6 ms·

This is also why chemical batteries that can react in milliseconds are such useful buffer tech, even if they can't store more than a few hours of demand.

kragen 3y ago
chemical batteries don't take nearly that long to react!
- ok_dad 3y ago
  The control systems do though
  - kragen 3y ago
    i doubt that very much indeed. it isn't practical to run a switching power supply with feedback latencies of multiple milliseconds
    - toomuchtodo 3y ago
      Tesla Megapacks detect and respond within ~100ms when operating in grid support mode. Unsure how that compares to what you’d expect. https://www.aurecongroup.com/-/media/files/downloads-library/thought-leadership/aurecon-hornsdale-power-reserve-impact-study-2018.pdf https://www.aurecongroup.com/-/media/files/downloads-library...
      - kragen 3y ago
        they have internal control systems that respond a lot faster than that. 100ms is 6 entire cycles of a 60-hertz sine wave, an utter eternity compared to the control mechanisms required to follow that sine wave up and down with pwm/pdm. the 150ms number in that brochure probably has to do with computer networking software, not the battery pack control system itself but the power conversion control system is relatively slow, taking hundreds of nanoseconds to respond (ten thousand times faster than a few milliseconds). the battery chemistry itself responds enormously faster than that when the current drawn from it changes; the only thing that limits the electrochemical response is the capacitance of the electrical double layer, which in a battery (as opposed to a supercapacitor) is on the order of one nanofarad per ampere. so the tiny changes in the overpotential that turn the electrochemical reactions on or off can take place in well under a single nanosecond so saying 'chemical batteries that can react in milliseconds' is understating the speed by a factor of ten million. that's like saying 'jet planes that can fly meters per day', 'supercomputers that can do dozens of multiplications per minute', or 'skyscrapers that can reach tens of microns in height'
        totallyabstract 3y ago
        I think this is a bit of a pedantic take. My reading of the GP is to distinguish systems based on chemical batteries from systems based on gravitational batteries rather than the reaction time of the chemical cells specifically.
        hn_go_brrrrr 3y ago
        Definitely pedantic, but also quite amusing.
        kragen 3y ago
        you are my kind of person
        dkbrk 3y ago
        > so saying 'chemical batteries that can react in milliseconds' is understating the speed by a factor of ten million. that's like saying 'jet planes that can fly meters per day', 'supercomputers that can do dozens of multiplications per minute', or 'skyscrapers that can reach tens of microns in height' I checked your analogies for jet planes and skyscrapers, and found them to be well within an order of magnitude of ten million, but you're off on your supercomputer analogy by a factor somewhere in the range of a billion. I couldn't find a hard definition of what constitutes a supercomputer. The bottom of the Top 500 list is in the single-digit petaflop range and an RTX 4090 has 70 or 80 single-precision teraflops, and presumably you need more than a handful of graphics cards to constitute a supercomputer, so let's say 1 petaflop is our threshold for a supercomputer. The ratio of a petaflop to 12 flops/minute is 5e15, or 5 million billion. Saying "a factor of ten million is like saying 'supercomputers that can do dozens of multiplications per minute'" is like saying 'the moon orbits meters above the Earth's surface'.
        kragen 3y ago
        thanks for checking my calculations! unlike the other examples, the category of 'supercomputer' has shifted over time. initially, i was thinking of a cray-1, which is the machine that popularized the term 'supercomputer' https://books.google.com/ngrams/graph?content=supercomputer&year_start=1800&year_end=2019&corpus=en-2019&smoothing=3 https://books.google.com/ngrams/graph?content=supercomputer&.... it was only about 160 megaflops, which is ten million times 960 calculations per minute. that wouldn't normally be described as 'dozens'; it's about an order of magnitude high. so i should have done my calculations a bit more carefully up to the late 90s, when the term 'supercomputer' had largely fallen out of use, top-end supers were still in the low-digit gigaflops: https://www.hpcwire.com/1997/04/04/tera-mta-computer-posts-benchmark-for-integer-sorting/ https://www.hpcwire.com/1997/04/04/tera-mta-computer-posts-b... you are of course correct that current machines are many orders of magnitude faster than that, and even a single video card is about three orders of magnitude faster

kragen 3y ago

chemical batteries don't take nearly that long to react!

