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.

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.