I play in the street.
Join Date: Jul 2005
Location: College: K-State; Home: Overland Park, KS
Bikes: 2005 Allez Triple, 1971 Schwinn Varsity Fixed Gear Conversion
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As michigander pointed out, it depends on the amperage of the bulb. Divide the wattage of the bulb by the voltage (6V) to get the amperage. Divide the battery capacity (in amp-hours) by the amperage, and voila! Run time in hours! As a rough estimate, the capacity of an alkaline AA might be around 1 or 2 amp-hours. Note: The capacities do not add up, as all the current flows through all the batteries at the same time.
Also, AA's are actually rated at 20 milliamps for 107 hours. Since your current will likely be much greater than this, the batteries will probably not perform ideally. Hopefully they will not get super hot.