How many mAH do you need for a battery rating to run a 75 watt bulb?

Question

I'm planning on making a 75 watt 12 volt halogen bike light.I'm planning to make a battery pack out of 10 rechargeable 2800mAH AA batteries. I want it to last atleast an hour. Would I need to sets of the 10 batteries to make 12volts and 5600mAH or would 2800 be enough? If you have any other ideas preferably cheaper please let me know.

Answer 1

You will need something like 10,000 to 20,000 mA-hrs, or 10 to 20 A-hr. Forget about AA batteries, they are not made that high.  A sealed lead acid battery is what you need, plus a recharger.  The battery would weigh 8 to 20 pounds, and you will need some way to mount it to your bike.  Is it really worth it?

The previous answerer had good advice: use a smaller wattage light.

p.s. if you're really determined to use a 75W light, here are links to 10 and 20 Ah, 12V batteries:

http://www.batterymart.com/p-12v-10ah-se...

http://www.batterymart.com/p-12v-20ah-se...

Answer 2

That is h*** of a bike light, 75W at 12V is over 6A, too much for a 2800mAh cell, and draining at a 1 hour rate you will not get the full capacity from the battery, they are rated a lower discharge, perhaps a 20hr. rate.

I found a data sheet for an Energizer ® 2500mAh NiMH cell that is down to 1.0V in less that 1/2hr at 5000mA (which is less than you need) , it is hard to tell how much less because the graph has limited resolution, but it is less than 1/2 hr.

6A would require 6000mAh to run for an hour, not allowing for the capacity loss  due to the high discharge, though as the voltage drops the current (and power) through the lamp will drop so you might get light a little longer than the numbers suggest,  it doesn't look like even 2 sets,, 5600mAh,  is going to be enough for a full hour, and that is with new cells, as they age the run time will decrease.

As for suggestions, do you really need that much power? I have a 15W bike light that is pretty bright, 75W seems like a lot,  50W might give you an hour with 5600mAh (allowing something for loses and aging) and would still be pretty impressive for a bike (a single auto head light is not much more that 50W)