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Allowing that there are all different stipulations about what makes an android, consider the following android type: a synthetic organic that requires nourishment in the form of basic amino acids to keep the fleshy bits alive. Beneath the flesh, a cybernetic/robotic substrate, integrated with the organic parts. I've heard of several different options for power, including nuclear, solar, etc.

The question: Could hydrogen fuel cells be used as power for the cybernetics (assuming an infrastructure exists in a hypothetical world to produce it and the cells are small enough to be stored in the chassis of the android), and what might a typical battery life be for an average humanoid platform with slightly enhanced physical abilities?

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    $\begingroup$ (a) The model is one-specific-question/one-best-answer, you've asked two and neither one is specific at all. (b) Since we do not have androids in our world, the only viable answer to your first question is "yes." I don't know what else you might be expecting (an argument against HFCs?)> And therein lies the fundamental problem here, your question is subjective (read: primarily opinion-based). If you have the tech for A, why not B? It's your world, only you can answer that. So, VTC:POB until you clarify specifically what it is you're asking. $\endgroup$
    – JBH
    Sep 25, 2019 at 21:11
  • $\begingroup$ Got it, thanks. $\endgroup$
    – Brian
    Sep 25, 2019 at 21:30
  • $\begingroup$ Honestly, I'd just whack a nice big chunk of Plutonium in there and you've got yourself an Android with a 50+ year lifespan. $\endgroup$
    – Arkenstein XII
    Sep 26, 2019 at 1:52
  • $\begingroup$ @ArkensteinXII the weight of a RITG is pretty excessive. Its fine for stationary emplacements, or things in space, but it'll take up most of the mass of a human-like robot and it'll need most of the energy it outputs just to move it around. Maybe if your android was just hella lazy? $\endgroup$
    – Starfish Prime
    Sep 26, 2019 at 8:38
  • $\begingroup$ @StarfishPrime RITGs can be constructed quite a bit smaller, and if the cybernetics are designed for energy efficiency, they need not require a huge amount of power. $\endgroup$
    – Arkenstein XII
    Sep 26, 2019 at 19:59

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Inevitable answer: "it depends".

A human uses about a hundred watts of power at rest, and maybe double that for extended periods if they're fairly fit. A world-class cyclist might manage as much as a kilowatt (possibly aided by pharmaceuticals).

The power-to-weight ratio of current fuel cells is quite variable, but by the time you've got actual androids to power I'm sure that the higher-end figures will be commonplace. A kilowatt fuel cell could, therefore, be only a kilo, and will probably fit inside a replica human torso, no problem. Even more pessimistic estimates aren't that bad, with a 10kg cell providing enough oomph.

Fuel cell vehicles are about 50-60% efficient. This figure probably won't rise that much in the future, so lets assume the worst for simplicity. Hydrogen has an energy density of 140MJ/kg, giving you a maximum of 70MJ of useful energy per kilo of fuel, or 19.5kWh. That's enough to run an idle human for over a week, or a world-class athlete running at full power for a bit under a day.

Modern high-pressure hydrogen tanks are filled at 700 bar. At this pressure, and at room temperature (handwave, handwave) hydrogen has a density of about 38kg/m3. That 1kg of hydrogen, then, could fit in a cylinder of 10cm radius and 83cm height. That's quite big! You'll have difficulty fitting that in a person-sized hull and still have space for all the other useful androidy things you'll be wanting.

Take home message: hydrogen fuel cells good, hydrogen fuel bad. I'm not sure what alternatives would be best in your situation, but obviously depend very much on what you want your android to do and for how long. Other types of fuel cell do exist with much more energy-dense fuel, but current efficiencies are low. Naturally, as the author you may handwave your future tech as you see fit. A methanol fuel cell with a 50% efficiency (current real world figures are more like 10%!) would be a good start... the fuel has 20 times the density of 700bar hydrogen and is a lot easier to work with. Even with a usable energy density of only 11MJ/kg, a 6.4kg fuel tank isn't a big load to haul around and fits in 1/3rd the space.

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    $\begingroup$ According to this site from the DoE, the current state of the art for storage of ~20kWh of hydrogen would be on the order of 14.3 kg and 25 L in size. (In old money, a little over 30 pounds of tank, in a cube about a foot on a side.) You might be able to fit that in an android but it'd be rather obtrusive. $\endgroup$
    – Cadence
    Sep 25, 2019 at 22:27
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    $\begingroup$ It's also worth noting that having to haul that around would increase the power consumption of your hypothetical android. $\endgroup$
    – jdunlop
    Sep 25, 2019 at 22:29
  • $\begingroup$ @Cadence, so your android is a little more humanoid by virtue of having a bit of a gut on it. I don't think anyone would really notice! $\endgroup$
    – Separatrix
    Sep 26, 2019 at 8:07
  • $\begingroup$ @Cadence current state of the art robotics does not include androids, so there's wiggle room ;-) $\endgroup$
    – Starfish Prime
    Sep 26, 2019 at 8:25
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    $\begingroup$ @Separatrix I really, really like the idea of a super high-tech super-fast, super-strong, super-intelligent android being a bit on the chubby side. Hell, they'd probably blend in a little better with regular humans if they didn't all look like terminators. $\endgroup$
    – Starfish Prime
    Sep 26, 2019 at 8:31

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