Battery-powered Pi?

To some extent the answer depends .on your level of electronics and soldering experience. Note you can buy as a module what I'm about to describe from specialist development board type outlets such as Sparkfun, Adafruit, Mikroelectronica, Meccanique etc. E-14 doesn't as yet sell exactly this type of thing but will have the individual components.

If you want simple and as few components as possible then look out for a low dropout voltage regulator (which E-14 and other component sellers will have) in addtion at minimum all you will need is a small output capacitor. A standard 7805 regulator is not fit for your purpose due to the facts the input voltage must be at least 2v higher than the output and it's current requirements are relatively high, something like a LP2957AIS/NOPB.LP2957AIS/NOPB.. LM2940, L119, there are others from Micrel for example.

However to get the most from your battery in terms of efficiency you need to look at a dc to dc switchmode regulator these will require more components but your battery charge will last longer and there are also swm chips that can double as a battery charger chip. It all depends on what you want exactly.

Either solution will work.

I haven't checked the exact specifications of the series regulators I referenced in regards of current output capability as they are just off the top of my head.

While this wouldn't work for your existing 6V battery, there are many high capacity and relatively inexpensive battery powered USB chargers from companies like Duracell and New Trent.  I've been pleased with the Duracell Instant Charger and Duracell Powerhouse Charger for battery-operated Arduino projects.  The eLinux wiki reports 12 hours of Pi operation on a 7000mAh New Trent charger: http://elinux.org/RPi_VerifiedPeripherals#External_Batteries. 

Over on http://www.raspberrymod.com/viewtopic.php?f=60&t=640&sid=b006909c691d78e07788ff03dbf8526c I wrote a little piece about Lithium-Polymer batteries and the rising trend to integrate Li-Po chargers on microcontroller boards.

This doesn't help at all with Malcolm's question of course, but I'd like to ask a supplementary question in case someone here knows the answer.  Is there a 2-cell Li-Po charger module (2 x 3.7V = 7.4V) available from anywhere, suitable for integration into 5V-powered equipment?

Interesting idea of putting them in series.  I had been wondering how the Duracell Li-ion battery-powered USB chargers I have work.  I suppose it could either have cells in series and then regulate down to 5V or have a boost converter which brings a single cell up to 5V.  I wonder which method, in general, is more efficient.

Alternatively we could ply Pete with crates of beer until he submits and makes future Pi board operable standalone from 3.7V.

Sounds good   I can't remember exactly what post but I recall from RPi forum either Pete or Gert saying that the Pi not designed with battery operation in mind at all (understandably given their goal).  The impression I had is the way it handles power is not super efficient, but it doesn't matter as much when plugged into wall and not trying to squeeze out battery life.

But yeah, I just started using 3.3V 8MHz Arduino Pro from SparkFun and I'm digging it for LiPo operation.  The main embedded project I have in mind for the Pi would unfortunately be battery powered, so I'll be interested in figuring out an efficient solution once Pi's are available in quantities.

What are the RPi current requirements? How much current can it draw at 100%CPU?

The numbers I've seen from someone actually measuring current draw was 550 mA, but the official spec is 700 mA. I think this may not include power drawn by USB devices connected to the Pi's two USB ports.  The Pi has three linear regulators on board (check RG1, RG2, RG3 on the schematic), so you can guess it wasn't designed for the highest efficiency. The CPU core voltage is generated by a switcher integrated on the SoC, though.

By the way there are significant differences between switching power supply modules, if you care about efficiency. I tested two small ones shown below (much smaller than what you need to power a R-Pi). They aren't directly comparable but I found the efficiency of the Pololu one to be higher at *every* output current than the Sparkfun design. And the output noise was also lower. (and the Pololu one was actually cheaper!)

https://picasaweb.google.com/lh/photo/PzArmv5OEmDtBuyC2cBFQ9MTjNZETYmyPJy0liipFm0?feat=directlink

@ Morgaine,

Li-Po batteries are fussy things and you might suffer from tolerance stack up or balance problems if you charge in series (although lot's of battery packs must do this).

Why not use a boost converter to generate the 5V you need from the battery's 4V or so?

Boost converters are simple, small, cheap and quite efficient.

If you can give me a  spec I'll suggest something specific.

Michael Kellett