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Comment by speeq | Hacker News Reader

Power consumption of the Model A+ with 1Wh sounds impressive. Does the Pi also have a sleep mode where it draws even less?

Using two Panasonic NCR18650B 18650 batteries (each ~12Wh) and a 3V to 5V step-up converter it should be able to run for a day!

Add a solar panel + charging circuit and you have a little Linux computer which is able to run continuously on solar power :)

EDIT: 1Wh is with a monitor plugged in and running a GPU-intensive demo, I wonder what the power consumption is like without a GUI but just a WiFi dongle and the Pi-camera sending snapshots periodically.

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> and a 3V to 5V step-up converter

You may want to step it up to 3.3V only for the main power supply and do a little surgery on the Pi to feed it directly to its 3.3V 'rail', foregoing the 5V->3.3V linear regulator. (I'm just saying this from reading the given page, I haven't done this myself, so YMMV.)

http://www.bitwizard.nl/wiki/index.php?title=Reducing_power_...

BTW it's not clear to me whether the Pi or the Beaglebone Black use less power [when idle]. The latter would provide more CPU power, which would make it more interesting for a general purpose solar computer.

PS. I was thinking about the B+ when writing the above (stupid me, should actually read the article before posting). The A+ likely has lower power consumption than the Beaglebone (but also only 256 MB RAM, which is really going to be limiting for many tasks). Also, perhaps they have replaced the 5V->3.3V linear regulator with a step-down regulator which would be more efficient (still, doing only one transformation instead of two would give you a bit lower losses).

Thanks! They seem to have replaced the linear regulator with a switching regulator (which is more efficient) on the A+ & B+ models.

I agree though, doing only one transformation would be more efficient - but I'm not sure which USB devices would work on 3.3V - it depends on the individual use-case I guess.

I now noticed that the NCR18650B batteries are Li-ion batteries (i.e. 2.7-3.7V range for one cell; I was thinking NiMH which are below 1.5V per cell). This may be harder to regulate to 3.3V as you can't simply choose a step-up nor step-down circuit because the source voltage crosses the target voltage during operation; I think I've seen chips that claim to do this but I don't know how efficiently[1]. You may be better off with the original plan of converting to 5V after all.

[1] well, actually an inverting buck-boost converter should be fine and seems like it should be as efficient as a step-up (https://en.wikipedia.org/wiki/Buck-boost_converter). Whatever, my lack of enough experience in that area shows.

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