Originally Posted by dgholmes59
A 35 Watt bulb is going to eat up those small batteries pretty quick. If you are going to run that large of a bulb, I would advise a SLA battery of 7AH or use a smaller wattage bulb of less than 20 watts. 10 to 15 watt would give you 1.5 to 2.0 hours of runtime.
P (power) =I(Current) * E(voltage)
35W/12V = 2.9 amps which is what the bulb will draw at 12V.
2.5 AH /2.9 amps is approximately 52 minutes but it will be more like 35 min. or less and that will not be full brightness for the 35 minutes.
I would recommend a larger battery capacity or a smaller wattage bulb if you are worried about the weight of a SLA battery.
These are only calculated runtimes. I would experiment with the runtimes on the bench before going out with the light because you sure don't want to get out miles from the house and run out of light.
I run two 20 watt halogen lights with a 6 Ah or 7 ah SLA battery. I only run one light until it gets real dark and then I run both. But I try to ensure I am almost done with my ride because I know my battery is going to run down twice as fast with both.
Once you know the aH and the amp draw, how do you calculate the run-time ? ??