Car battery

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Lead-acid car battery

A car battery is a type of rechargable battery that supplies electric energy to an automobile. It can be a SLI battery (Starting - Lighting - Ignition) to power the starter motor, the lights and the ignition system of a vehicle’s engine or a traction battery (used for the main power source of an electric vehicle).

They are usually lead-acid batteries that provide a nominal 12-volt (actually 12.6 volts) potential difference by serially connecting six cells that each produce about 2 to 2.1 volts. As other batteries of its type, it is made up of plates of lead and lead oxide. These plates are submerged into a 35% sulfuric acid and 65% water solution called the electrolyte solution. This process causes a chemical reaction that releases electrons, allowing them to flow through conductors thus producing electricity. As a lead acid battery discharges, the materials of the lead plates react with the acid of the electrolyte, changing the surface of both plates to lead sulphate. When the battery is recharged, the chemical reaction is reversed. The lead sulphate reforms into lead oxide and lead, restoring the plates to their original condition, allowing the process to be repeated.


Car batteries have different uses and various other 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 being 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 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 can be dual purpose (starting and deep cycling).

Use and maintenance

Fluid level

The majority of batteries today are maintenance free and don't require top up. If the battery has easily detachable tops then a top up may be required from time to time. In this case the tops are simply removed and the cells topped up with distilled or deionised water just above the visible plates.

Tap or rain water should never be used as they both can contain high levels of minerals which will impair battery performance.

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 the vast majority of 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 this was different, some cars had 'positive ground', but such vehicles were found to suffer worse body corrosion and, sometimes, blocked radiators due to deposition of metal sludge. Care should be taken when changing acid as acid can have a negative effect on human skin.

When removing a car battery, the ground connection should be removed first and the other connection second. This ensures that a short circuit will not occur by a wrench touching grounded engine parts while disconnecting the other terminal. When connecting a battery, connect the live (or positive) connection first and then the grounded one.


Because of "sulfation" (see lead-acid battery), one should never buy a battery that is more than six-months old. 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). 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).

Exploding batteries

Maintenance free (MF) Batteries rely on valves fitted to each cell which can vent hydrogen if over-pressurisation occurs. Generally however, oxygen and hydrogen recombine in the space above the electrolyte, so that over-pressurisation rarely occurs. However, should such a condition occur, and the valves fail to operate (through being blocked for example), then there is a possibility of an internal explosion if the oxy-hydrogen mixture is ignited. Just a slight jolt can cause a spark to jump between the posts, and the gas explodes. Personal injuries can result. The condition can be assessed if any swelling in the cell walls of the battery is visible. The swelling from internal pressurisation varies from cell to cell, that at the battery ends being most obvious, because the plastic is unsupported by cells at either side. It is surprising how powerful an explosion can be caused in the small air space above the electrolyte can occur, but when one cell explodes, it sets off a chain reaction in the rest. Such batteries should be isolated and discarded, taking great care using protective personal equipment (goggles, overalls, gloves etc) during the handling.

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 amount of total energy 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.
  • 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 ~ State-of-charge
12.65 V 100 %
12.45 V 75 %
12.24 V 50 %
12.06 V 25 %
11.89 V 0 %

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.

See also


External links