Power Leakage from DC Power Supply

[richguy82]

I am trying to use an 850 watt DC power supply plugged into the bed ProPower outlet. I have a watt meter on this cord and it says it’s pulling just under 900 watts. When I go into my truck it says it’s pulling 1400! Any idea what this is? Does this value include the fans inside the truck potentially trying to cool the inverter?

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[RickLightning]

I am trying to use an 850 watt DC power supply plugged into the bed ProPower outlet. I have a watt meter on this cord and it says it’s pulling just under 900 watts. When I go into my truck it says it’s pulling 1400! Any idea what this is? Does this value include the fans inside the truck potentially trying to cool the inverter?

No, it doesn't include the truck itself running fans.

 

[jetfixr1]

Clamp on an amp meter and see how much current is flowing

 

[K6CCC]

My guess is that the power supply is a VERY non power factor corrected power supply. What that means is that the current does not follow the voltage. This is common on DC power supplies. A Real or True RMS multimeter can handle that and correctly display the actual power. Many meters can not. Never measured the Lightning's ability to deal with seriously non PF loads, but it would not surprise me to find that it reads the power poorly. Pretty sure I have seen the same issue.

 

[richguy82]

Clamp on an amp meter and see how much current is flowing

I don’t have one of those unfortunately.

 

[jetfixr1]

If you have a harbor freight around theyre usually around $30-$45. Cheap and will give you a somewhat close reading.

I don’t have one of those unfortunately.

 

[richguy82]

My guess is that the power supply is a VERY non power factor corrected power supply. What that means is that the current does not follow the voltage. This is common on DC power supplies. A Real or True RMS multimeter can handle that and correctly display the actual power. Many meters can not. Never measured the Lightning's ability to deal with seriously non PF loads, but it would not surprise me to find that it reads the power poorly. Pretty sure I have seen the same issue.

Interesting. So you think the Lightning is confused by this power supply? I mean it’s a cheap power supply for sure. The watt meter is reading as I would expect as it’s losing roughly 7% due to the fan and whatever else.

 

[K6CCC]

OK, let me explain this a bit. Warning, this will get technical.

When dealing with Alternating Current ( AC ) power, the voltage wings back and forth many times per second - 120 times per second in each direction in the US for 60 complete cycles per second. For 120 volt AC power, that cycle varies from about 165 volts positive to 165 volts negative in reference to the neutral wire. It follows a sinusoidal curve - also known for this discussion as a sine wave. In a purely resistive load (a heating element for example), the current exactly follows the voltage. However very few things are a purely resistive load. If the load is somewhat capacitive or inductive (motors for example), the current leads or follows the voltage. A simple wattmeter can't handle the current and voltage peaks not being at the same time and will give inaccurate readings. A good True RMS (Root Mean Squared) watt meter knows how to handle the time difference. This is not exactly what is happening with your cheap DC power supply, but it gives some background.

With a simple AC operated DC power, the voltage supplied to the power supply is still following the sine wave, but the current is radically different. What happens is that all the current flows during a small percentage of time (when the voltage is near peak), and for the rest of the sine wave, there is almost zero current flow. Because the current is flowing only for a very small percentage of time, the peak current really high. Most meters have fits over that and give quite inaccurate readings. Good DC power supplies have additional circuitry to correct for this peak current issue - called power factor correction. But that costs extra money so cheap power supplies generally do not have this additional circuitry. For most purposes, the peak current is not a major problem, but especially when the power source is a small generator or inverter, it can really cause problems. One of the issues is that if you have an inverter that is paying attention to the peak current (such as our Lightnings), it can trip off on overcurrent, even though the total wattage is well under the limit. That is the exact situation I mentioned in my earlier post where I had the Lightning inverters trip off even though I knew the total wattage was well under the limits.

If you want a longer explanation about power factor, you can check out: https://en.wikipedia.org/wiki/Power_factor

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