through this tech article you are still not sure about your battery, or if you are not 100% comfortable with performing the tests above, you can always take your car to an auto electrician for diagnosis. Also most roadside assist services offer a battery service that will test your battery and electrical system for free, and replace your battery with a high quality item if needed.

1 Response

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