An automotive battery is a type of rechargeable battery that supplies electric energy to an automobile. Usually this refers to an SLI battery (starting, lighting, ignition) to power the starter motor, the lights, and the ignition system of a vehicle’s engine.
Automotive SLI batteries are usually lead-acid type, and are made of six galvanic cells in series to give a 12 volt system. Each cell provides 2.1 volts for a total of 12.6 volt at full charge. Heavy vehicles such as highway trucks or tractors, often equipped with diesel engines, may have two batteries in series for a 24 volt system, or may have parallel strings of batteries.
Lead-acid batteries are made up of plates of lead and separate plates of lead dioxide, which are submerged into an electrolyte solution of about 35% sulfuric acid and 65% water. This causes a chemical reaction that releases electrons, allowing them to flow through conductors to produce electricity. As the battery discharges, the acid of the electrolyte reacts with the materials of the plates, changing their surface to lead sulfate. When the battery is recharged, the chemical reaction is reversed: the lead sulfate reforms into lead oxide and lead. With the plates restored to their original condition, the process may now be repeated.
Battery recycling of automotive batteries reduces resources required for manufacture of new batteries and diverts toxic lead from landfills or improper disposal.
Lead-acid batteries for automotive use are made with slightly different construction techniques, depending on the application of the battery. The “flooded cell” type, indicating liquid electrolyte, is typically inexpensive and long-lasting, but requires more maintenance and can spill or leak. Flooded batteries are distinguished by the removable caps that allow for the electrolyte to be tested and maintained.
More costly alternatives to flooded batteries are “valve regulated lead acid” (VRLA) batteries, also called “sealed” batteries. The absorbed glass mat (AGM) type uses a glass mat separator, and a “gel cell” uses fine powder to absorb and immobilize the sulfuric acid electrolyte. These batteries are not serviceable: the cells are sealed so the degree of charge cannot be measured by hydrometer and the electrolyte cannot be replenished. They are typically termed “maintenance-free” by proponents, or “unable to be maintained” by skeptics. Both types of sealed batteries may be used in vehicular applications where leakage or ventilation for vented gasses is a concern. However, this article deals with the classic, flooded-type of car battery.
The starting (cranking) or shallow cycle type is designed to deliver large bursts of power for a short time, as is needed to start an engine. Once the engine is started, the battery is recharged by the engine-driven charging system. Starting batteries are intended to have a low depth of discharge on each use. They are constructed of many thin plates with thin separators between the plates, and may have a higher specific gravity electrolyte to cut internal resistance.
The deep cycle (or motive) type is designed to continuously give power for long periods of time (such as in a trolling motor for a small boat, auxiliary power for a recreational vehicle, or traction power for a golf cart or other battery electric vehicle). They can also be used to store energy from a photovoltaic array or a small wind turbine. Deep-cycle batteries have fewer, thicker plates and are intended to have a greater depth of discharge on each cycle, but will not provide as high a current on heavy loads. The thicker plates survive a higher number of charge/discharge cycles. The specific energy is in the range of 30-40 watt-hours per kilogram.
Some cars use more exotic starter batteries–the 2010 Porsche 911 GT3 RS offers a lithium-ion battery as an option to save weight over a conventional lead-acid battery.
Use and maintenance
Filling a (flooded lead-acid type) car battery with distilled water
Car batteries using lead-antimony plates would need regular watering to replace water lost due to electrolysis on each charging cycle. By changing the alloying element to calcium, more recent designs have lower water loss, unless overcharged. Modern car batteries have reduced maintenance requirements, and may not give caps for addition of water to the cells. Such batteries include extra electrolyte above the plates to allow for losses during the battery life. If the battery has easily detachable caps then a top-up with distilled water may be required from time to time. Prolonged overcharging or charging at excessively high voltage causes some of the water in the electrolyte to be broken up into hydrogen and oxygen gases, which escape from the cells; this is called gassing. If the electrolyte liquid level drops too low, the plates are exposed to air, lose capacity, and are damaged. The sulfuric acid in the battery normally does not need replacement since it is not consumed even on overcharging. Impurities or additives in the water will cut the life and performance of the battery. Manufacturers usually recommend use of demineralized or distilled water, since even potable tap water can contain high levels of minerals.
Charge and discharge
In normal automotive service the vehicle’s charging system powers the vehicle’s electrical systems and restores charge used from the battery during engine cranking. When installing a new battery or recharging a battery that has been accidentally discharged completely, one of several different methods can be used to charge it. The most gentle of these is called trickle charging. Other methods include slow-charging and quick-charging, the latter being the harshest.
