Kill a watt question

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  • hoosierdoc

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    I have a P3, on a whim I plugged my 1a Apple charger in and charged my phone. It read 0.09A. I plugged my CPAP in which should draw about 2.5A and was getting 0.28A even with heater on.

    it’s on AMP setting and displays “amps” on the screen. Voltage says 123V.

    Any ideas?
     

    K_W

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    I have a P3, on a whim I plugged my 1a Apple charger in and charged my phone. It read 0.09A. I plugged my CPAP in which should draw about 2.5A and was getting 0.28A even with heater on.

    it’s on AMP setting and displays “amps” on the screen. Voltage says 123V.

    Any ideas?

    I think you are trying to directly calculate transformer MAX input volts and amps into actual usage output volts and amps (measured by kill-a-watt).

    Power supplies are sized to supply surge current to start motors, charge capacitors, and some heating elements that draw less as the reach temperature.

    If your CPAP supply is 12V DC 2.5A MAX output to the machine then a 120V 0.25 AC input to the power supply when running sounds about right.

    I have a few devices on a UPS that combined have a max draw of 60W AC according to their power bricks... but only draw 9 watts in actual use.
     
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    hoosierdoc

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    Reading about the cpap it’s supposed to draw about 2.5a during use.

    the iPhone charger is a 2a charger (I was wrong, using one for iPad) and looking at the mAh battery rating it should spit out two amp per hour into my phone right? The 2900mAh battery charges in about 1.5a on the 2a charger. How can it do this if it draws 0.09a from the wall?

    i’m trying to calculate how quick i’ll Drain my boat battery. Am I getting confused on the voltage change with transformer and how that affects amps? I think the iPhone charger is 5v 10w which may be accounting for the 24X difference i’m Seeing in input amps vs what I expected.

    the boat battery bank is 12v so instead of 0.22a input at 124v I should see about 2.2a from the 13v bank or so right?
     
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    eldirector

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    V=IR
    Volts = Amps * Resistance

    Edit:
    Forgot that USB is 5V

    5V = 2.5A * R

    110V = 0.25A * R

    Assuming Resistance doesn't change (and it shouldn't - we are talking about the same charger. Just the input and output sides), that sounds about right. Basically an "order of magnitude" moving from 110V to 5V (or 12V).
     

    femurphy77

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    The ratings on equipment are typically at full load. Rarely will you see that in use. Heck if it wasn't for in rush currents a 5k generator would damn near be sufficient for an average American home during a power outage. I've taken amp readings at my panel with everything on and it's nowhere near the panel rating, in rush is the driving issue though.
     

    schmart

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    the iPhone charger is a 2a charger (I was wrong, using one for iPad) and looking at the mAh battery rating it should spit out two amp per hour into my phone right? The 2900mAh battery charges in about 1.5a on the 2a charger. How can it do this if it draws 0.09a from the wall?

    i’m trying to calculate how quick i’ll Drain my boat battery. Am I getting confused on the voltage change with transformer and how that affects amps? I think the iPhone charger is 5v 10w which may be accounting for the 24X difference i’m Seeing in input amps vs what I expected.

    the boat battery bank is 12v so instead of 0.22a input at 124v I should see about 2.2a from the 13v bank or so right?

    Answering the first question is that electrical power (Watts) is a product of Volts and Amps (Watts = volts * amps). So, in the first question, the iPhone charger outputs 5 volts and as you say 1.5 amps giving a total power of 7.5 watts. To determine how many amps that takes at 120 volts, take the 7.5 amps and divide by 120 to get .0625 amps. In any energy transformation device, there is always losses caused by inefficiencies so your measurement of .09 amps is very reasonable. The extra power will be converted to heat in the charger and is why it gets warm.

    As far as draining your boat battery, obviously it will depend on how big the battery is, but a typical car battery has about 45 Amp/hours of capacity (45000 mAh). However, this can't simply be compared to a device battery or external charger bank because again the voltages are different. The car battery at 12 volts has 540 watt hours. The external charger banks use about 4v batteries so it would take a 135 amp hour charger bank to be equivalent.

    Going back to the iPad, it has a 32 watt hour battery, so you should be able to charge your ipad about 16 times (540/32) from your boat battery, assuming it is equivalent to a typical car battery. Unless it is a sail boat and the battery isn't critical on getting an engine started and back to shore, I wouldn't want to charge it more than 3 or 4 times before I got the engine started and recharge the battery! :)

    --Rick
     

    jkaetz

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    Answering the first question is that electrical power (Watts) is a product of Volts and Amps (Watts = volts * amps). So, in the first question, the iPhone charger outputs 5 volts and as you say 1.5 amps giving a total power of 7.5 watts. To determine how many amps that takes at 120 volts, take the 7.5 amps and divide by 120 to get .0625 amps. In any energy transformation device, there is always losses caused by inefficiencies so your measurement of .09 amps is very reasonable. The extra power will be converted to heat in the charger and is why it gets warm.

