Car battery

Car battery

A car 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. 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 each, the actual voltage is roughly 12.6 V (2.1 × 6 = 12.6 V) at full charge. Lead-acid batteries are made up of plates of lead and separate plates of lead oxide, 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.


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 no free liquid electrolyte 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 energy to weight ratio, or specific energy, is in the range of 30 Wh/kg (108 kJ/kg).

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

Use and maintenance

Fluid level

Formerly 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, 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. If the battery has easily detachable caps then a distilled water 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 sulfuric 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.

Some manufacturers (primarily the old Delco-Remy)include a built-in hydrometer to show the state of charge of the battery. This lucite "eye" has a float immersed in the electrolyte. When the battery is charged, the specific gravity of the electrolyte increases, 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 is primarily accomplished with slow, low-current (trickle) charging.

In emergencies a battery can be jump started, by the battery of another vehicle or by a hand portable battery booster. Generally, this is not recommended on newer, computer-controlled vehicles as damage could easily occur.

Battery Storage

When not in active use batteries need to be monitored and periodically charged to prevent damage and retain capacity. Make sure to fully charge the battery immediately prior to storage. Clean the battery with soap and water to remove any leakage paths on the battery case due to acid or sulphation buildup on the case exterior. Dry the unit and place in a cool environment (below if possible) High temperatures increase the self discharge rate. Lead-acid batteries must always be kept in a fully charged condition. Check the battery voltage with a good voltmeter every 6 months and charge battery to restore power lost due to self discharge. Recommended Charge methods: Constant voltage to 2.40/cell, followed by float charge held at 2.25 V/cell. Battery cells must remain cool during charging, preferably below , charging causes heating and temperatures above increase plate corrosion rates.

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.

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. 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.

Recycling the old battery

In the United States, about 97% of lead from used batteries is reclaimed for recycling. 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). 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. 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 should be 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.

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 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. 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.
  • 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. Originally this was the only cranking rating used, and is still considered the standard method of rating cranking current.
  • 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 determined are by the Battery Council International organization.
  • 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 6.3 V 100% 1.265 g/cm3
12.45 V 6.2 V 75% 1.225 g/cm3
12.24 V 6.1 V 50% 1.190 g/cm3
12.06 V 6.0 V 25% 1.155 g/cm3
11.89 V 6.0 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.

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). For 42 V systems, an 18-cell lead acid battery with a nominal 36 V is proposed.

See also



External links

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