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Old 11-05-2012, 06:00 PM   #1
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How many amps at 12 volt?

If I use a 115 volt 23 watt compact florescent light with an inverter from a 12 volt battery, how many amps will it draw and how long will it run on a 625 amp 12 volt battery?
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Old 11-05-2012, 06:20 PM   #2
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I = (23 W /115 V) = 0.2 amps.

That 625 amps is probably cold cranking power - 30 seconds of power. The reserve power of the battery is more specific to what you want to know. That is harder to figure. A 106 amp hour battery would run your .2 amp light for about 22 days or over 500 hours I believe.

This does not account for any loss in the inversion of power.
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Old 11-05-2012, 06:39 PM   #3
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But doesn't the amps change when you drop the voltage to 12 volts, or doesn't that apply since I am using an inverter to change 12 volts to 115 volts?
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Old 11-05-2012, 06:44 PM   #4
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23 watts is just a hair under 2 amps at 12 volts. Given the inefficiency of your inverter, 3 amps would be a safe guess.
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Old 11-05-2012, 08:28 PM   #5
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This might sound dumb, but why not simply get a 12 volt fluorescent fixture? Usually, a two tube light is 16 watts, with no loss, and no Rube Goldberg 12 volt to 120 volt to fluorescent tube setup.
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Old 11-05-2012, 08:29 PM   #6
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3 amps from +12v is a good estimate. I've not looked at the efficiency of invertors, but I would be surprised if they are better than 75% efficient.

This means to get 23W out, you need to put 30.6W in. If the battery voltage as measured at the input of the invertor is +12v then it draws 2.6A.

To answer your question, you need to know the capacity of the battery - this is measured in amp-hours. i.e. how many amps for how many hours.
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Old 11-05-2012, 08:33 PM   #7
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I would use 2.5 amps as a realistic current flow. For a 106 amp-hr battery, you don't want to drop the voltage below 50% of the rating, so I would say that you could operate the 23 watt light for about 20 hrs (53/2.5).

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Old 11-06-2012, 07:14 AM   #8
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Thanks for the info. A 12 volt fluorescent fixture would be a better bet. I just wanted to use a table lamp outside using an extra battery I have.
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