How many amps is my light using?

watts = volts x amps. so amps = watts/volts.

so your 100w bulb is using 8.3 amps.

You'll want something no greater than 45w to get 2 hours out of your battery.

Have a look here for a decent bike light:
Monster Quartz Halogen Bike Light - www.dansworkshop.com

I'd be more apt to look at real-world results from use.

You could get an inline meter to use such as the E-Flite power meter from Horizon Hobby to measure exact amp draw but you can also calculate it from the info you've given.

That 7.5AH (Amp Hour) battery should run for an hour if discharged at 7.5 Amps. This is the battery's capacity or C rating.

You got 3/4 of an hour (45 of 60 minutes) of use so you're obviously pulling more than 7.5 Amps.

60 minutes in an hour divided by 45 of minutes of use gives you a 1.3C discharge rate.

Now take your discharge rate and multiply that by the battery's capacity of 7.5AH and you will find that you have a real-world amp draw of 10 Amps.

So it looks as if a 20AH battery is what you'll need for two hours of light with the current bulb. You could wire three 7.5AH batteries in parallel for a combined AH rating of 22.5AH at 12V.

A 50W bulb should give about an hour and a half of light on the 7.5AH battery.

Here's another crazy idea.

Use high output LEDs.

jasonh
I looked at your link. that is some serous lights. Good work.

No, that's not my setup. It's just something I stumbled on while searching for various stuff on the net.

I use High Power LEDs and 55W Halogen for headlights. Best of both worlds and reassurance of not having no light because of a dead battery.

I use the LEDs most of the time, because they have a dedicated 3.7V Li-ion battery and last forever.

I also have a lightweight 12V 3.9AH NICD pack (made myself from drill batteries) for the 55W light, 12V car horn, 12V LED tailight, and also it automatically recharges the Li-Ion 3.7V pack from this 12V pack, so I only have to worry about charging one battery.

A typical 55W light uses 5A, and I get approx 45mins of bright light with it, and it starts to slowly dim, but still has enough power to see. 45mins of bright light isn't bad, if you want a longer lasting light, then use LED.

100W is a LOT of power, and drains the battery faster, so if you go 55W and LED, you can use the bright light in high-traffic or badly lit areas, and use the LED for everywhere else, and have plenty of light for hours. I only have 3.9AH and do fine, so your 7.5AH is plenty with the right setup. I find that a 55W is so bright that it's not needed all the time anyway.

I was looking into LED's and found these which come with a torch housing.

DealExtreme: $9.04 3V~18V Input 3W Cree Drop-in Module (supports Surefire 6P)