# Light amperage draw questions



## Brett (Jul 16, 2008)

Watts / Volts = Amps is correct

18 amp-hours is more of a time reference on a battery

18 hours at 1 amp of draw, or 9 hours at 2 amps
or 1 hour at 18 amps or 1/2 hour at 36 amps.

Get the picture?


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## TomFL (Oct 2, 2007)

So my math is right or wrong? 

16.6 amps for one hour = 16.6 amps drawn from an 18 amp/hour battery. 

Life expectancy is just over an hour---right?

-T


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## Brett (Jul 16, 2008)

Assuming fully charged new battery, your answer is right.
Old batteries won't meet the A/H rating.


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## firecat1981 (Nov 27, 2007)

why not get some newer LED lights? you'll be extending the battery by 4x that way with the same amount of light.


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## iMacattack (Dec 11, 2006)

Amp Hours

The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.

Reserve Minutes

Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.)


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## Brett (Jul 16, 2008)

I've been wrong for a long time.   

I'll blame it on the guy who taught me!  ;D

I do enjoy the variety of topics that show up on site.
       Learn something new here all the time!

http://www.windpowerunlimited.com/batteries/Amp_Hours.htm


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## TomFL (Oct 2, 2007)

OK, so what I'm learning is that perhaps the best way to decipher my needs is to hook up the lights and leave them run at my house for an hour to see what happens before committing to a particular battery size for the build. 

Too many variable with load/temps, etc to be sure. 

Anyone have a better idea?

-T


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## Brett (Jul 16, 2008)

Add a charging kit to your outboard.
Even a pull start can have an alternator.
That way you can use the battery until dead,
then fire up the outboard to power the nav lights to get home.

http://rainboat.com/batterychargingkit.aspx


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## DuckNut (Apr 3, 2009)

Do you really need 200watts? You rode in my boat with a single amber fog light (because the other is broken off) with 25 watts. May not light up the world but can't outdrive it. Think of the speed you are looking at and then make the decision.

Use a hand held spot light in addition and I believe that is all of the light you would need.


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## TomFL (Oct 2, 2007)

> Do you really need 200watts?  You rode in my boat with a single amber fog light (because the other is broken off) with 25 watts.  May not light up the world but can't outdrive it.  Think of the speed you are looking at and then make the decision.
> 
> Use a hand held spot light in addition and I believe that is all of the light you would need.


Not really, but I have 'em so why not. Plus, more light is like more speed. Never have enough...

-T


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## johnboy (Feb 2, 2009)

Here's a nice online calculator.

http://www.altronix.com/app_notes/calc.php

Looks like 0.4 hours. A 50 ah would give you over an hour.

Hope this helps.


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