Everything You Need To Know About Vehicle Batteries

What does a car battery do, exactly?

It is often said that the battery is the main source of electrical power on any vehicle, but this is not correct. While the entire electrical system on a vehicle draws electrical power from the battery, the alternator is the primary source of electrical power.

The alternator replaces the battery’s “stock” of electrical energy as it is used up by all the systems that make the vehicle work. If the battery is in a good condition, it will supply sufficient electrical power to all the vehicles’ electrical systems.

It is important to mention that a faulty alternator can often masquerade as a faulty battery to the uninitiated, as the most common symptoms are that their car won’t start or a battery warning light appears on their dash.

Instead of taking their vehicle to a professional mechanic for proper investigation, many motorists will often go ahead and purchase a new battery, which at first, appears to solve their problem. Unfortunately, after a couple of days of driving, they find themselves back in the same scenario because the faulty alternator isn’t replenishing what the vehicle’s electrical systems are taking out.

How many types of car batteries are there?

Also known as “LSI” batteries (Lights, Starting, Ignition), car batteries are different from the high-voltage battery packs that power hybrid and electric vehicles. Also, LSI batteries comprise a wide range of technologies, chemistries, and constructions to meet different power demands on different types of vehicles.

Batteries are also made in a wide range of sizes, capacities, and different terminal configurations to meet car manufacturers’ design specifications. Here are some details on the three main types of LSI batteries that are in common use today, starting with -

Flooded lead-acid batteries

“Flooded” means that the battery’s lead plates are all fully submerged in a mixture of distilled water and sulphuric acid. Flooded batteries produce electricity when the electrolyte reacts with the lead and other metals in the plates. Flooded lead-acid batteries are the most widely used type of battery today, although they are the least efficient of all common battery types.

Enhanced flooded lead-acid batteries

An enhanced flooded LSI battery is a car battery in which the plates are thicker than in a standard flooded LSI battery. The plates are also separated by porous insulators. Enhanced flooded car batteries also feature polyester sheets between the separators and the plates to improve contact between the plates and the electrolyte. These enhancements increase the useful life of an enhanced flooded battery and nearly double its efficiency when compared to a simple flooded battery.

Enhanced flooded batteries can also endure more charging cycles than a simple flooded battery, and they can be discharged more deeply. However, an enhanced flooded battery cannot be discharged completely without damaging the battery.

Because of their high efficiency and long service lives, enhanced flooded LSI batteries are used in vehicles with simple start-stop systems that operate automatically. Simple flooded batteries can sometimes be replaced by enhanced flooded batteries, but both types of batteries must have the same capacities and ratings.

AGM (Absorbent Glass Mat) batteries

AGM batteries are designed to meet the high-power demands of high-end luxury vehicles that feature a high number of electrical systems.

However, unlike flooded batteries that use liquid electrolyte, the electrolyte in an AGM battery is contained within a woven mat of glass fibres. This is to ensure a large contact area between the lead plates and the electrolyte. This construction method translates into a vastly increased efficiency and power output as compared to even the most advanced flooded lead-acid batteries.

AGM batteries can withstand almost three times the number of charge/discharge cycles than even the most advanced enhanced flooded batteries can. Therefore, AGM batteries are typically used in vehicles with advanced stop-start systems, which are common in high-end luxury vehicles.

What is a deep cycle car battery?

Normal LSI car batteries are designed to deliver short bursts of high amperage current to start the car. Deep cycle batteries, on the other hand, are designed to deliver steady currents over longer periods. One example would be running loads over long periods such as might be found in a caravan or recreational vehicle.

Because deep cycle batteries have thicker plates than conventional LSI car batteries, deep cycle batteries can supply more power over longer periods than conventional batteries can. Therefore, unlike conventional batteries that usually do not discharge more than about 15% during normal use, deep cycle batteries can be run flat completely without suffering damage.

In practice, deep cycle batteries have about three times more RC (Reserve Capacity) than normal batteries, but only 50% to 75% of a normal battery’s CCA (Cold Cranking Amp) rating.

What are car battery size groupings?

