AC/DC wiring question about mattress pad...

[Robbie]

Hey guys, so after our first camp trip of the year on Easter weekend I've been thinking about how we'll keep warm in our teardrop for those early and late year camp trips. I've been looking at personal space heaters but the only decent ones I can find run on AC power. I'd like to find something that runs on DC power so it can run off the battery at night so I won't have to keep the generator running through the night. I know they make 12v heated mattress pads but all I can find are smaller pads that will only cover about half of our mattress and they are pretty pricey for being so small. I found the one below which runs on AC power but it says and shows that it has a power box that turns the AC power into low-voltage DC power. My question is, could I just cut the little adapter box off and hard wire a 12v plug to the end of the wires and plug it directly into my 12v outlets?

http://www.amazon.com/Soft-Heat-Thread- ... 890&sr=1-2

Robbie

[GuitarPhotog]

The power supply on mine says it puts out 18VDC, so you'd better check your specific pad.

<Chas>

[Robbie]

Aaah... Good to know. For some reason when I see or hear DC volts I automatically assume its 12v. Turns out Target and Walmart carry this pad so I'll have to stop in one day and check out the power ratings on that adapter.

If this plan doesn't work, I guess I could run the pad off of an inverter plugged into the 12v outlet right? Assuming the power consuption (watts) is rated less than that of the inverter, correct?

Robbie

[sgip2000]

Most low voltage heated blankets/matress pads are 18V-24V. I do have a 12V heated blanket, but it is fairly small. Those can be found on Amazon.

[Shadow Catcher]

As a rule of thumb Any resistance heat appliance sucks major watts/amps. If you wish to go that route shop for a 12V pad and figure out what it will suck out of your battery. An alternative is http://cozywinters.com/shop/ew-rvhmp.html which uses about 3A. The 18V might work but if it does not you are out the price.

[eamarquardt]

I looked at: http://cozywinters.com/shop/ew-rvhmp.html . It says the "average" current usage is 3 amps. Not to be a skeptic but manufacturers usually advertise what they think will sell their product. I looked at the pad on our queen bed and each side is rated at 3.5 amps at 16.5 volts d/c. As you're just driving a resistance heater (no motors) I'm guessing that it would work fine on 12 volts but would not have quite the heat output it normally would.

You might buy one, run it on 12 volts, see how it works, and measure the current. Yeah I know a PITA. If it doesn't meet your needs you pack it up nice and pretty and return it as I doubt that running it on 12 volts will harm it.

After you determine if it works, how much heat it will put out on 12 volts, and the current draw you can figure out how much juice it will take to run all night. I'm betting that it will take 25-30 amp hours. About half the capacity of a 100 amp/hour deep cycle battery as you don't want to discharge the battery more than about 50%.

You also have to have an idea on how much juice (technical term for electricity) you'll need for your other devices. Now you have to figure out how much juice you're going to put back into your battery each day. If you are only staying out one night you can do it when you return home. If you're out in the boondocks you can charge by connecting it to your tow vehicle and running the engine (maybe while toodling about), solar, generator, thermonuclear, or whatever. You shouldn't charge a battery at more than about 10% of its capacity so it's gonna take 3-5 hours to properly charge it back up.

A human body will put out about 100 watts when resting. So 3 amps at 12 volts is about a third of that.

These are all things that need to be considered to make it work. A lot of folks are "overly optimistic" about what and for how long they can run something on a battery.

Look before you leap.

Hope this helps.

Cheers,

Gus

[sgip2000]

On a side note, the manuals for both "Sunbeam" blankets I have say not to use them with generator power. Not sure why. Doesn't say anything about inverters though.