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12 V, 40 Ah Lead-acid 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. Each cell provides 2.1 volts for a total of 12.6 volt at full charge. 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 sulphate. 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.



Lead-acid batteries for automotive use are made with slightly different construction techniques, depending on the application of the battery. The typical battery in use today is of the "flooded cell" type, indicating liquid electrolyte. AGM or absorbed glass mat type batteries have less liquid electrolyte than older style batteries and are gaining acceptance by consumers in SLI applications. This article deals with the flooded type of car battery.

The starting (cranking) or shallow cycle type is designed to deliver large bursts of energy, usually to start an engine. The SLI batteries 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 recharged by the engine-driven charging system. 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, 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 photo voltaic 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. The specific energy is in the range of 30-40 watt-hours per kilogram.[3]

Batteries intended for SLI systems are intended to deliver a heavy current for a short time, and to have a relatively low degree of discharge on each use. They have many thin plates,thin separators between the plates, and may have a higher specific gravity electrolyte to reduce internal resistance. 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.[4]

Some battery manufacturers claim their batteries are dual purpose (starting and deep cycling).

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.[5]

Use and maintenance


Fluid level

Filling a car battery with distilled water

Car batteries using lead-antimony plates would require regular watering top-up 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 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 fat cunts

Charge and discharge

In normal automotive service the vehicle's charge system, also referred to as charging system, consisting of the engine-driven alternator and the voltage regulator 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 most cars, the voltage regulator of the charge system is unaware of the relative currents charging the battery and for powering the car's loads such as engine control, fans and lightning. The charge system provides essentially a fixed voltage of typically 13.8 to 14.4 V (Volt), unless the alternator is at its current limit. An empty battery draws a high current of typically 20 to 40 A (Ampere). As the battery gets charged the charge current decreases to typically 2 A to 5 A. A high load results when multiple high power systems such as heaters and entertainment system in addition to the lights are running. In this case, the battery voltage will decrease and the charge current as well.

Some manufacturers include a built-in hydrometer to show the state of charge of the battery. This acrylic "eye" has a float immersed in the electrolyte. 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 if 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. Batteries will last longer if not stored in a discharged state.

Sulfation occurs when a battery is not fully charged, and the longer it remains in a discharged state the harder it is to overcome the sulfation. This may be overcome with slow, low-current (trickle) charging. Sulfation is due to 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.

Jumper cable connected to battery post. Hydrometer window visible by jumper clamp. White powdery corrosion products visible on top of battery. This BCI Group 24F battery claims 525 cold cranking amperes and 125 minutes reserve capacity.

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

Whenever the car's charge system is inadequate or too slow to fully charge the battery, a battery charger can be used. Simple chargers will 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.

Battery Storage

Batteries should be monitored and periodically charged if in storage, to retain their capacity. Batteries intended to be stored should be fully charged, cleaned of corrosion deposits, and left in a cool dry environment. High temperatures increase the self discharge rate and plate corrosion. Lead-acid batteries must always be kept in a fully charged condition. The terminal voltage can be measured as an indication of state of charge. Batteries may be charged periodically by a constant voltage method, or attached to a "float" charger.

Changing a battery

In 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.[citation needed]

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 or ATV battery.

Recycling the old battery

In the United States, about 97% of lead from used batteries is reclaimed for recycling.[6] Many cities offer battery recycling services for lead-acid batteries.

In several U.S. states and Canadian provinces, purchasers of new lead-acid batteries are charged a small deposit fee, refunded when the replaced battery is returned. This encourages recycling of old batteries instead of abandonment or disposal with household waste. Businesses which sell new car batteries may also collect used batteries (and may be required to do so by law) for recycling. Some businesses will accept old batteries on a "walk-in" basis (not in exchange for a new battery). Most battery shops and recycling centers will pay for scrap batteries. This can be a lucrative business, enticing especially to risk-takers because of the wild fluctuations in the value of scrap lead that can occur literally overnight. When lead prices go up, scrap batteries can become targets for thieves.