    As far as draining your boat battery, obviously it will depend on how big the battery is, but a typical car battery has about 45 Amp/hours of capacity (45000 mAh). However, this can't simply be compared to a device battery or external charger bank because again the voltages are different. The car battery at 12 volts has 540 watt hours. The external charger banks use about 4v batteries so it would take a 135 amp hour charger bank to be equivalent.

    Going back to the iPad, it has a 32 watt hour battery, so you should be able to charge your ipad about 16 times (540/32) from your boat battery, assuming it is equivalent to a typical car battery. Unless it is a sail boat and the battery isn't critical on getting an engine started and back to shore, I wouldn't want to charge it more than 3 or 4 times before I got the engine started and recharge the battery! :)

    --Rick
    This, you'll also want to use dc -> dc (Think cigarette lighter chargers) converters to charge devices like phones and tablets. A lot of things now days also run directly from 12v power. Anything that has a wall wart usually converts to 12vdc or 5vdc.

    Final notes, you state "battery bank" if you have multiple 12v batteries connected in parallel you get to add their collective amp hours. We have a runabout with a single starter battery and can play in the lake all day while running the radio, external audio amp, and charging phones without killing it.
     

    JettaKnight

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    Reading about the cpap it’s supposed to draw about 2.5a during use.

    the iPhone charger is a 2a charger (I was wrong, using one for iPad) and looking at the mAh battery rating it should spit out two amp per hour into my phone right? The 2900mAh battery charges in about 1.5a on the 2a charger. How can it do this if it draws 0.09a from the wall?

    i’m trying to calculate how quick i’ll Drain my boat battery. Am I getting confused on the voltage change with transformer and how that affects amps? I think the iPhone charger is 5v 10w which may be accounting for the 24X difference i’m Seeing in input amps vs what I expected.

    the boat battery bank is 12v so instead of 0.22a input at 124v I should see about 2.2a from the 13v bank or so right?

    Are you running these two devices from a power inverter when on the boat?

    Otherwise, why are you measuring 120 VAC current, because that not the actual intended use.



    I'll wait for an answer before I continue my thoughts.
     

    hoosierdoc

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    Are you running these two devices from a power inverter when on the boat?

    Otherwise, why are you measuring 120 VAC current, because that not the actual intended use.



    I'll wait for an answer before I continue my thoughts.

    Eventually. I have a 12v system and will be charging phones from a 12v outlet and have a dc-dc converter for the cpap. Claims it’s 90w.

    i have always been thinking of amp-hours. I’ll have two 135AH six-volt batteries in series. So I figure I have about 65AH to play with.
     

    Cameramonkey

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    Stop thinking in amps, think in watts, and add 10-15% loss if you are using an inverter.

    This, you'll also want to use dc -> dc (Think cigarette lighter chargers) converters to charge devices like phones and tablets. A lot of things now days also run directly from 12v power. Anything that has a wall wart usually converts to 12vdc or 5vdc.

    Final notes, you state "battery bank" if you have multiple 12v batteries connected in parallel you get to add their collective amp hours. We have a runabout with a single starter battery and can play in the lake all day while running the radio, external audio amp, and charging phones without killing it.


    THIS. DC -> AC Inverters are stupidly inefficient, and unless you are drawing 90%of the inverter's max load, you lose more energy to heat from the process than you get out the other side in useable electrons. I fought with this on a network switching rack trying to squeeze 24 hours of battery life for critical infrastructure. I was going to need something like 2 1/2 times the amp hours I was going to actually consume on the other side because I was only utilizing 10% of the inverter's load. It was nuts. (and I gave up)
     
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    JettaKnight

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    Eventually. I have a 12v system and will be charging phones from a 12v outlet and have a dc-dc converter for the cpap. Claims it’s 90w.

    i have always been thinking of amp-hours. I’ll have two 135AH six-volt batteries in series. So I figure I have about 65AH to play with.
    OK, you've thought farther ahead than I thought you did.

    Ideally, DC-DC converters should be the most efficient path. Avoid 120 VAC if at all possible. But, I would advise against connecting the CPAP directly to the battery.
     

    Cameramonkey

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    One other suggestion. Add one more battery to the bank behind an isolator? That way if you accidentally run your main bank down, you have one more bank in reserve. Kinda like the reserve tank on a motorcycle. When in isolation mode, the extra battery is charged by your charger, but it will not discharge with the rest of the batteries until you switch over to that one battery.
     

    PistolBob

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    OK, you've thought farther ahead than I thought you did.

    Ideally, DC-DC converters should be the most efficient path. Avoid 120 VAC if at all possible. But, I would advise against connecting the CPAP directly to the battery.

    Depends on the CPAP. Mine runs off a 12v battery just fine with the 12v DC cable from Respironics. On 12 vdc it pulls 3A.
     

    hoosierdoc

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    The boat has a 5hp pull start outboard. If I lose all electrical i’m Fine. We sailed this year without a battery. Just a comfort thing. I got a marine stereo i’m Hoping to rock out with.

    If I ever get it installed...

    my DC-DC power supply came with clips for battery terminal and a 12v plug. Should be Ok. That’s From manufacturer
     
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