The length of time a 12volt battery powers a 12volt light depends on two things: the battery’s capacity and the lamp’s current consumption. Batteries come in all shapes and sizes, and lights have a range of technologies and brightnesses that affects how much electricity they use. A simple calculation using the capacity and current consumption figures tells you exactly how long a battery will run a light.
Battery Capacity

Manufacturers rate battery capacity in units called amphours  a figure that multiplies amperes of current by the number of hours the battery can deliver current to a device. For example, a 100 amphour battery delivers 1 amp 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 lightemitting 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 amphour rating by the light's current consumption. For example, to determine the time for 100amphour battery with a 4amp light, divide 100 by 4 to obtain 25 hours. Divide 100 amphours by the 20milliamp consumption of an LED and you get 5,000 hours.
Parallel Connection

You can connect 12volt batteries in a parallel circuit to increase their overall capacity; in this configuration, the resulting capacity is the sum of each battery’s individual amphour rating. For example, if you connect a 100amphour battery with a 50amphour one, the total capacity is 150 amphours. Connecting lamps in parallel increases the total amount of light you get, but it increases the overall current consumption. Two 5amp 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.
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