Power-supply

[sinatra39]

Hello,
I´ve decided to buy a Raspberry Pi B+ and I want to use it mobile, so for this purpose I want to use a 5 volts, 2.5 amps, 5000 mAh battery pack (http://www.amazon.de/EasyAcc-Schnellade ... v+usb+akku)
Would this work? (eventuall, since it delivers 2.5 amps also with a webcam together => I want to use a USB hub, plug the battery into 1 port and the webcam + the Pi into others)
Thanks for your suggestions!

[W. H. Heydt]

The output (5v) is correct and the 2.5A is certainly enough for the Pi.

I'm a little unclear on why you would plug the batter into one of the hubs ports. Normally there is a power connection point for a hub. However, since a B+ has 4 USB port and can support up to 1.2A output on those ports, which ought to be considerably more than a web cam needs to operate, why not dispense with the hub, connect the battery to the Pi and the web cam to a USB port on the PI?

Bear in mind that you can't boot a B+ by back-powering it.

[sinatra39]

But I can boot it, if I plug the battery in at the micro USB port, right? and if yes what's the reason, that it does not work via USB B, is there a diode or something in it, which only allows current out? Thanks : )/

[sinatra39]

I eventually found an even better solution, which has a capacity of 15 ah, delivers 3.1 amp @ 5 volts and also works via micro-USB. Would this work with 3.1 amps over micro-USB (does the Pi withstand the current?) and since 2 amps is actually recommended, do you think it would even be possible to attach an Arduino-Uno to the Pi without an auxiliary external power supply? (http://www.amazon.de/POWER-BANK-Powerba ... v+usb+akku) Thanks very much!

[davidcoton]

Would this work with 3.1 amps over micro-USB (does the Pi withstand the current?) and since 2 amps is actually recommended, do you think it would even be possible to attach an Arduino-Uno to the Pi without an auxiliary external power supply?

See the (long) discussion http://www.raspberrypi.org/forums/viewt ... 63&t=96187 in part about the current rating for the micro USB. The connector spec seems to be 1.8A, so 2A is probably safe, 3A will probably work but maybe not in the long term. Basically I would not try to exceed a total design load of 2A through the microUSB. There are other options for powering the Pi, Google search this site is your friend. You need to consider the load current of each part of the system, and what fuse protection you want for the Pi and other hardware. "Its working, it doesn't need a fuse" is a very poor and somewhat dangerous argument.

[klricks]

Would this work with 3.1 amps over micro-USB (does the Pi withstand the current?) and since 2 amps is actually recommended, do you think it would even be possible to attach an Arduino-Uno to the Pi without an auxiliary external power supply?

See the (long) discussion http://www.raspberrypi.org/forums/viewt ... 63&t=96187 in part about the current rating for the micro USB. The connector spec seems to be 1.8A, so 2A is probably safe, 3A will probably work but maybe not in the long term. Basically I would not try to exceed a total design load of 2A through the microUSB. There are other options for powering the Pi, Google search this site is your friend. You need to consider the load current of each part of the system, and what fuse protection you want for the Pi and other hardware. "Its working, it doesn't need a fuse" is a very poor and somewhat dangerous argument.

If you attempt to draw more than 2A the 'polyfuse' in the B+ will begin to activate and the RPi will become unstable.

[sinatra39]

So the Pi works needs at least 0.8 amps and it can draw safely <= 2 amps?
Another question => at which voltage below 5 volts dies the Pi turn off? Thanks!

[klricks]

So the Pi works needs at least 0.8 amps and it can draw safely <= 2 amps?
Another question => at which voltage below 5 volts dies the Pi turn off? Thanks!

The 0.8A would be the RPi itself + a typical wired keyboard and mouse and maybe a WIFI adapter.

The specified operating range is 5.00V +- 5% or 4.75V to 5.25V
At 4.65V and below the Red Power light (B+) will turn off as a warning.
The RPi has no logic (software or hardware) to turn itself off at a certain point. It just becomes unstable.. especially USB devices, crashes, reboots, corrupt SD card etc......... The actual voltage at which that happens would depend on the power supply and what other devices are connected to the RPi.

[W. H. Heydt]

So the Pi works needs at least 0.8 amps and it can draw safely <= 2 amps?
Another question => at which voltage below 5 volts dies the Pi turn off? Thanks!

The Pi doesn't actually need 0.8A. A B+ at idle draws around 0.4A (IIRC...might be less). The reason for higher current supply is to provide power to auxiliary devices...camera, display (some day..), HDMI, and--the big one--USB. Regardless of the current that can be supplied by source, the Pi will never draw more than 2A because the polyfuse will trip.

As for voltage... When you get below 4.75v, USB devices will begin to fail to work. The Pi itself can operate down to around 3v (again, IIRC), but things attached to it won't. The processor actually runs on 1.8v.

So the question "What are the power requirements for a Raspberry Pi?" doesn't have an exact answer. It depends on what you're doing with it, what is connected to it, and what you expect of it. The "safe" answer for a Model B+ or A+ is "5v+/-5% and at least 2A". If you can supply that, your Pi will do anything that is within its specifications.

Some minor data points... I have a power supply from MCM that is marked as 5v 2200mA. That's pretty much perfect for a B+ or A+. Last weekend at a Jam, I had a Cubieboard1 running from a NewIT 2A supply with the Adafruit power monitor hooked up. The Cubieboard with an attached SSD was drawing just a bit over 0.4A--it mostly ran 0.41 to 0.42A, and the voltage was reading 5.07 to 5.08v. Again, that's as near to perfect as you'd want for a B+ or A+. I was also running a B+, with keyboard, mouse and WiFi dongle attached using an Adafruit 1A, 5,25v supply. The B+ ran flawlessly.