car battery

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.

They are usually lead-acid batteries that provide a nominal 12-volt (actual 12.6 volt) 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 lead plate gets thinner. When the battery is recharged, the lead is redeposited on the plates allowing the process to be repeated.

Types

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.

• The starting (cranking) 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

Fluid level

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

Do not use deionised water, or tap water as they both can contain high levels of minerals which will impair battery performance.

Charge and discharge

There are several different methods used to recharge a depleted battery. The most gentle of these is called trickle charging. Other methods include slow-charging and quick-charging, the latter being the harshest.

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 connection first and then the grounded one.

Freshness

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) with I being skipped. 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 ratio to time that a battery can deliver a certain amount of current. Generally expressed in the base value of number of hours discharging a current of 1 A.
• 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 more a 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. The value is calculated by Peukert's equation:

where: C is the theoretical capacity of the battery, I is the current, T is time, and n is the Peukert number, a constant for the given battery. The equation captures the fact that at higher currents, there is less available energy in the battery. The Peukert number is determined empirically and reflects the internal resistance of the battery. To calculate Peukert's exponent for a battery it is discharged twice using two different currents and the time is taken for each. The two currents and the two times are entered into the following equation:

A value close to 1 indicates a well-performing battery with little loss. A higher number reflects a less efficient battery. The Peukert number of a battery is exponential and is between 1.3 and 1.4 for lead acid types.

• The hydrometer measures the amount of sulfuric acid in the electrolyte. A low reading means that sulfate is stuck to the battery plates. 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 %

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).

Formats

• Lead-acid battery

The most commonly used battery. Has one of the lowest energy-to-weight ratios; and the weight of the battery can be beneficial for traction on the vehicle.

• NiMH

Used on some electric cars.

References

• Autobatteries.com FAQ website
• Johnson Controls battery ratings information

External links

• Car battery maintenance on Popular Mechanics
• Car battery FAQ on repairfaq.org
• How to Change a Car Battery
• Voltage of a car battery
• Auto Battery Article