An automotive battery is a type of rechargeable battery that supplies electric energy to an automobile.[1] 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. An automotive battery may also be a traction battery used for the main power source of an electric vehicle.
Automotive SLI batteries are usually lead-acid type, and are made of six galvanic cells in series to provide 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.[2] 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.
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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 "Sealed" or "Valve regulated" battery of the absorbed glass mat (AGM) type which 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 (typically termed "maintenance-free") and do not require replenishment of electrolyte under normal use. Both types of sealed batteries may be used in vehicular applications where leakage 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 reduce internal resistance.[1]
The deep cycle (or motive) type is designed to continuously provide power for long periods of time (for example 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.[2] Some battery manufacturers claim their batteries are dual purpose (for both starting and deep cycling). This may include "marine" type batteries that may be labeled "deep discharge", which is slightly different than "deep cycle".
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.[3]
Car batteries using lead-antimony plates would require 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 provide 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. 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 require replacement since it is not consumed even on overcharging. Impurities or additives in the water will reduce 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.
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.
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 retain 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.
When changing a battery, battery manufacturers recommend disconnecting the ground connection first to prevent accidental short-circuits between the battery terminal and the vehicle frame. 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 appropriate 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 purchase, 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, ATV, or airplane battery.
Because of "sulfation", lead-acid batteries stored with electrolyte slowly deteriorate. Car batteries are date coded to ensure 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 alphanumerical code. The first character is a letter that specifies the month (A for January, B for February and so on).[4] 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 minimize the load on the vehicle charging system.
Common battery faults include:
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 facilitate such repairs; an internal fault generally requires replacement of the entire unit.[1]
Any lead-acid battery system when overcharged will produce hydrogen gas (gassing) 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.
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.
The open circuit voltage, is measured when the engine is off and no loads are connected. It can be approximately related to the charge of the battery by:
| Open circuit voltage | Approximate charge |
Relative acid density |
|
|---|---|---|---|
| 12 V | 6 V | ||
| 12.65 V | 6.32 V | 100% | 1.265 g/cm3 |
| 12.45 V | 6.22 V | 75% | 1.225 g/cm3 |
| 12.24 V | 6.12 V | 50% | 1.190 g/cm3 |
| 12.06 V | 6.03 V | 25% | 1.155 g/cm3 |
| 11.89 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: