>Cell phone batteries seem like a
>possible fit, but it seems difficult to connect to them, and I don't
>have documentation on the ones I've seen with 4 terminals (I'm assuming
>that extra terminals are connections to thermistors for temp
>monitoring).  Any recommendations?

I have recently had a similar issue with my laptop battery, no info on the 
connections and no markings. Basically I wanted to hook the battery directly 
to a solar panel for charging when in the middle of nowhere. I eventually 
found out that most laptop batteries have six terminals, outer are usually 
+/- , two are the SMBus (by the way, microchip have an app note and code for 
interfacing with SMBus using a PIC16F) and the other two are /enable and 
thermistor connection. I think it's a good bet that with the four terminal 
phone batteries, the outer two will be +/- and the other two will be /enable 
and thermistor (to figure out which was which, I just used a multimeter on 
the outside two, and used a >500 ohm resistor from each outer terminal 
touching each inner terminal until it registered a voltage)
 There's info on Lithium Ion charging floating about on the web, and as long 
as you take the correct precautions, I think they are hard to beat for 
energy-to-weight-ratio/low-self-discharge/no-memory-effect etc. The new NiMH 
low self discharge look good, but the lower discharge comes at the price of 
less capacity (I think the larger seperator area that stops discharge also 
lowers capacity) and according to wiki, the highest that are available are 
2500mAh.
 Here's a site with a few Li-Ions and others that might interest you:
http://www.batteryspace.com/3.7V-Modules-3200-9000-mAh.aspx 


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