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Battery Capacity
Manufacturers rate battery capacity in units called amp-hours -- a figure that multiplies amperes of current by the number of hours the battery can deliver current to a device. For example, a 100 amp-hour battery delivers 1 for 100 hours or 5 amps for 20 hours. The battery contains a finite amount of current; a higher current drain results in a shorter battery lifetime.
Lamp Current Consumption
The current consumption of 12 volt lamps varies widely due to differences in use and technology. For example, a single xenon bulb used for car headlights may consume 35 watts of power. As power is current times voltage, dividing 35 watts by 12 volts gives 2.92 amps of current. By contrast, a 12 volt light-emitting diode may use only 20 milliamps of current, although it puts out less light than the halogen bulb. In order to determine how long a battery can run a 12 volt light, you must have the light's current consumption data.
Calculation
To calculate the maximum time a 12 volt battery can run a light, divide the battery's amp-hour rating by the light's current consumption. For example, to determine the time for 100-amp-hour battery with a 4-amp light, divide 100 by 4 to obtain 25 hours. Divide 100 amp-hours by the 20-milliamp consumption of an LED and you get 5,000 hours.
Parallel Connection
You can connect 12-volt batteries in a parallel circuit to increase their overall capacity; in this configuration, the resulting capacity is the sum of each battery̵7;s individual amp-hour rating. For example, if you connect a 100-amp-hour battery with a 50-amp-hour one, the total capacity is 150 amp-hours. Connecting lamps in parallel increases the total amount of light you get, but it increases the overall current consumption. Two 5-amp lights connected in parallel consume a total of 10 amps of current. Although this doubles the amount of light produced it uses up the battery in half the time.