Providing 12V Power to RV-Based Hardware?

jp93023 asks: "I am putting together a mobile computer lab in a converted RV. RVs have a 12v DC system for running most interior lights, built-in appliances, fans and so forth. They have a parallel 120v AC system for providing normal outlets, which is great when you are plugged in. It strikes me that when I am running from battery power (which will be most of the time) I will be expending precious watts converting the 12v from the marine batteries up to 120v, only to convert it right back down to 5, 9, 12, and 20v DC for all the PCs, laptops, etc. The equipment package will include desktops, laptops, digital tape decks, etc., so I'd be planning on bypassing the built-in transformers for everything but the laptops. Have any slashdotters put together a unified low voltage DC power distribution system for such a situation? Would the power savings be worth it? Any pointers to products or plans would be great!"

Comments

The Boaters have lots of documentation...

[Ocelot Wreak]

I echo the comment on boats - there are a LOT of "How To" manuals complete with schematics for designing your own 12V systems in boats, right down to the size of wire to use. [I will post a title and ISBN when I get home to check the bookshelf.] Same principles apply to cars.

I would be very afraid of the 12V coming out of the cigarette lighter adapter - I heard a story of someone working away on a laptop just fine using the car's 12V battery system and the car ignition turned off. However, they had their laptop fried when they turned the car ignition switch on. It send some crufty power through the cig lighter and into the equipment.

Switchers

[Matt_Bennett]

If battery conservation is important, stay away from the inverter- a large portion of the power will go away as heat- look into switching power supplies, which can run up to 80-90% efficiency. Look through surplus catalogs and so forth, buying them factory new will probably be prohibitively expensive, but they can be quite reasonable in surplus.

Also, try to stay away from linear power supplies- they also will convert a lot of your precious power to heat.

Running a desktop computer off a 12V system looks to be pretty easy at first, especially when you look at the common voltages (+/- 12, 5, 3.3V) But the problem starts when you look at how much current each one of those voltage rails use, it's pretty amazing- 30, 40 amps is not unusual.

With lower voltage DC supplies, you have high currents for the same amount of power delivered, so you're also going to want to use pretty thick wire to distribute the power around. This will minimize the loss of power to the resistance of the wire.

Try amateur radio sites...

[MaggieL]

...because this is a project that a lot of amateur radio folk undertake in building emergency communications or stormchaser vans. Most amateur radio equipment runs on 12v DC, and these vehicles also usually pack a number of computers for digital text communications modes, processing weather info and satellite tracking.

Suggested keywords: "communications van", "emergency communications", "stormchaser", "RACES", "ARES"

Re:RV power

[t]

This is not informative. The poster is asking specifically if there are power savings to be had by avoiding the 12VDC to 120VAC to 5,9,12,20VDC process and instead using a DC-DC transformer to go directly from 12VDC to 5,9,12,20VDC as needed.

All the above "Informative" post says is "Duh like dude yeah, plug that baby into the cigarette lighter dude. and yeah like dude, plug it into the AC plug too. Like noooo problem."

I would think some benefits would be that by not using the typical AC power bricks you'll avoid the continuous drain that they cause. For some devices it can be a considerable quantity. Sloppy design I guess. I'm not sure how some of the latest tech in DC-DC converters works but I would expect there to be less electrical intereference generated too. Helpful if you're parked near a wifi point, just don't run the microwave.

Another issue is that some devices want low power AC for odd reasons. In those cases it may be possible to find a 12VDC to 9VAC converter that would run cooler (waste less).

t.

Re:Why not use a generator?

[pete-classic]

Also, the efficency of a generator powering a small load is TERRIBLE! If you are running loads totaling a couple of KW gens are bad, but for a 200 watt computer the generator will not run well at all.

I've only ever worked with gens in the Army, but you should always run a gen loaded. If you don't have a sufficent load you add what we just called a load, which is nothing but a heating coil AKA a big ass resistor.

Then it comes down to what you mean by efficiency. Pound-for-pound and dollar-for-dollar gens and gas are pretty hard to beat, and you can piss a lot of gas away in heat before batteries start looking good.

-Peter