How many amps is my light using?

rgvkid

New Member
Sep 21, 2008
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Los Angeles,CA
I installed a small fog light conected to a 12volt 7.5H battery. The battery was fully charged but it went out in about 45min. How do i tell how many amps the light bulb is using. I want to get a smaller light for the bike but i need it to run for at least 2 hours. The bulb reads 12V 100W 199. Not sure what the 199 is, maybe a part number. Harbor freight has some smaller light but if the light bulb is also 100W will i have the same problem?

I have the power cable plugged directly to the battery with a fuse and the ground of both the light and battery connected to the battery rack. I don't have a switch, i just unplugged the light when i didn't need it on.

Any advice would be appreciated. Thank You.
 

Long&Black

New Member
Nov 24, 2008
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Georgia
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.
 
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rgvkid

New Member
Sep 21, 2008
110
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Los Angeles,CA
Thanks For the replies and info. Makes things alot clearer. I picked up the 55W Blue Fog lamps from Harbor freight, but after checking out Jasons link i think i'll take them back and go that route. It looks like those lights are even brighter then the original 100w that i have on now. Plus i like the idea of a low and High to project the light further. If i can pull an Hour and a Half i'll be happy. I don't want to add any more batteries due to the weight which is already on the bike.

Thanks for the advice, its greatly appreciated.
 

ZnsaneRyder

New Member
Nov 21, 2008
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FLORIDA
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.
 
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