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).[7] 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. The top-up charge can be considered complete when the terminal voltage is just above 15.1 V DC. 15 V DC is the voltage level where any sulphation that may be present is driven from the plates back into the electrolyte solution. A new battery can have some sulphation even though it has never been in service. If the top up charge cannot be done it is not harmful to place the battery in immediate service.


Corrosion at the battery terminals can prevent a car from starting, by adding resistance. The corrosion is caused by a small crack in the casing, or by leakage past the seal, allowing sulfuric acid vapor to corrode the battery terminals (which are made of lead). To prevent corrosion, during regular battery service the terminals may be cleaned with a wire brush and a solution of baking soda and water and corrosive products washed away with water. When the battery terminals are re-assembled, they are often coated with petroleum jelly (grease is not desired) or a commercially available anti-corrosion product to reduce the rate of corrosion accumulation. The corrosive white powder sometimes found around the battery terminals is usually lead sulfate which is toxic by inhalation, ingestion and skin contact. It is also corrosive to the eyes, skin and any metal parts of the automobile with which it may come in contact.

Battery defects

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 building up below the plates of the cell
  • 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 a low or zero charged state

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. [8]

Exploding batteries

Car battery after explosion

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 or the battery is faulty, a flammable concentration of hydrogen may remain in the cell or in the battery enclosure. Any 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 severely 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. In most of the cases when positive and negative leads are connected interchangeably, they may lead to severe injury caused by explosion. When severely overcharged, a lead-acid battery gases at a high level 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 put out a high level of charge, and can quickly ruin a battery. A swollen, hot battery is very dangerous, and should not be handled until it has been given sufficient time to cool and any hydrogen gas present to dissipate.

Car batteries should always be handled with proper protective equipment (goggles, overalls, gloves), and make certain there are no sparks or smoking close by.

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 amperes), 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, except in Europe where it is required by law.
  • Cranking amperes (CA), also sometimes referred to as marine cranking amperes (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 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.[9]
  • 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 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.

Terminal voltage

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 (12 V) Open Circuit Voltage (6 V) Approximate charge Relative acid density
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:

  • 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
  • Wait at least 12 hours after charging to measure open circuit voltage, the resting time allows surface charge to dissipate and enables a more accurate reading.
  • All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes.

See also


  1. ^ Horst Bauer Bosch Automotive Handbook 4th Edition Robert Bosch GmbH, Stuttgart 1996 ISBN 0-8376-0333-1, pages 803-807
  2. ^ David Linden (ed), Handbook of Batteries Third Edition,McGraw Hill, New York, ISBN 0-07-135978-8, page 23.6
  3. ^ David Linden, Thomas B. Reddy (ed). Handbook Of Batteries 3rd Edition. McGraw-Hill, New York, 2002 ISBN 0-07-135978-8, table 23-1 page 23.2
  4. ^ * Horst Bauer Bosch Automotive Handbook 4th Edition Robert Bosch GmbH, Stuttgart 1996 ISBN 0-8376-0333-1 pages 803-804
  5. ^ Wert, Ray (2009-08-19). "2010 Porsche 911 GT3 RS: Track-Ready, Street-Legal And More Power". Retrieved 2009-09-18. 
  6. ^ Battery Recycling retrieved 2008 May 1
  7. ^ From the Mopar Mailing List archives.. "Battery Date Codes". Retrieved 2009-09-18. 
  8. ^ Horst Bauer Bosch Automotive Handbook 4th Edition Robert Bosch GmbH, Stuttgart 1996 ISBN 0-8376-0333-1, pages 806-807
  9. ^ Battery Council International (1996), BCI Battery Replacement Data Book


See also

  • AD-X2, the story of a battery additive and consumer fraud

External links

Simple English

Most car batteries are lead acid batteries, although some electric cars can have other batteries.

Other pages


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