The voltage regulator of the charge system does not measure the relative currents charging the battery and for powering the car’s loads. The charge system essentially provides a fixed voltage of typically 13.8 to 14.4 V (Volt), adjusted to ambient temperature, unless the alternator is at its current limit. A discharged battery draws a high charge current of typically 20 to 40 A (Ampere). As the battery gets charged the charge current typically decreases to 2—5 A. A high load results when multiple high-power systems such as ignition, radiator fan, heater blowers, lights and entertainment system are running. In this case, the battery voltage will begin to decrease unless the engine is running at a higher rpm and the alternator/generator is delivering at least enough current to power the load.
Some manufacturers include a built-in hydrometer to show the state of charge of the battery, a transparent tube with a float immersed in the electrolyte visible through a window. When the battery is charged, the specific gravity of the electrolyte increases (since all the sulfate ions are in the electrolyte, not combined with the plates), and the colored top of the float is visible in the window. When the battery is discharged, or the electrolyte level is too low, the float sinks and the window appears yellow (or black). The built-in hydrometer only checks the state of charge of one cell and will not show faults in the other cells. In a non-sealed battery each of the cells can be checked with a portable or hand-held hydrometer.
A positive (red) jumper cable connected to battery post. An optional hydrometer window is visible by the single jumper clamp. (The black negative jumper clamp is not shown)
In emergencies a vehicle can be jump started by the battery of another vehicle or by a portable battery booster.
Whenever the car’s charge system is inadequate to fully charge the battery, a battery charger can be used. Simple chargers do not regulate the charge current, and the user needs to stop the process or lower the charge current to prevent excessive gassing of the battery. More elaborate chargers, in particular those implementing the 3-step charge profile, also referred to as IUoU, charge the battery fully and safely in a short time without requiring user intervention. Desulfating chargers are also commercially available for charging all types of lead-acid batteries.
Batteries last longer when stored in a charged state. Leaving an automotive battery discharged will shorten its life, or make it unusable if left for a long time (usually several years); sulfation eventually becomes irreversible by normal charging. Batteries in storage may be monitored and periodically charged, or attached to a “float” charger to keep their capacity. Batteries are prepared for storage by charging and cleaning deposits from the posts. Batteries are stored in a cool, dry environment for best results since high temperatures increase the self discharge rate and plate corrosion.
Changing a battery
When changing a battery, battery manufacturers recommend disconnecting the negative ground connection first to prevent accidental short-circuits between the battery terminal and the vehicle frame. Conversely the positive cable is connected first. A study by the National Highway Traffic Safety Association estimated that in 1994 more than 2000 people were injured in the United States while working with automobile batteries.
The majority of automotive lead-acid batteries are filled with the proper electrolyte solution at the manufacturing plant, and shipped to the retailers ready to sell. Decades ago, this was not the case. The retailer filled the battery, usually at the time of buy, and charged the battery. This was a time-consuming and potentially dangerous process. Care had to be taken when filling the battery with acid, as acids are highly corrosive and can damage eyes, skin and mucous membranes. Fortunately, this is less of a problem these days, and the need to fill a battery with acid usually only arises when purchasing a motorcycle or ATV battery.
Because of “sulfation”, lead-acid batteries stored with electrolyte slowly deteriorate. Car batteries are date coded to make sure installation within one year of manufacture. In the United States, the manufacturing date is printed on a sticker. The date can be written in plain text or using an alphanumeric code. The first character is a letter that specifies the month (A for January, B for February and so on). The letter “I” is skipped due to its potential to be mistaken for the number 1. The second character is a single digit that indicates the year of manufacturing (for example, 6 for 2006). When first installing a newly purchased battery a “top up” charge at a low rate with an external battery charger (available at auto parts stores) may maximize battery life and lower the load on the vehicle charging system.
Common battery faults include:
Shorted cell due to failure of the separator between the positive and negative plates
Shorted cell or cells due to build up of shed plate material below the plates of the cell
Broken internal connections due to corrosion
Broken plates due to vibration and corrosion
Low electrolyte level
Cracked or broken case
Sulfation after prolonged disuse in a low or zero charged state
Corrosion at the battery terminals can prevent a car from starting due to electrical resistance. The white powder sometimes found around the battery terminals is usually lead sulfate which is toxic by inhalation, ingestion and skin contact. The corrosion is caused by an imperfect seal between the plastic battery case and lead battery post allowing sulfuric acid to react with the lead battery posts. The corrosion process is also expedited by over charging. Corrosion can also be caused by factors such as salt water, dirt, heat, humidity, cracks in the battery casing or loose battery terminals. Inspection, cleaning and protection with a light coating of dielectric grease are measures used to prevent corrosion of battery terminals.
Sulfation occurs when a battery is not fully charged. The longer it remains in a discharged state the harder it is to overcome sulfation. This may be overcome with slow, low-current (trickle) charging. Sulfation is the formation of large, non-conductive lead sulfate crystals on the plates; lead sulfate formation is part of each cycle, but in the discharged condition the crystals become large and block passage of current through the electrolyte.