Size really does matter- especially when it comes to car batteries. Therefore, the BCI (Battery Council International) has developed a standard system to classify batteries into different size groupings. Essentially, size groupings are based on the physical dimensions, as well as on the CCA (Cold Cranking Amp) and RC (Reserve Capacity) rating of batteries.

This makes it important to know what the recommended size grouping for your particular vehicle is. Here are some examples of common size groupings-

• Group 75: Most General Motors vehicles
• Group 65: Large-bodied Ford vehicles
• Group 35: Most late model Honda, Nissan, and Toyota vehicles
• Group 34: Most Chrysler vehicles
• Group 34/78: Some Chrysler and General Motors vehicles

As mentioned previously, physical dimensions are only one defining characteristic of a particular battery size grouping. Other and perhaps more important aspects are the ratings and pole configurations of batteries within a size grouping. Therefore, fitting the wrong battery could mean the premature failure of the replacement battery. Fitting the wrong battery could also damage the charging system, starter motor, and other electrical components.

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 is 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 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.

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 many volts is a car battery?

Many people believe car batteries hold 12 volts. However, if a battery's open-circuit voltage measures only 12 volts, that battery is significantly discharged.

In practice, a car battery has six cells, each of which has a typical resting voltage of 2.1 volts. Therefore, since the cells are connected in series, the total rest voltage of a fully charged battery should be at least 12.6 to 12.8 volts.

The above resting voltage value is known as the open-circuit voltage. However, this value could be misleading or dead wrong if the measurement is taken immediately after the vehicle is switched off. Thus, to get a reasonably accurate test result, flooded lead-acid batteries must rest for at least 8 hours. Preferably, though, the battery must rest for 24 hours before you take a voltage reading with a multimeter.

This is because lead-acid car batteries can “recover” after use, meaning that a battery’s resting voltage can increase immediately after use. For example, the voltage of a lead-acid car battery that contains calcium can rise by between 5% and 8% after the vehicle is switched off.

Such momentary battery voltage increases can be cancelled out by briefly discharging the battery. However, there is no accurate way of telling just how much the battery should be discharged to remove the effect of the momentary battery voltage increase.

Things are a little different with AGM (Absorbent Glass Mat) batteries. The minimum rest voltage of an AGM battery is 12.8 volts. If this voltage drops down to 12.6 volts, the battery is only 75% charged. If it drops down to 12.3 volts, the battery is only 50% charged.

Note that when an AGM battery’s resting voltage is at or below 11.80 volts, the battery is effectively flat.

How to charge a lead-acid car battery

The charging of LSI batteries has become a rather exact science because different types of LSI batteries require different charging profiles. In simple terms, a charging profile consists of three phases, each of which is described below-

Constant current charge

In this phase, a controlled current replaces almost all the car battery's lost capacity, which can take between 5 hours and 8 hours. This time is determined by the battery's state of charge, ambient temperature, the temperature of the battery, and the strength of the charging current. Most modern car battery chargers will charge a car battery only up to about 70% of its capacity.

Topping off charge

This phase can take an additional 5 to 7 hours and uses a reduced current to charge the battery up to its full capacity.

Float charge

Most modern battery chargers can supply a current that is smaller than 1 ampere to maintain the battery’s full charge for indefinite periods. However, the float current is not supplied all the time. Any high-quality battery charger detects when the battery's nominal voltage drops for any reason. The battery charger will then apply a float current as required to restore and maintain the battery’s full capacity.

The charging time for a lead-acid battery can be reduced to about 8 hours by using a higher current during the first phase of charging. However, doing this will reduce the battery’s useful service life.

So, what does this mean for the average car owner who needs to charge their car battery? It means you should only use a high-quality car battery charger that is recommended for your specific type of LSI battery.

How long should I charge a car battery?

Different batteries require different charging times. The type of battery, its age, states of charge and health; the ambient temperature, and the capabilities of the charging equipment all affect charging times.

The details of how each variable affects charging times are highly technical, but the fortunate thing is that most modern battery chargers are highly sophisticated. Modern battery chargers detect not only a battery’s overall state of health, but also the type of car battery installed and automatically apply an appropriate charging strategy.

If you use the correct or recommended car battery charger, the table below provides some rough estimates on how long it should take to charge a car battery-

Note that this table is based on charging currents that do not exceed 10% of a battery's capacity. For instance, a 60-ampere battery requires 6-ampere charging current, while an 80-ampere requires an 8-ampere current.

How long should a car battery last?

Depending on the type of battery installed and how the vehicle is used, battery warranties generally vary from 6 months to 3 years. However, the length of a warranty period is no guarantee that any particular battery will outlast its warranty period.

Nonetheless, lead-acid batteries usually last for an average of about 42 months. However, this period can be somewhat extended, or greatly reduced by many things, including one or more of the following-

• Using an unsuitable battery for a particular application
• The condition of the vehicle’s charging system
• Vehicle use, such as frequent short trips that do not allow the battery to achieve or maintain a full charge
• Extreme hot and/or cold temperatures
• Excessive discharges, such as forgetting to switch off the headlights. In such cases, a lead-acid battery might suffer fatal damage
• Excessive sulphating of the plates that result from never allowing the battery to reach a full charge
• Prolonged parasitic power draws resulting from faults in wiring or improper fitting of aftermarket accessories such as audio systems or auxiliary driving lights

There are many other possible reasons why a car battery might fail prematurely. However, regular servicing and proper maintenance of the vehicle, and the battery will largely prevent premature battery failures. Moreover, batteries in well-maintained vehicles often outlast their warranty periods by wide margins.

How do I know when to replace my car battery?

Common symptoms of old, defective, or damaged batteries are much the same across all vehicle types, and these could include one or more of the following-

• The cranking, or engine turning speed has reduced noticeably when you are starting the car
• The battery may not crank the engine when it is cold but cranks the engine fine in hot weather, or when the engine is hot
• The battery runs down overnight, or even if the vehicle is parked for a few hours
• The battery’s casing is distorted, or the casing seems to “bulge” out on one or more sides
• You get the smell of rotten eggs or battery acid when the engine runs. This is often accompanied by a build-up of acid on the positive terminal.
• The battery warning light is on. The light could be on because the alternator is not working, or because the battery had failed. However, a battery warning light can also be caused other problems in the charging system.
• When you experience strange behaviour from the anti-theft/security system, and/or the central locking system
• When you notice that power windows do not open or close as quickly as before

We also re-iterate that many charging system faults mimic the effects of a defective or flat battery and encourage motorists to find a professional mechanic to assist them with diagnosing and fixing the root cause(s) of the problem.

How to replace a car battery

It is certainly possible to do a car battery replacement on a DIY basis. You could also call for roadside assistance when you get stuck on the road, but a DIY car battery replacement is not really a recommended procedure. Mistakes can cause serious short circuits, and/or the vehicle could lose vital computer programming from one or more control modules.

Some control modules on modern vehicles are always supplied with battery power even when the vehicle is switched off. One such module, the Keep Alive Memory, contains information on the idling speed, basic ignition timing settings, and control parameters for the electronic throttle control system- amongst other things.

Thus, when you remove battery power from the Keep Alive Memory by disconnecting the battery, the information it contains could be lost or corrupted. The only way to repair corrupted programming is to have the vehicle partly or completely reprogrammed.

Losing programming can be prevented by installing a temporary power supply known as a “Memory Saver” before you disconnect the battery. In practice though, memory savers are not created equal, and you might still experience software issues after the car battery replacement.

There is one other problem to consider, which is the fact a car battery’s performance decreases as the battery ages. To compensate for this, most alternators can increase or decrease charging currents to suit the battery’s capacity or condition.

This is a very useful feature, but it does require that the new battery be integrated into the charging system after the battery replacement. This is called a Battery Reset and if this is not done, the alternator and power demand module will not recognise the replacement battery. Therefore, the alternator might overcharge or undercharge the new battery, which typically leads to the premature failure of the replacement battery.

Battery reset procedures are performed with high-end diagnostic equipment, and sometimes with equipment that is specific to a particular vehicle make or model. Buying an expensive scan tool to save a few dollars on labour charges for a battery replacement just does not make any kind of sense. Instead, it makes sense to engage a professional mechanic with the correct equipment, skills, and knowledge to replace the battery in your vehicle.

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 (July 2022), 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.