ExternalBatteryPack
Adding an external battery pack
FYI I found a "mobile power cord" made by Belkin (about US$8 at Walmart) which allows charging the battery from a 12v source. You could use it with a 12v gell cell...
Also, I stumbled on to a graph display on the N770 showing the battery's charge level vs time. But can't remember how I got there nor have I been able to bring it up again. Anyone know anything about that ?
This is really just a placeholder to encourage research and commentary.
The stock Nokia power supply on my German 770, an AC-4E, puts out 5v at 890mA - older Nokia supplies I've looked at, with the bigger plug, put out 5.7v.
NiMh rechargeable AA batteries are often in the 1800 to 2400mAh range, at 1.2v. I'm wiring up a pack with four of them, giving you 4.8v. The stock Nokia battery is rated at 1500mAh at 3.7v. I'm hoping to double the battery life of the 770 - power conversion will of course reduce efficiency.
So far, I haven't found anybody selling just the new small Nokia power plug on its own, so I'll probably have to sacrifice a cheap charger before I can hook my AA's up and test this all out.
Single USB ports put out a nominal 5v. Low power ports can supply 100mA, high power can put out 500mA. You're not supposed to draw 500mA without communicating over USB and requesting this from the host, something that USB power supply cables for gadgets do not ever do in my experience. USB ports on your computer are supposed to handle these problems and shut down without damage if you draw too much. USB plugs can lower the voltage, so by the time the power hits your first actual circuit board, it could have dropped half a volt.
What's against buying a second battery ? It works, it costs about 10€ (new ones at 65€, but replica and used one's about that price). Ok doesn't work for usb-host-mode-+voltage, but does the one described above ?
I don't trust the cheapest third party ones. Also, many of the cheaper ones are 750 to 1000mAh, which is rather low. Finally, you need to turn it off; I'm planning to use my 770 while wandering around a city with wifi on, so swapping batteries would annoy me. - GopiFlaherty
How about contacting the people at e.g. Batterygeek (http://www.batterygeek.net/)? They seem to have the necessary expertise in the field and maybe they already have a compatible model.
After checking on their site, it appears that their external battery for the Sony PSP seems the most promising: output 5V at max 2A and 6000 mAh. It probably only needs a Nokia-style tip (which might already be available: see checkout). The price is little more than a genuine Nokia internal battery.
Those are pretty interesting, but fairly expensive. They're Lithium Ion, so they've got more power per gram - I weighed four NiMh AAs, and they were ~120g, giving you 1800 mAh at 4.8v. That makes the 5v @ 6000mAh output sound pretty good. If it's regulated, then it could also feed the USB system. The battery pack is about as thick as the 770, and as tall, but a bit less than half as wide - 58.5mm, compared to the 770's 141mm width. Of course, my solution costs €5 instead of $70. :) - GopiFlaherty