Hacker News Reader

Comment by danuker | Hacker News Reader

Rather than inverting 12V DC to 240V AC and back, you could skip a step and use a laptop car charger, converting 12V DC straight to ~20V DC.

View on HN

You could also use a desktop with one of these ATX power supplies that runs on 12VDC instead of 120/240VAC:

https://www.cartft.com/catalog/il/2302

Wow, I was not expecting that price! Is this just a rare item or is there something particularity expensive inside it?

Definitely are cheaper options too: https://www.mini-box.com/picoPSU-160-XT

It's not a mass-market item so they're expensive to begin with, but I'd bet its current price is a function of the chip shortage we've been hearing about.

How much labor does it save?

How much trouble shooting does it eliminate?

How much experience does it make unnecessary?

Or to put it another way, at the point where 1500w of 12v source computer power supply makes sense, the price is not unreasonable.

Not at all. At that absurd price it's cheaper to buy more solar panels and ignore the inefficiency.

cheaper

Price sensitive shoppers are not in the product's market segment.

Looking at the size of this thing, I sort of doubt it's much more efficient than doing a DC->AC->DC conversion. In fact, I'll betcha that's exactly what's happening inside this beast to both get the right voltages and stabilize them.

It says 72% efficiency, so over 400W of heat at full load. Inverter to a normal PSU would be about the same, if not worse. It could be that it raises the voltage then drops it back down to 12V, but i'd sooner think it's just cheaper mosfets.

Anyway, these kinds of converters can usually regulate themselves.

That's true. If both conversions are 90% efficient, we are talking a 81% efficiency in both process, wasting nearly 1/5 of the energy used. That's not good at all.

Don't DC-DC transformers use AC internally?

Yes but it's kHz-range PWM square-wave AC; there's no 60Hz sinewave involved. Converting to/from a 60Hz sinewave requires extra circuitry that decreases efficiency. And in this application a 60Hz AC sinewave serves no useful purpose.

If you want electrical isolation between the input and the output, they use AC in the middle to achieve that, but if you don't want isolation, you could use a boost DC converter: https://en.wikipedia.org/wiki/Boost_converter

Which use alternating voltage, across the inductor, but the current is always (with load) positive. Neat!

In power electronics terms, no. AC is defined as waveforms that have an average value of zero, which you won't find in a DC-DC converter.

That wasn't the point of OP though. Their point was that you could remove some inefficient steps to improve the overall efficiency and energy capture of the system.

The point is to remove the laptop power brick. Let's say the laptop charger is 90% efficient, which is fairly typical. The post claims their 12VDC to 240VAC inverter is 92% efficient. A 12VDC to 20VDC voltage booster would only have to be >83% efficient to beat that setup.

Which isn’t that hard for a switch mode converter. Those are usually 85% or more IIRC.

Not necessarily. See voltage doubler:

https://en.m.wikipedia.org/wiki/Voltage_doubler

I did this for a year on my previous macbook and it destroyed the battery. The inverter creates cleaner power. Good luck finding a quality one that can handle the voltage changes from 14.6 -> ~10.5 the battery is going to spit out with the MPPT attached..

Maintaining a regulated voltage with input voltage changes is a primary feature of a DC-DC converter, like a boost converter. With a buck-boost converter, you can maintain a regulated voltage when the input voltage goes above or below the output voltage.

Wait, you tried to do it without a DC/DC converter to regulate it? Sounds like a bad idea.

A good off-the-shelf isolated DC/DC converter that would clean it all up and give you a nice, consistent, filtered DC voltage should be able to be had for $100 or a bit less.

I wonder how much the efficiency loss from a laptop car charger would compare to just using something like a NUC -- IIRC some of their models have really wide input voltage ranges (12v-20v or something like that, depends on the model). The laptop is really just bringing a battery, compared to a NUC, for this application, and they are going panel->regulator->battery->converter anyway so the laptop battery seems redundant.

Odroid H1/H2 iirc can run from like 10-22VDC, the issue is finding a solar panel that has an open circuit voltage less than 22V. I found a buck-boost that works with my old laptop and a "car charger" with my admittedly oddball solar panels, so this is possibly, but takes time shopping and reading specifications.

The design in the article (which does seem to have some redundant parts, so...) instead goes:

Panel -> regulator -> battery -> inverter -> laptop

So, the voltage of the panel shouldn't really matter too much, I think (I mean, you size the regulator input range as appropriate). OTOH, solar panels are a little magical from my point of view, so maybe that regulator ought to be replaced by some solarpanel specific thing, which might be more constricting.

It isn’t a regulator, it’s a charger - depending on the design, you can get even higher voltage spikes. 12V batteries charge at 13-14V, but most chargers design for lead acid can get away with reallly really noisy voltage transients due to the way lead acid works. It’s a pretty insensitive chemistry and dampens them normally.

Some chargers with ‘equalize’ or even worse ‘desulphation’ can intentionally go even higher in voltage than normal charging voltages.

So basically ‘if you just assume it wouldn’t kill your laptop to directly connect it, you’re playing Russian roulette with your laptop’.

With a decent spec sheet (and oscilloscope) to verify nothing too crazy that the charger is doing, some decent power filtering capacitors, and good DC-DC power supply you’d be fine though.

That's not a way to get good power out of the PV panel, though. An MPPT tracker (https://en.m.wikipedia.org/wiki/Maximum_power_point_tracking) is needed to get the best results, and the room for improvement is indeed substantial. There are good and cheap chinese ones available, such as Epever brand.

A very cheap, bad-practice, but effective solution is to run the inverter to AC and back to DC again with the ATX power supply, but to connect the 12V from the battery direct to the motherboards 12v rail.

The motherboard uses most of its power from the 12v rail, and due to the galvanic isolation in the ATX power supply, it should be safe to do.

The 12VDC from the battery may not be in spec for the motherboard, but typically they'll work anyway.

It's also worth noting that many "12 volt" DC batteries do not store and output at exactly 12 volts.

Depending on the tolerances of your board, you might want to throw a buck converter in the circuit and set the output voltage set to exactly 12 volts to account for any potential over-voltage coming from the battery/panel setup.

They are pretty cheap if you want to DIY (7 for $10 on Amazon). Worth the peace of mind, imo.

I imagine this is what most car-laptop chargers are, but with branded packaging.

the efficiency savings with this approach will outweigh any other effort

I don't think that's true. There are plenty of low-power setups that consume less than 2 watts, for an instant gain of 6/7ths or 85%. DC-to-AC-to-DC conversion is less than ideal, but even at a low estimate of only 90% efficiency at each step, that'd still be only a 1.0-(0.9^3)= ~28% loss.

I don't think % efficiency tells the whole story, because those numbers are measured under load. A 12V->240V inverter probably burns a lot of power just sitting idle, so the efficiency curve starts at 0%. You really need to know (or measure) that curve's overall shape.

Or use a 20v MPPT with a higher-voltage panel (or two smaller panels in series) and skip the car charger.

Wouldn't the battery voltage be the controlling factor here? Probably could go with 2 12v batteries in series? I assume a 20v laptop charger is fine with that.

You really can’t assume much of anything here frankly. Voltages here are nominal, and open circuit voltages can be quite high, or when under load can drop quite a bit. MPPT charger will attempt to maintain and output voltage within it’s target range, but voltage transients and ripple can be a problem - they aren’t designed as a live power supply, they are design to charge huge energy sinks with specific chemistries (aka batteries), so they usually cut corners that would matter in this case but don’t for batteries.

Better: a DC-DC converter that is designed for solar panels.

Reply on news.ycombinator.com