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A car battery is a type of electric battery that supplies electric energy to the starter motor and the ignition system of a vehicle’s engine. The term is also used for the main power source of an electric vehicle (traction battery).
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 amperage for short period of time. Once the engine is started, they are being continuously recharged.
- The deep cycle type is designed to continuously provide power for long periods of time (for example in a golf cart). 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.
Use and maintenance
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 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.
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).
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 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 108 kJ/kg (30 W·h/kg).
- The most commonly used battery. Has one of the lowest energy-to-weight ratios; although the weight of the battery can be beneficial for traction on the vehicle.
- Used on some electric cars.
- Car adapter
- Jump start (vehicle)
- Autobatteries.com FAQ website
- Johnson Controls battery ratings information
- Car battery maintenance on Popular Mechanics
- Car battery FAQ on repairfaq.org
- Voltage of a car battery
- Car Battery Fitment Guide
Categories: Electric batteries | Auto parts