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Why do car batteries fail?

Graham Reynolds | 10th Aug 2023 | 7 minutes to read

A flat battery has ended many vacations, 4x4 adventures, and long-term relationships with first cars, but while you might be familiar with battery failure, the reasons behind it can be more complex than leaving the headlights on overnight.

Figuring out why a car battery has failed can mean avoiding a full replacement, which can save you time and money, and it can also help you better protect your battery against future failures.

In this article, with help from our friends at, we’ll go over some of the reasons a car battery fails, as well as how batteries are tested, and price ranges for batteries:

  1. How and Why Do Car Batteries Fail?
  2. How Are Car Batteries Tested?
  3. How Much Does a Car Battery Cost?
  4. Are Cheap Car Batteries Always Bad Batteries?

How and Why Do Car Batteries Fail?

While factory faults can and do cause flooded lead-acid batteries to fail, factory defects do not cause many battery failures. Aside from the usual life cycle of a battery, how a vehicle is used is often the main reason why its battery fails or is not holding a charge.

Here are some reasons how vehicle usage causes battery failures:

A Full Charge Is Not Achieved or Maintained

Every time we start a vehicle, the starter motor consumes a large amount of the battery’s stored electrical energy. If the battery is working correctly, the alternator will replace all the energy that the battery had lost by starting the engine very quickly.

Of course, this assumes that the trip is long enough for the alternator to recharge the battery. However, using the headlights, or wipers, may prevent the alternator from replacing all the energy that was lost during starting the car.

Fortunately, one or two short trips should not cause a car’s battery to fail. However, many short trips in quick succession will progressively damage the battery until it can no longer crank the engine. This happens because sulphation prevents the battery from accepting or holding a charge. You can, however, usually keep the battery fully charged by doing at least one 10 km-long trip per week.

Acid Stratification (Acid Layering)

Acid layering happens when the liquid electrolyte separates into different layers in the battery. In practice, the acid in the mixture becomes more concentrated towards the bottom of the battery. This can happen when the battery’s state of charge stays below about 80%. The main cause of a low state of charge is because the battery is never fully charged or is not holding a charge.

Flooded lead-acid batteries perform best when the electrolyte is mixed evenly throughout each battery cell. This ensures that electricity is produced at the same rate over the entire surface area of each set of plates in each battery cell.

Here is how acid layering affects a flooded battery:

The parts of the plates that are in contact with acid-poor electrolyte produce less electricity and corrode faster than other parts of the plates. On the other hand, the parts of the plates that are in contact with acid-rich electrolyte increase the open-circuit voltage of the battery.

A stratified battery might appear to be fully charged when its open-circuit voltage is measured across its poles. However, it has, in fact, significantly less capacity than specified available because electricity production is uneven. Noting that "open-circuit voltage" refers to the voltage of a battery or battery cell from which no current is drawn.

Sometimes, it is possible to reverse acid layering to restore a battery’s efficiency. However, whether the reversal process is successful depends on the degree of acid layering, and the amount of corrosion in the battery.

How Are Car Batteries Tested?

First and foremost, when it comes to identifying difficult starting or no-start situations, it is pivotal to ensure that a vehicles charging system is operating correctly. Once this has been ascertained, the testing and diagnosis of a potentially faulty battery can commence.

It is important to point out that the limitations of all hand-held battery testers often make them unsuitable for home use. For instance, some handheld testers can test a battery's CCA (Cold Cranking Amp) ability but cannot measure RC (Reserve Capacity) at the same time. Similarly, some electrical battery testers can test a battery's resting voltage, but cannot measure the other two important parameters, CCA and RC at the same time. Therefore, the biggest secret of successful battery testing lies in correctly interpreting the limited information that current testing methods produce.

Of course, this does not mean that a car battery cannot be tested on a do-it-yourself basis with a digital multimeter to measure the battery’s resting voltage. You can also use a hydrometer to gauge the specific gravity of liquid electrolyte. As a practical matter, the specific gravity of the electrolyte is a good indicator of the battery’s state of charge.


Battery testing can highlight an array of battery issues

However, flooded car batteries can sometimes produce two different rest voltage readings in two tests that are performed within minutes of each other. Moreover, interpreting test results obtained with inadequate equipment accurately and correctly is as much an art as it is a science. Therefore, it is easy to make mistakes or jump to wrong conclusions if you want to test your battery on a do-it-yourself basis.

LSI car batteries of all types can be tested at many workshops with diagnostic battery chargers. These chargers automatically detect the type of battery being tested, before applying a modified charging profile to detect faults, defects, and shortcomings in the battery.

Apart from simply reading a car battery voltage, It can be very difficult to assess the overall condition of a car battery without doing at least two different tests. Therefore, we recommend that you have your battery tested professionally twice a year with an advanced battery tester such as a diagnostic battery charger.

One final thought about battery testing is that LSI batteries can fail with no warning signs or symptoms. Therefore, we recommend that if your battery is older than about two years, you have it tested professionally. The few dollars you may spend on doing this could save you a ton of money in repair bills should the battery fail when you least expect it.

How Much Does a Car Battery Cost?

Car battery prices vary greatly between the different battery brands, types, size groupings, warranties and capacities.

As a result, we cannot provide price estimates that will always be accurate for all vehicle types. Nonetheless, below are some “ballpark” prices that should give you a better idea of what you can expect to pay for a new car battery. Note, though, that while these estimates were accurate at the time of writing, they do not include labour charges to install a new car battery, or to do a battery reset procedure-

Hatchbacks and Small Vehicles

If you own a hatchback or compact sedan vehicle, you can usually expect to pay between about $150 to around $250 for a replacement battery.

Mid-Sized Sedans

Lead-acid batteries typically cost between about $180 to about $250 for high-tech LSI batteries that are suited to high-tech stop-start systems. Note that many mid-sized sedans now require advanced AGM (Absorbent Glass Mat) batteries, which usually cost from around $400 to $600.

4WD Vehicles, High-End Luxury Sedans, and SUV’s

Typical prices of lead-acid batteries for these vehicles vary from about $250 - $350. However, heavy duty deep cycle batteries that power high-demand accessories and equipment can cost as much as $500.

Are Cheap Car Batteries Always Bad Batteries?

When you are shopping for a car battery replacement, it is normal to find small price differences between different battery brands and size groupings. However, be very careful of car batteries that are a lot cheaper than other batteries of the same size and capacity.

Cheap car batteries are cheap for a reason. You may be offered a so-called “rebuilt” battery, or worse, a used battery that was merely cleaned off and repainted.

The internet abounds with such offerings. However, there is as yet no reliable way to refurbish a car battery to make it last as long as a new battery will. So yes, a cheap battery is usually a bad battery, because if batteries could be successfully rebuilt, all reputable battery manufacturers would be doing it.

Avoid buying a used, substandard, or inherently defective battery by dealing only with a reputable dealer that will issue an appropriate warranty. Remember that when a cheap car battery fails, it is often not the only thing that fails. Battery failures can cause other problems and failures, so buying a cheap car battery might just be inviting trouble that could easily have been avoided.

Check Your Battery Fitment With Repco

Battery failure can be a serious problem that may require a full replacement of your battery. Knowing the correct fitment for your vehicle is highly important, so head into your local Repco store to get advice on your battery fitment from our friendly staff or use our online Rego Check to instantly see which batteries are compatible with your vehicle.

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