Light amperage draw questions

Discussion in 'Power it up with Electronics' started by TomFL, Jan 20, 2010.

  1. TomFL

    TomFL Well-Known Member

    OK, I've got a battery idea for the duck boat. Trying to keep it as light as possible and still work what I need electronics-wise.

    The battery is an 18 amp-hour battery.

    I want to run a pair of 100w halogen lights as well as a bow and stern nav light. I need them to run for about an hour max.

    If I do the math on the lights ( Divide the total number of watts by the system's volts. For example, a 100-watt bulb in a 12-volt system will draw 8.3 amps.
    (100w / 12v = 8.3a) ) I'm thinking this would (barely) make it at 16.6 amp/hours.

    And that would really leave me no capacity for the running/nav lights which I doubt draw much but they draw something...

    Am I doing my math right??

  2. Brett

    Brett > PRO STAFF <

    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?   ;)

  3. TomFL

    TomFL Well-Known Member

    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?

  4. Brett

    Brett > PRO STAFF <

    Assuming fully charged new battery, your answer is right.
    Old batteries won't meet the A/H rating.
  5. firecat1981

    firecat1981 BBA Counselor

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

    iMacattack busy, too busy

    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.)
  7. Brett

    Brett > PRO STAFF <

    I've been wrong for a long time.   :eek:

    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!

  8. TomFL

    TomFL Well-Known Member

    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?

  9. Brett

    Brett > PRO STAFF <

    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.
  10. DuckNut

    DuckNut Brandon, FL

    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.
  11. TomFL

    TomFL Well-Known Member

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

  12. johnboy

    johnboy local yokal