Reducing power consumption


12 posts
by tim124 » Mon Feb 20, 2012 1:42 am
I'm looking to use a R-Pi board to do some logging and data analysis in a remote location, powered by battery. I've seen different quoted power requirements for the two versions (A Vs B). Is this simply the peak power being quoted? And will the B boards use the same power with the Ethernet disbaled? If you're not using the Ethernet, could you get away with a 300ma supply?

I don't know a huge amount about embedded linux, but are there other ways that I'll be able to reduce power consumption like disabling the graphics processor?
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by drgeoff » Mon Feb 20, 2012 11:21 am
The power consumption will depend on what the board is doing.  There will also be some variations between boards because of chip tolerances.  The B model has an extra chip which converts one USB output to two USBs plus ethernet so even with the ethernet disabled the B board will consume more power.  Sorry, can't  give you any precise figures.
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by mobeyduck » Mon Feb 20, 2012 12:32 pm
The worst case power consumption for model B would be 700mah

The worst case power consumption for model A would be 500mah

But this was from one of the early posts so it might be less. Its a good starting point.

You can underclock and undervolt the chip that will make it use a bit less power.

See http://www.raspberrypi.org/for.....config-txt

Disabling the video will cut a little bit, it will all be micromanagement.
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by teh_orph » Mon Feb 20, 2012 12:57 pm
ARM and other vendors often make a point of talking about how much power it costs to access main memory. So, if your application suits it,

- buy the smaller memory model ('A') as less memory will need to be refreshed
- run as few background processes as possible - ideally just your program!
- keep your application's data set (and code size) as tiny as possible
- enable the L2 cache for CPU access
- disable the display
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by tim124 » Mon Feb 20, 2012 1:01 pm
Thanks. This is a good start! I imagine as more people get their hands on hardware and starting hacking, more information will come out.

Will kernel drivers mean I can change some of these settings while the system is running? It would also be great if I could enable and disable the power to individual USB ports at runtime.
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by ledow » Mon Feb 20, 2012 2:51 pm
5v, 700mA in the worst case?

You could run it for about 2 hours, then, from one of these:

http://www.maplin.co.uk/rechar.....es-27088 

Hell, say the max power is vastly overestimated and you could get 20 hours.  Still not very much for a device that's in a remote location without basically sticking it on a car battery.

The battery my company just paid £70 for to run a 1500W server UPS could probably manage only a week once you took voltage conversion and uneven power usage into account.

I don't think it would really be very suitable for that job (unless you have a huge solar panel to keep that thing charged all the time), but obviously more accurate usage numbers would be available once they start shipping.
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by RaTTuS » Mon Feb 20, 2012 3:03 pm
http://www.raspberrypi.org/for.....olar-power < for some helpful solar power info
http://www.catb.org/esr/faqs/smart-questions.html <- ask smart Questions
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by tim124 » Mon Feb 20, 2012 3:06 pm
This will most likely run off of a sealed lead-acid battery with solar charging.

I would be using the Raspberry Pi boards to lower the costs really. I could design a low power ARM board (Cortex M3) but I'd be paying about £6-8 for the IC's in single volumes and about another £100+ for a populated board.

Raspberry Pi A's are going to be about £16 and I can connected a small USB sampling board which can also be tested with Windows PC's and software.

This should reduce the dev time and cost substantially.
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by mobeyduck » Mon Feb 20, 2012 4:50 pm
Looking at those batterys from maplin that were linked I cant stop thinking that a lipo would be better

http://www.seeedstudio.com/dep.....th=178_183 6ah

add a rider and a 3w solar panel, note this panel probably wont be enough to power it all the time. (all same site) will cost $58 against the $63.42 for only a 7ah 12v
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by jamesh » Mon Feb 20, 2012 5:26 pm
The GPU automatically shuts down areas that are not being used - so you don't need to worry about that. All told it uses less than 10mw playing something like an MP3. Its the Arm that will use most of the power.
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by tim124 » Mon Feb 20, 2012 5:38 pm
I've thought of some more power related questions.

Will it support wake-on LAN, wake-on USB or wake on any of the GPIO pin interrupts?

Does the hardware support any ACPI sleep modes?
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by jamesh » Mon Feb 20, 2012 5:55 pm
You need to search the forum - these Q's have already be asked and answered.
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