What kind of battery is this thing going to have, and how do you plan on having the device charge itself? Will it have replaceable batteries or any kind of standard power connector to allow for external power (such as a wall outlet plug)? And, of course, as a hacker, I have to ask; will it be easy to split open and wire a new, more powerful battery to it as battery tech improves?
I ask because I already have many motorized projects in mind, and as we all know, motors use a great deal more power than processors. (My first reaction to the device as a whole was "Wow, it's the 21st Century Furby!, and I almost instantly conjured up ideas of it moving and shaking and possibly walking/driving around and my cats attacking it).
Thanks and Good luck!
FURTHER EDIT: The circuit below should not due to an oversight that causes it not to function properly with two cells. Please use it only as a reference for your own designs.
EDIT: Simple Lithium Ion Battery Charger circuit for Chumby follows
Changes for Chumby Daughtercard
Cells should be in parallel to be charged with this charging circuit, and as such only provides 3.7V, not enough to drive a(n) (Alpha) Chumby which requires about 6.5.
BILL OF MATERIALS for changes
1 ADP2291 Analog Devices Lithium Ion Charger IC.
1 BCPS1 Philips PNP Transistor (Max P. Dissipation @ 25dC = 1.3W, Beta @ 1A = 50, Vce(saturation) = 0.5v). Replace with any transistor that fits criterion for circuit ( ~0.7V Vce, .7W dissipation, 25 beta). See Analog Devices ADP2291 datasheet for details.
1 Small LED (blue for ChumbyRice )
1 100k resistor (optional, paired with thermosistor)
1 100M resistor
1 (330) resistor (any resistor suitable for controlling voltage for LED to prevent burnout)
1 10uF capacitor
1 100uF capacitor
1 470k thermosistor (automatic shutdown in case of battery overheating, optional)
2 LG 18650 2400mAh, 3.7V lithium ion cells, connected in serial, tabs on.
--Implementation #2 also includes
1 Schottky Diode (use an extra A240, 40V 2A diode like that of Chumby Mainboard)
Total cost: (pennies for resistors & caps + a few more pennies for the thermosistor + $1.70 w/ ship for one ADP2291 + maybe a buck for 10 of the transistors + ricer LED [you should have this laying around]) + 2 LG batteries @ $6.50/cell = approx. $15.
When the external power source is connected, the ADP2291 pulls some of the filtered (!!!!!!!!) voltage and uses it to charge the lithium ion cells. The chip is timed by the 100uF capacitor which automatically shuts the charger off when the battery's full, or after 3 hours, whichever occurs first. The device powers back on when the battery voltage drops by 150mV (and trickle charges to full, where it cuts off after 30 minutes). Like said above, the thermosistor protects the batteries from overcharging by cutting off the charger if the circuit gets too hot from the adjacent batteries (it reduces the power input to the batteries until a certain temp when the chip itself cuts off). The LED connected to the charge circuit tells us when the battery is charging.
If using a different battery (I actually changed planned batteries because of this), you MUST access the charge information of the battery. The circuit is designed around using these batteries and using different batteries (of greater, lower, or possibly equal power) will change the circuit. The ADP2291 datasheet has all of the information about choosing the right resistor (here played by the 100M ohm resistor) and the right transistor (here played by the PNP BCPS1) to match the batteries. It's also important to note that the transistor might get hot, so use wider traces on the PCB to soak up some of the heat.
Now, if only I had a Chumby to test it on... (actually, before anyone tests it out on a Chumby, it'd probably be smart to replicate the whole power circuit and try it on that alone to make sure you don't fry anything of real value. The ADP2291 is only a buck a chip (which is a lot to ask to be added to Chumby, but pretty cheap for a hobbyist) and if you're super kind the Analog Devices reps might even send you a free one or two. They're also teensy tiny buggers (3mm x 3mm) so use a pair of tweezers while soldering if you've got a free hand. Barring I did everything with my math right, the circuit should work fine. The biggest thing I can imagine that would go wrong would be the transistor burning out, but if that happens I'll let you know and I'll step up to a beefier switch. To the Chumbyista: this would be a nice addition that a lot of hobbyists might want, you might want to consider making a bit of room on the PCB for it if at all possible, it's only a few mm^2 :-D).
Future modifications include: Either adding a second A/D or removing the squeeze-o-meter/ambient light sensor and adding a battery voltage analog in for a battery-life meter. Using a digital input to indicate whether the Chumby is in battery mode or is plugged in (via the STAT pin on the LTC4412). (possibly) NiCad/NiMH charger for R/C car hobbyists.
Second implementation is the quick and dirty move to the daughtercard as expressed by bunnie. Filter could be made better by adding a fuse to protect charging circuit and a R-C filter to clip out the remaining noise in the line. I'll be testing this circuit individually as soon as I recieve all of the components I need to do so (most are ordered, though it looks as if I'll have to pick up those diodes...).
Power-wise, in an ideal setting, 2400mA with a steady 200mA drain would last 12 hours, but reality is that the device would likely cut out at about 8 at best (peak power usage at 300mA, and capacity losses due to heat from all of the devices). It's worthy of noting the battery pack provides almost 4 times the amount of power as compared to the iPod 3/4G battery, but roughly 1.5x that of the 1st gen's Lithium Polymer battery. That being said, Chumby with a LiON pack will probably be hefty.