21 ms·

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.

tonymet 5y ago
the efficiency savings with this approach will outweigh any other effort
- WJW 5y ago
  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.
  - p1mrx 5y ago
    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.
na85 5y ago
Don't DC-DC transformers use AC internally?
- dreamcompiler 5y ago
  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.
- jagger27 5y ago
  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.
  - colejohnson66 5y ago
    Which isn’t that hard for a switch mode converter. Those are usually 85% or more IIRC.
- reportingsjr 5y ago
  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.
- 0xfaded 5y ago
  Not necessarily. See voltage doubler: https://en.m.wikipedia.org/wiki/Voltage_doubler https://en.m.wikipedia.org/wiki/Voltage_doubler
- jmrm 5y ago
  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 https://en.wikipedia.org/wiki/Boost_converter
  - nomel 5y ago
    Which use alternating voltage, across the inductor, but the current is always (with load) positive. Neat!
dreamcompiler 5y ago
Or use a 20v MPPT with a higher-voltage panel (or two smaller panels in series) and skip the car charger.
- sfblah 5y ago
  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.
  - lazide 5y ago
    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.
- deleted 5y ago
  [deleted]
coding123 5y ago
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..
- nomel 5y ago
  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.
- stephen_g 5y ago
  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.
ananonymoususer 5y ago
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 https://www.cartft.com/catalog/il/2302
- consumer451 5y ago
  Wow, I was not expecting that price! Is this just a rare item or is there something particularity expensive inside it?
  - fragmede 5y ago
    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.
  - tyrells 5y ago
    Definitely are cheaper options too: https://www.mini-box.com/picoPSU-160-XT https://www.mini-box.com/picoPSU-160-XT
  - brudgers 5y ago
    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.
    - goodpoint 5y ago
      Not at all. At that absurd price it's cheaper to buy more solar panels and ignore the inefficiency.
      - brudgers 5y ago
        cheaper Price sensitive shoppers are not in the product's market segment.
jmrm 5y ago
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.
londons_explore 5y ago
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.
- kempbellt 5y ago
  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.
bee_rider 5y ago
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.
- genewitch 5y ago
  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.
  - Maakuth 5y ago
    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 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.
  - bee_rider 5y ago
    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.
    - lazide 5y ago
      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.
goodpoint 5y ago
Better: a DC-DC converter that is designed for solar panels.

tonymet 5y ago

the efficiency savings with this approach will outweigh any other effort

WJW 5y ago
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.
- p1mrx 5y ago
  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.

na85 5y ago

Don't DC-DC transformers use AC internally?

dreamcompiler 5y ago
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.
jagger27 5y ago
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.
- colejohnson66 5y ago
  Which isn’t that hard for a switch mode converter. Those are usually 85% or more IIRC.
reportingsjr 5y ago
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.
0xfaded 5y ago
Not necessarily. See voltage doubler: https://en.m.wikipedia.org/wiki/Voltage_doubler https://en.m.wikipedia.org/wiki/Voltage_doubler
jmrm 5y ago
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 https://en.wikipedia.org/wiki/Boost_converter
- nomel 5y ago
  Which use alternating voltage, across the inductor, but the current is always (with load) positive. Neat!

dreamcompiler 5y ago

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

sfblah 5y ago
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.
- lazide 5y ago
  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.
deleted 5y ago
[deleted]

coding123 5y ago

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..

nomel 5y ago
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.
stephen_g 5y ago
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.

ananonymoususer 5y ago

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

consumer451 5y ago
Wow, I was not expecting that price! Is this just a rare item or is there something particularity expensive inside it?
- fragmede 5y ago
  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.
- tyrells 5y ago
  Definitely are cheaper options too: https://www.mini-box.com/picoPSU-160-XT https://www.mini-box.com/picoPSU-160-XT
- brudgers 5y ago
  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.
  - goodpoint 5y ago
    Not at all. At that absurd price it's cheaper to buy more solar panels and ignore the inefficiency.
    - brudgers 5y ago
      cheaper Price sensitive shoppers are not in the product's market segment.

jmrm 5y ago

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.

londons_explore 5y ago

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.

kempbellt 5y ago
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.

bee_rider 5y ago

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.

genewitch 5y ago
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.
- Maakuth 5y ago
  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 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.
- bee_rider 5y ago
  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.
  - lazide 5y ago
    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.

goodpoint 5y ago

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