A friend posed this question a while back and despite searching the internet and our collective Star Wars books, we were unable to come up with an answer. I even made this reddit post, but with no real luck. I understand that droids are machines and that they need to "rest" every 100 hours or so to recharge themselves, but what are they recharging? Do they use some kind of batteries? Do they create their own energy somehow? In Episode IV, we see Threepio "switch off", presumably to store power, and in Episode V, Luke plugs Artoo into some sort of power inverter to power his camp. Does anyone have any documentation on what exactly keeps a droid running?

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    I saw mentions of power cells somewhere. But don't recall the source. – DVK-on-Ahch-To Mar 13 '13 at 18:54
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    Magic. Star Wars isn't hard enough science fiction to have anything close to believable technology backing it. Besides the mumbo-jumbo surrounding the Force, we never see spacecraft refueled in the movies, nor do we see anyone put a battery or fuel cell into any device. Meanwhile, a lightsaber would need to have a megawatt power source to do what it does, and Yoda can levitate a multi-ton spacecraft with no energy source other than his mind. Magic. – Kyle Jones Mar 13 '13 at 21:32
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    @KyleJones, you are just naming an excuse to not think. – JMD Mar 13 '13 at 21:47
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    Beer. On no wait, that's Futurama.. ;) – Andrew Thompson Mar 14 '13 at 1:43
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    @KyleJones - We actually do see X-Wings and Y-Wings being refueled as they are getting ready to launch the attack on the Death Star in ANH. And the Science channel ran a series called Sci Fi Science where they talked about how several science fiction technologies, including Star Wars lightsabers, could be made in reality using current or relatively near future tech. science.discovery.com/tv-shows/sci-fi-science/videos/… – BBlake Mar 14 '13 at 13:22

According to this in-universe schematic, R2-D2 is running Kerdon Aerosystems (or KevDan Aerosystems, depending on the source) hydro-glycolic fuel cells.

R2-D2 Schematic
(Click for large version)

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    This is great but I see a small, potential flaw. Wookieepedia describes the fuel cells as some kind of mix of glycol and water. Ethylene Glycol is something that is commonly used today as coolant or anti-freeze in automotives, and the location of the fuel cells in the image lead me to believe that those are the fuel for the jets that Artoo uses in the prequel trilogy, and not what powers him overall. This is a great find, but it might be a little misleading. Thanks for doing the research! – Rathenix Mar 13 '13 at 21:44
  • Although, 25 in that image is listed as "Powerbus Cables" that go from the fuel to 48: the drive motor. So the fuel might also be for propulsion. I also see that 3 and 46 are how he interfaces with a ship, so maybe there is some kind of battery that is charging like that. – Rathenix Mar 13 '13 at 21:55
  • Now that you mention it, it's probably more likely that the fuel cell is a propellant of some kind, rather than the energy core. Especially since the starwars.com page is from 2006 and Attack of the Clones came out in 2002, which is after we found out R2 could fly unassisted. – Force Flow Mar 14 '13 at 2:39
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    In flipping through a few of my pre-prequel star wars "cross-section" and "technical journal" books, it looks like for ships all the way down to datapads and lightsabers there are "power cells", "power packs", "power generators","ion fission reactors", "fuel cells" and "radioactive gas" for fuel in some cases. – Force Flow Mar 14 '13 at 2:49
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    Good point. I think we can safely say that droids must be using a power pack of some kind. It seems like everything in Star Wars does. It just strikes me as odd that no source has ever just put it down as canon that, for example, a standard R2-series astromech droid is powered by a power cell located in its chassis or something. – Rathenix Mar 14 '13 at 13:52

I would say, apart from the lack of canonicity, that there are multiple answers due to multiple types of power supply used:

  • Some were externally powered.
  • A microfusion pile powered at least one (Vuffi Raa, although its origin was extra-galactic so unlikely to apply to 'local' droids.
  • This is a somewhat speculative extrapolation, but I'd hazard a guess that a Micrel Power Supply would work, given that they're small & work for prostheses.
  • I checked Wookieepedia for broad references to droids as a whole, but since there aren't any, these are some great finds. The Iziz generators sound to me like they were used for powering structures or buildings rather than droids, but the aarticle is small, so it's hard to say. I would call Vuffi Raa's power source the perfect example of what I was looking for if it weren't for its origin, and I'm not sure about the Micrel Power Supply because of the lack of information on it, but it's the closest thing to an explanation that I've seen in all my searching so far. Thanks. – Rathenix Mar 13 '13 at 20:24
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    The Iziz generator wasn't specifically for droids, true; I just saw it as an example of droids running on power from outside themselves ("During the Clone Wars, King Sanjay Rash utilized the power generator to provide energy for his battle droid forces in the city."), suggesting such a power architecture being more broadly applicable. – Ghillie Dhu Mar 13 '13 at 20:47

From an electrical engineers point of view, here are some additional info regarding @Force Flow's answer:

A fuel cell is a real thing, similar to batteries. Ethylene glycol fuel cells exist today and can apparently be quite efficient.

"Bus" means data signals, needed to control something. So "Powerbus" might be a kind of current loop: meaning signals that are both capable of transferring information, while at the same time delivering supply current. There are two wires, so one could be for the current loop and the other for ground. The wires are quite thick, which would allow for higher currents. If it was a pure data bus, they wouldn't need to be thick. Possible these could be some multi-signal machine cables as well, in which case there would be several isolated wires in one.

Perhaps the purpose of this being a "bus", is for R2-D2 to monitor his own power supply and see how much time he's got before needing to recharge. Or perhaps this is how he controls the speed of the tracks.

As for why the wires are mounted externally, it doesn't make much sense. They seem to be attached through connectors or maybe cable glands, for some reason the cells should be detachable. Perhaps a human operator is expected to replace them manually. Or perhaps it is just some manner of optional backup power source.

It would be an awkward place to mount fuel for the propulsion rockets, particularly if flammable. If so, they would be named fuel tanks and not cells. And as shown above, ethylene glycol fuel cells are power sources existing in the real world, today. So I would dismiss the rocket fuel idea.

Rather, it would seem that the wires are mounted close to the track engine on purpose, likely to reduce electromechanical interference. Electrical engines are "electrically noisy" and most of the current consumed by R2-D2 might be needed for driving the track. By placing the energy source close to the engine, more sensitive electronic systems wouldn't have to suffer from transient voltage spikes and ground currents originating from driving the engines. So this would be good electronics design practice.

Perhaps the fuel cells only drive the tracks, and the rest of the droid is powered by an internal battery. If so, maybe the fuel cells are detached when the droid is placed in a spaceship, where it has no use of the tracks. Maybe the droid is then powered from the spaceship reactor, without the need of using its own fuel cells.

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