Hacker News Reader

Comment by zirgs | Hacker News Reader

Wouldn't it make more sense to install 230V on a boat? Modern electronics handle 100-240 anyway and you can save on cables.

View on HN

12VDC is the standard on boats, campervans, RVs, etc because it can be delivered straight from the batteries. In order to deliver 240/120VAC you'd need an inverter to generate the signal, and this obviously introduces loss during conversion. For cable runs through the length of my boat (52ft), the cables aren't particularly thick - the thickest cables are in the engine bay, going from the alternator on the diesel engine to the batts, and again from the Solar controller (MPPT) to the batts.

There's a plethora of stuff you can use with 12VDC to the extent that the only things I run off 240VAC is the vacuum cleaner and the TV (12VDC TVs are available but the cost vs quality payoff isn't really worth it).

230VAC flat screen TVs tend to be powered by 14VDC at the motherboard level, might well function if fed 12VDC from an external power source, bypassing the built-in step-down converter entirely.

Typically you don't want lethal potential in such wet environments. Ever wondered why emergency lights on ferries are different than the standard ones? Also, you don't want any earth breaker since you really want the lights, pump and motor to be working no matter how much water you have on the wrong side of the hull.

I would think 110v would be a little bit safer in the wet environment. Not perfectly safe but better.

Boats are typically so small that you can use smaller cables without problem even at 120 volts. Many of them run only 12 volts (often most things like lights and radios are run on 12 volts, but the owner as an inverter for a couple appliances they can't find in 12 volt). Of course nobody runs smaller cables at 120 volts because you would need an engineer to figure out the proper rating - it is cheaper to just use the standard code wires. If you do pay an engineer you will probably discover on boats you need to use the larger wires anyway for mechanical reasons.

Having wired up a couple travel trailers and a boat (and, incidentally, being an EE who can theoretically run those calcs), I agree with the parent: It would be better if the industry switched to 120V/220V AC for more parts, and away from 12V.

The problem is that the ampacity in a 12V DC light circuit is 10x that of an equivalent-power 120V AC system. And worse yet, power dissipation in a resistive element is equal to I^2 R, so you're heating the wiring harness with power losses are 100x worse than in a 120V system and almost 200x worse than a 220V system. So you have to oversize the wires to reduce R, which adds weight and cost. And if you drop 2V to line losses in your 12V system, your device is seeing 83% of the nominal voltage, while if you drop 2V in a 120V system (you won't, because Ohm says voltage drop is equal to IR, and I is 1/10th that of a 12V system), but still, you're 98% nominal.

They're running 12V because that's the voltage of a classic 6-cell lead-acid battery, and you probably didn't want 220V in your Model T, even with felted asbestos insulation, but I'd be confident enough to lick a modern XLPE wire carrying 220V. A century of using those lead acid batteries (and worse, 3-cell 6V systems...can't endorse converting your classic car to 12V enough...) created now-entrenched economies of scale for lamps, and switches, and pumps, and radios, and fans, and most of the other things you need in a boat or camper. The only advantage of using those 12V parts is that their manufacturers put a little more effort into making them efficient. No one (sadly) will notice or care if the AC-DC converter that runs the clock on your stove burns 5W at idle even though your wall clock can run on one AA battery for years, but they will notice and care if their camper battery is dead after leaving it parked for a couple weeks.

Today, I recommend using LiFePO4 batteries in whatever series cell arrangement gets you the required watt hours, regardless of that output voltage. Then run a modern, high-efficiency (high-frequency) digital inverter to bump the voltage up to whatever your local AC grid runs at. You can go 220V AC in the US if you really want more efficiency and smaller wires, and are willing to deal with the hassle of finding the right lightbulbs and international power cords and so on.

Doesn't a 48V DC installation provide a very reasonable compromise?

That's what I use for my off-grid solar installations, and it works pretty well. 4x less amps, and you still get a safe voltage.

Yes, but there aren't many 48V DC appliances -- radios, lights, gauges, etc.

Many larger boats have 24V DC systems, and there are quite a few boat electronics that run on either 12V or 24V (though you will still need either a 24->12v converter for many things, or be able to tap 12v from the battery bank).

DC-DC buck converters got incredibly cheap and efficient, every small appliance can have one.

Yep, that's a great compromise. 40V, 56V, 80V, 48V outdoor power tools use the same voltage for many of the same reason (they're slowly moving away from 18V battery packs for anything high-power). Maybe after a few decades, RV suppliers will standardize on 48V DC lighting circuits, a standard 48V DC receptacle, and provide fans and smoke detectors and pumps and sensors and so on that run on 48V DC. Integrated circuits will emerge in quantity designed to rectify 48V down to whatever the LED drivers and microcontrollers and so on use, and eventually compete in economies of scale with those used in the 12V automotive world.

But good luck finding those parts today.

Reply on news.ycombinator.com