The Physics
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Electric Current from a Car Battery

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Johnson, Kenneth, and John D. Cutnell. Physics. 4th ed. New York: Wiley, 1998: 619, A-17. "A car battery has a rating of 220 ampere*hours (A*h)…. Determine the maximum current that the battery can provide for 38 minutes." 350 A
1985 Volvo 740 GL, GLE, Turbo Owners Manual. Volvo Cars of North America: 111. "Capacity 450 A, 90 min" 450 A
Batteries Catalog. Dayton, Ohio: AC Delco, 1999. "1979, Chevrolet, CCA… " [very large table] 275–505 A
Hinckley, Zack. What You Need To Know About Your Battery. Spider Cañon Comment. 16 July 1999. "By itself, the figure for battery rating doesn't mean much, but seen in terms of these typical electrical loads in your car it does:"
headlights 10-15 A
parking lights 3-5 A
interior light 1-2 A
radio 1.5 A
air conditioner 15-30 A
1–200 A

The potential difference of a typical car battery is 12 V. The potential difference is set up by the electromotive force generated by chemical reactions. The electrolyte used in automotive batteries is a strong solution of sulfuric acid and distilled water. The chemical reaction inside a car battery can be reversed. Therefore, a car battery is known as a secondary cell. A secondary cell battery can be recharged by applying a reverse voltage.

Car batteries have three main jobs. Starting the car is most demanding of these jobs. In order to start a car the battery must supply 100 to 200 amps of current over a short period of time. This can become very difficult during the winter. The chemicals in the battery can become too cold to stir up the power necessary to start the car motor. The second job of the car battery is to keep itself charged by providing current for the car generator's field. Finally, the battery of a car must supply a medium level of current for a long period of time in order to keep the car's electrical accessories working while the engine is not running.

There are several conventions used when measuring the current through a battery. The Cold Cranking Amps rating (CCA) indicates the amperes of electricity that can be delivered at 0 °F for 30 seconds by the battery to the car. Another way the current through a battery could be measured is the rate of discharge. This is the ability of a battery to deliver power continuously over a given period of time known as the discharge time. The discharge time of most car batteries is several hours. Batteries can also be rated in terms of useful capacity. Useful capacity is measured in ampere-hours. A 80 amp-hour battery will provide any combination of current and hours whose product is 80. The useful capacity of a battery changes depending on the discharge rate. Therefore, a battery could have a useful capacity of 42 amp-hours at the C/10 rate of 4.2 amps, but only 30 amp-hours at the C/1 rate of 30 A.

Aron Fisch -- 2000