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Old 04-26-2017, 10:50 AM   #21
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Quote:
Originally Posted by pmsherman View Post
Your math is a little off. As you stated, power losses in wire are proportional to the current squared. If you have 0.1 of the current at 120V, you will have 0.1 * 0.1 or 0.01 as much losses in the wire. That's 1/100, not 1/10! This difference is why we have high voltage (300,000V and higher) long distance power transmission lines and local power distribution lines running 3300V with transformers feeding clusters of houses.

The downside to converting to 120VAC is that there will be losses in the converter. When the 120VAC is converted back to (usually) 12VDC for use in the CPAP, there's additional losses. It's always more efficient to use 12V directly form the battery instead of including two voltage conversions.

Phil
You're right, I said current squared, but then messed up the math. thanks.

I didn't address the two conversions because I "assume" the OP's CPAP is probably 120VAC already (i.e., if it IS 12VDC, he can't easily access that part of the circuit).

If one does decide to go with straight 12VDC, they should use wire that's as large as possible in order to reduce the resistance per foot, since they can't change the current. AND keep the wiring as short as possible. (Same for solar panels.)
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Old 05-02-2017, 08:34 AM   #22
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Not all CPAPs are 12 volt, mine is 24. They sell a 12 to 24 volt adapter for it.

Frank
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