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Car battery


A car 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. This also may describe a traction battery used for the main power source of an electric vehicle.

Automotive starter batteries (usually of lead-acid type) provide a nominal 12-volt potential difference by connecting six galvanic cells in series. Since the cells naturally produce about 2.1 V, the actual voltage is roughly 12.6 V. Lead-acid batteries are made up of plates of lead and lead oxide, which are submerged into an electrolyte solution of 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 sulphate reforms into lead oxide and lead. With the plates restored to their original condition, the process may now be repeated.

Additional recommended knowledge



Lead-acid batteries have different uses. Several elements are alloyed with the lead such as calcium, cadmium or strontium to change density, hardness, or porosity of the plates and to make the plates easier to manufacture.

  • The starting (cranking) or shallow cycle type is designed to deliver quick bursts of energy, usually to start an engine. They usually have a greater plate count in order to have a larger surface area that provides high electric current for short period of time. Once the engine is started, they are continuously recharged. See Jump start (vehicle).
  • 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, 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. They usually have thicker plates in order to have a greater capacity and survive a higher number of charge/discharge cycles. See battery pack.

Some batteries are claimed by their manufacturers to be dual purpose (starting and deep cycling).

Use and maintenance

Fluid level

Formerly car batteries using lead-antimony plates would require regular top-up to replace water lost due to electrolysis on each charging cycle. By changing the alloying element, more recent designs have lower water loss unless overcharged. Modern car batteries have low maintenance requirements, and may not provide caps for addition of water to the cells. If the battery has easily detachable caps then a top up 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 cells can be topped up with distilled or deionised water just above the visible plates. The sulphuric acid in the battery normally does not require replacement since it is not consumed even on overcharging.

Impurities 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.

Charge and discharge

In normal automotive service the vehicle's engine-driven alternator 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.

In emergencies a battery can be jump started, by the battery of another vehicle or by a hand portable battery booster.

Changing a battery

In most modern automobiles, the grounding is provided by connecting the body of the car to the negative electrode of the battery, a system called 'negative ground'. In the past some cars had 'positive ground'. Such vehicles were found to suffer worse body corrosion and, sometimes, blocked radiators due to deposition of metal sludge.

The recommended practice when removing a car battery is to disconnect the ground connection first and then other terminal. This ensures that a short circuit will not occur by a wrench touching grounded engine parts while disconnecting the other terminal. Similarly, the ground should be connected last when installing a battery.

Care should be taken when first filling the battery with acid, as acids are highly corrosive and can damage eyes, skin and mucous membranes. A 1994 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.


Because of "sulfation" (see lead-acid battery), lead-acid batteries stored with electrolyte slowly deteriorate. Car batteries should be installed 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). 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).


Corrosion at the battery terminals can prevent a car from starting. To prevent corrosion, during regular battery service the terminals may be cleaned with a wire brush and corrosion products washed away with water. When the battery terminals are re-assembled, they are coated with vaseline/petroleum jelly (grease is not desired) to reduce the rate of corrosion accumulation.

Battery defects

Common battery faults include:

  • Shorted cell due to failure of the separator between the positive and negative plates
  • Broken internal connections due to corrosion
  • Broken plates due to vibration and corrosion
  • Low electrolyte
  • Cracked or broken case
  • Broken terminals
  • Sulfation after prolonged disuse.

In addition, 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. [2]

Exploding batteries

Any lead-acid battery system when overcharged will produce hydrogen gas. 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, a flammable concentration of hydrogen may remain in the cell or in the battery enclosure. Any spark can cause an explosion, which will damage the battery and its surroundings and which will disperse acid into the surroundings. Car batteries should always be handled with proper protective equipment (goggles, overalls, gloves).

Terms and ratings

  • Ampere-hours (A·h) is the product of the time that a battery can deliver a certain amount of current (in hours) times that current (in amps), for a particular discharge period. This is one indication of the total amount of charge a battery is able to store and deliver at its rated voltage. This rating is rarely stated for automotive batteries.
  • Cranking amps (CA), also sometimes referred to as marine cranking amps (MCA), is the amount of current a battery can provide 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 amps (CCA) is the amount of current a battery can provide at 0°F (−18°C). The rating is defined as the amperage 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 amps (HCA) is the amount of current a battery can provide at 80°F (26.7°C). The rating is defined as the amperage 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 determined are by the Battery Council International organization. [3]
  • Peukert's Law expresses the fact that the capacity available from a battery varies according to how rapidly it is discharged. A battery discharged at high rate will give fewer amperehours 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, measured when the engine is off. It can be approximately related to the charge of the battery by:
Open Circuit Voltage (12V) Open Circuit Voltage (6V) Approximate charge Relative acid density
12.65 V 6.3 V 100% 1.265
12.45 V 6.2 V 75% 1.225
12.24 V 6.1 V 50% 1.190
12.06 V 6.0 V 25% 1.155
11.89 V 6.0 V 0% 1.120

Open circuit voltage is also affected by temperature, and the specific gravity of the electrolyte at full charge.


The following is common for lead-acid batteries:

  • Quiescent (open-circuit) voltage at full charge: 12.6 V
  • Unloading-end: 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.

The energy to weight ratio, or specific energy, is in the range of 30 Wh/kg (108 kJ/kg).


The most commonly used battery for SLI applications. It has a low specific energy, but is cheaper than high-performance battery types.
Used on some battery electric cars.

Future Trends

Due to the increase of electric power payloads in today’s automobile, a 42 V power system has been considered and is being developed to replace the existing 14 V power system. (14 V and 42 V refer to the alternator charging voltage). See for example Modeling of 36 V Lead Acid Battery for the 42 V Automotive System Simulation. For 42 V systems, 18 cell lead acid battery with a nominal 36 V is proposed.

See also


  • FAQ website
  1. ^ Horst Bauer Bosch Automotive Handbook 4th Edition Robert Bosch GmbH, Stuttgart 1996 ISBN 0-8376-0333-1, pages 803-807
  2. ^ Horst Bauer Bosch Automotive Handbook 4th Edition Robert Bosch GmbH, Stuttgart 1996 ISBN 0-8376-0333-1, pages 806-807
  3. ^ Battery Council International (1996), BCI Battery Replacement Data Book
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Car_battery". A list of authors is available in Wikipedia.
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