The primary wear-out mechanism is the shedding of active material from the battery plates, which accumulates at the bottom of the cells and which may eventually short-circuit the plates.
Early automotive batteries could sometimes be repaired by dismantling and replacing damaged separators, plates, intercell connectors and other repairs. Modern battery cases do not help such repairs; an internal fault generally requires replacement of the entire unit.
Any lead-acid battery system when overcharged (>14.34 V) will produce hydrogen gas (gassing voltage) by electrolysis of water. If the rate of overcharge is small, the vents of each cell allow the dissipation of the gas. However, on severe overcharge or if ventilation is inadequate, or the battery is faulty, a flammable concentration of hydrogen may remain in the cell or in the battery enclosure. An internal spark can cause a hydrogen and oxygen explosion, which will damage the battery and its surroundings and which will disperse acid into the surroundings. Anyone close to the battery may be injured.
Sometimes the ends of a battery will be severely swollen, and when accompanied by the case being too hot to touch, this usually indicates a malfunction in the charging system of the car. Reversing the positive and negative leads will damage the battery. When severely overcharged, a lead-acid battery produces high levels of hydrogen and the venting system built into the battery cannot handle the high level of gas, so the pressure builds inside the battery, resulting in the swollen ends. An unregulated alternator can quickly ruin a battery by excessive voltage. A swollen, hot battery is dangerous.
Another potential cause of explosion is when the battery terminals are short-circuited via a very low resistance path (like a wrench lying across the terminals). Heating due to the internal resistance of the battery can cause the electrolyte to boil, also leading to explosion due to buildup of water vapor pressure (unrelated to electrolysis).
Persons handling car batteries should wear protective equipment (goggles, overalls, gloves) to avoid injury by acid spills. Any open flame or electric sparks in the area also present a danger of ignition of any hydrogen gas emanating from a battery.
Terms and ratings
Ampere-hours (A·h) is a measure of electrical charge that a battery can deliver. This measure is one indicator of the total amount of charge that a battery is able to store and deliver at its rated voltage. Its value is the product of the discharge-current (in amperes), multiplied by the duration (in hours) for which this discharge-current can be sustained by the battery. Generally, this value (or rating) varies widely with the duration of the discharge period (see: Peukert’s Law), therefore the value is typically only meaningful when the duration is specified. This rating is rarely stated for automotive batteries, except in Europe where it is required by law.
Cranking amperes (CA), also sometimes called marine cranking amperes (MCA), is the amount of current a battery can give at 32 °F (0 °C). The rating is defined as the number of amperes a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12 volt battery).
Cold cranking amperes (CCA) is the amount of current a battery can provide at 0 °F (−18 °C). The rating is defined as the current a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery). It is a more demanding test than those at higher temperatures.
Hot cranking amperes (HCA) is the amount of current a battery can provide at 80 °F (26.7 °C). The rating is defined as the current a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery).
Reserve capacity minutes (RCM), also referred to as reserve capacity (RC), is a battery’s ability to sustain a minimum stated electrical load; it is defined as the time (in minutes) that a lead-acid battery at 80 °F (27 °C) will continuously deliver 25 amperes before its voltage drops below 10.5 volts.
Battery Council International group size (BCI) specifies a battery’s physical dimensions, such as length, width, and height. These groups are determined by the Battery Council International organization.
Peukert’s Law states that the capacity available from a battery varies according to how rapidly it is discharged. A battery discharged at high rate will give fewer ampere hours than one discharged more slowly.
The hydrometer measures the density, and therefore indirectly the amount of sulfuric acid in the electrolyte. A low reading means that sulfate is bound to the battery plates and that the battery is discharged. Upon recharge of the battery, the sulfate returns to the electrolyte.
The open circuit voltage, is measured when the engine is off and no loads are connected. It can be about related to the charge of the battery by:
Open circuit voltage Approximate
12 V 6 V
12.60 V 6.32 V 100% 1.265 g/cm3
12.35 V 6.22 V 75% 1.225 g/cm3
12.10 V 6.12 V 50% 1.190 g/cm3
11.95 V 6.03 V 25% 1.155 g/cm3
11.70 V 6.00 V 0% 1.120 g/cm3
Open circuit voltage is also affected by temperature, and the specific gravity of the electrolyte at full charge.
The following is common for a six-cell automotive lead-acid battery at room temperature:
Quiescent (open-circuit) voltage at full charge: 12.6 V
Fully discharged: 11.8 V
Charge with 13.2–14.4 V
Gassing voltage: 14.4 V
Continuous-preservation charge with max. 13.2 V
After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V
Open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate and enable a more correct reading.
All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes (negative temperature coefficient – lower voltage at higher temperature).
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