Explain How Car DC Inverters Work - Page 2 - Fiberglass RV

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Old 07-22-2014, 06:33 PM   #15
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Name: Carol
Trailer: 22' Airstream Formerly 16' Scamp
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Originally Posted by Huck View Post
Everybody else knows. They just won't tell us!
A good place to start figuring it out is to read through the whole 12 Volt Side of Life website. The 12 Volt Side of Life - Part 2 has lots of info on inverters and what power usage you can expect as well as what the draw is on some typical appliances.

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Old 07-22-2014, 07:21 PM   #16
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We have a 1200 watt Inverter in our Scamp but never use it.

Instead we typically use a 150 watt cigarette lighter Inverter to run our TV and Sat dish when boondocking. They draw very little power.

We have installed three 12 volt outlets for Inverters in our Scamp, one under the dinette, one near the couch and one above the stove counter.

We also carry onw in the car for charging the laptop.

Our son's Cpap machine runs on AC or 12 volt DC. In his Scamp we installed a 12 VDC socket for his Cpap. He has 2 12 volt batteries and an 80 watt solar panel.

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2014 Honda Odyssey
1991 Scamp 16
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Old 07-28-2014, 01:17 PM   #17
Name: Tim
Trailer: Aliner folding & Weekend Warrior toy hauler
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Originally Posted by Huck View Post
...how can it support...4.2a usb ports? Where are all the amps coming from?
Easy, a USB port is 5vdc, not 110vac or 12vdc. Thus it can have 4.2a available at a 5vdc USB port.
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Old 07-29-2014, 10:12 AM   #18
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I am probably only going to confuse things more, but here is my best explanation.

Volts and Amps are almost meaningless on their own. Watts is power. Power is work, or what you want to make happen. Your Cpap machine is doing work, it requires a certain amount of power to do this. If they recommend a 150 watt inverter, the device likely only consumes ~100 watts. If you have a 300 watt inverter, it will work fine. It just wont be working as hard as it could.

Watts are a function of current, (amps) and voltage, (volts). 10 amps at 100 volts is 1000 watts, (10A x 100V = 1000W). This is always true in DC applications. It is also mostly true with AC, but AC can get a bit complicated when the current and voltage are not in phase. This happens with coils, and capacitors, but lets ignore that.

If your Cpap requires 120 watts to run, then at 120 VAC, it will draw 1 amp. Your inverter can almost be considered a transformer. At the 12 volt side it will draw 10 amps to make the same 120 watts, (12VDC x 10A = 120W). This ignores the power consumed by the inefficiency of the inverter.

The 5VDC is likely provided by DC to DC converter in the inverter. You can think of this as a transformer as well. DC to DC converters used to be expensive, but not so much anymore. This one is $2.27, free delivery:
P4PM DC12V Step Down to 5V 3A 15W Converter Car LED Display Power Supply Module | eBay
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Old 07-29-2014, 10:42 AM   #19
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Trailer: '88 Scamp 16, layout 4
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Originally Posted by David Tilston View Post
If they recommend a 150 watt inverter, the device likely only consumes ~100 watts. If you have a 300 watt inverter, it will work fine. It just wont be working as hard.

Yes but, just doing the math a diligent person would not know that the machine would run fine (and not let out the magic smoke) on an inverter half the size indicated. And, the 300 inverter had a cooling fan and was oversized (and therefore electrically wasteful) for the job. The smaller inverter is not only more closely matched to the load and more efficient but does not have a cooling fan drawing energy. When you are boondocking on batteries you have to watch every watt!

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