ok_dad 3y ago
The control systems do though
- kragen 3y ago
  i doubt that very much indeed. it isn't practical to run a switching power supply with feedback latencies of multiple milliseconds
  - toomuchtodo 3y ago
    Tesla Megapacks detect and respond within ~100ms when operating in grid support mode. Unsure how that compares to what you’d expect. https://www.aurecongroup.com/-/media/files/downloads-library/thought-leadership/aurecon-hornsdale-power-reserve-impact-study-2018.pdf https://www.aurecongroup.com/-/media/files/downloads-library...
    - kragen 3y ago
      they have internal control systems that respond a lot faster than that. 100ms is 6 entire cycles of a 60-hertz sine wave, an utter eternity compared to the control mechanisms required to follow that sine wave up and down with pwm/pdm. the 150ms number in that brochure probably has to do with computer networking software, not the battery pack control system itself but the power conversion control system is relatively slow, taking hundreds of nanoseconds to respond (ten thousand times faster than a few milliseconds). the battery chemistry itself responds enormously faster than that when the current drawn from it changes; the only thing that limits the electrochemical response is the capacitance of the electrical double layer, which in a battery (as opposed to a supercapacitor) is on the order of one nanofarad per ampere. so the tiny changes in the overpotential that turn the electrochemical reactions on or off can take place in well under a single nanosecond so saying 'chemical batteries that can react in milliseconds' is understating the speed by a factor of ten million. that's like saying 'jet planes that can fly meters per day', 'supercomputers that can do dozens of multiplications per minute', or 'skyscrapers that can reach tens of microns in height'
      - totallyabstract 3y ago
        I think this is a bit of a pedantic take. My reading of the GP is to distinguish systems based on chemical batteries from systems based on gravitational batteries rather than the reaction time of the chemical cells specifically.
        hn_go_brrrrr 3y ago
        Definitely pedantic, but also quite amusing.
        kragen 3y ago
        you are my kind of person
      - dkbrk 3y ago
        > so saying 'chemical batteries that can react in milliseconds' is understating the speed by a factor of ten million. that's like saying 'jet planes that can fly meters per day', 'supercomputers that can do dozens of multiplications per minute', or 'skyscrapers that can reach tens of microns in height' I checked your analogies for jet planes and skyscrapers, and found them to be well within an order of magnitude of ten million, but you're off on your supercomputer analogy by a factor somewhere in the range of a billion. I couldn't find a hard definition of what constitutes a supercomputer. The bottom of the Top 500 list is in the single-digit petaflop range and an RTX 4090 has 70 or 80 single-precision teraflops, and presumably you need more than a handful of graphics cards to constitute a supercomputer, so let's say 1 petaflop is our threshold for a supercomputer. The ratio of a petaflop to 12 flops/minute is 5e15, or 5 million billion. Saying "a factor of ten million is like saying 'supercomputers that can do dozens of multiplications per minute'" is like saying 'the moon orbits meters above the Earth's surface'.
        kragen 3y ago
        thanks for checking my calculations! unlike the other examples, the category of 'supercomputer' has shifted over time. initially, i was thinking of a cray-1, which is the machine that popularized the term 'supercomputer' https://books.google.com/ngrams/graph?content=supercomputer&year_start=1800&year_end=2019&corpus=en-2019&smoothing=3 https://books.google.com/ngrams/graph?content=supercomputer&.... it was only about 160 megaflops, which is ten million times 960 calculations per minute. that wouldn't normally be described as 'dozens'; it's about an order of magnitude high. so i should have done my calculations a bit more carefully up to the late 90s, when the term 'supercomputer' had largely fallen out of use, top-end supers were still in the low-digit gigaflops: https://www.hpcwire.com/1997/04/04/tera-mta-computer-posts-benchmark-for-integer-sorting/ https://www.hpcwire.com/1997/04/04/tera-mta-computer-posts-b... you are of course correct that current machines are many orders of magnitude faster than that, and even a single video card is about three orders of magnitude faster