gatto
Posts: 17
Joined: Fri Nov 11, 2011 7:17 pm

Re: Some power related questions

Fri Nov 11, 2011 7:25 pm

Basically, I have no idea about voltages and all that, so I have a few questions:

1. What would be an example of something that consumes the same amount of power as a Raspberry Pi? Specifically, model B.

2. In the perfect world where it was properly implemented into a netbook / laptop setup, roughly how many hours would you get on a 6 cell battery (general usage)?

3. Does the CPU have frequency scaling?

Thanks. :)

User avatar
abishur
Posts: 4477
Joined: Thu Jul 28, 2011 4:10 am
Location: USA
Contact: Website

Re: Some power related questions

Fri Nov 11, 2011 9:54 pm

1) A cellphone being charged

2) Depends on the mAh of the device

3) Not 100% sure about the CPU, but I do know that the GPU (which is the real power hog in this device) has very aggressive automatic scaling. If any part of it isn\'t actively being used it shuts that part down. :)
Dear forum: Play nice ;-)

gatto
Posts: 17
Joined: Fri Nov 11, 2011 7:17 pm

Re: Some power related questions

Fri Nov 11, 2011 10:14 pm

[quote]Quote from abishur on November 11, 2011, 21:54
1) A cellphone being charged

2) Depends on the mAh of the device

3) Not 100% sure about the CPU, but I do know that the GPU (which is the real power hog in this device) has very aggressive automatic scaling. If any part of it isn\'t actively being used it shuts that part down. :)[/quote]Cheers.

What about 4400 mAh? Are we talking over 12 hours, or am I completely off the mark? ;)

trogper
Posts: 10
Joined: Sun Oct 09, 2011 11:37 am

Re: Some power related questions

Fri Nov 11, 2011 11:42 pm

1,5w (max 2w, but not always)
http://www.powerstream.com/battery-capa ... ations.htm
you will need some dc/dc
http://goo.gl/nR17k
batteries in NBs are 10,8v Rpi accepts 5v

rpi takes:
2W:5V=0,4A

battery gives
1hour at 4,4A
so at 0,4A it is 11hours

you were so close
(please correct me if its bad/wrong)

NRoach44
Posts: 13
Joined: Thu Nov 03, 2011 9:13 am
Contact: Website

Re: Some power related questions

Sun Nov 13, 2011 1:07 pm

[quote]battery gives
1hour at 4,4A
so at 0,4A it is 11hours
[/quote]

Thats what I used to get on my netbook (Albeit with screen, wifi, bt off and it playing music) with it\'s 6 cell battery!

Dagger
Posts: 6
Joined: Sun Nov 13, 2011 1:26 pm

Re: Some power related questions

Sun Nov 13, 2011 3:25 pm

[quote]Quote from trogper on November 11, 2011, 23:42
battery gives
1hour at 4,4A
so at 0,4A it is 11hours

(please correct me if its bad/wrong)[/quote]

You\'re missing a factor of (10.8/5) from the voltage conversion.

I find the calculation easier to do by converting into Watt-hours: the 10.8 V, 4.4 Ah netbook battery provides 10.8 V * 4.4 Ah = 47.52 Watt-hours. The Raspberry Pi model A was reported to need 300 mA at 5 V, which is 0.3 A * 5 V = 1.5 Watts. So the total runtime would be 47.52 Wh / 1.5 W = 31.68 hours.

The model B was reported to need 700 mA; I\'ll leave the calculation for that as an exercise for the reader. Remember that these are peak currents; mean power consumption will probably be quite a bit lower.

trogper
Posts: 10
Joined: Sun Oct 09, 2011 11:37 am

Re: Some power related questions

Sun Nov 13, 2011 4:30 pm

and also screen, keyboard, mouse, wifi
also you can find on the internet PoE modules

wallabybob
Posts: 17
Joined: Thu Aug 18, 2011 2:08 am

Re: Some power related questions

Mon Nov 14, 2011 2:42 am

[quote]Quote from Dagger on November 13, 2011, 15:25
I find the calculation easier to do by converting into Watt-hours: the 10.8 V, 4.4 Ah netbook battery provides 10.8 V * 4.4 Ah = 47.52 Watt-hours. The Raspberry Pi model A was reported to need 300 mA at 5 V, which is 0.3 A * 5 V = 1.5 Watts. So the total runtime would be 47.52 Wh / 1.5 W = 31.68 hours.
[/quote]
As already pointed out this calculation is missing power consumed by peripherals. It is also missing power consumed in converting the battery voltage to the nominal 5V supply voltage required by the RaspberryPi. A simple linear regulator will consume more power than the RaspberryPi. A good switchmode regulator will provide over 90% of the energy from the battery to the RaspberryPi at 5V.

Return to “General discussion”