[Epic 4G General] dwallersv:
Well, this is something I know a thing or two about from my hobby with RC helicopters...

First, Lithium Polymer chemistry is, for purposes of charge density per unit volume, the same as traditional LiIon technology used in our phones. LiPo's are important where form-factor is critical (they can be "shaped" in just about any way needed), or where charge density per unit _weight_ is a critical consideration (like a model aircraft). They're much lighter than traditional LiIon chemistry.

We could definitely benefit from a large, custom shaped LiPo that covered most of the size of the phone. Problem is, it would be a custom, one-off job that would only be usable on the Epic, economies of scale would not apply to manufacturing, so it would be prohibitively expensive -- probably adding $100 thereabouts to the price of the phone. To gain about 75% more capacity. I'm guessing the marketing dept knows with pretty good confidence that this is not a winning proposition.

Then the are the other issues with LiPo because of their more sensitive chemistry that make charging and discharging much more challenging... man, you guys should see the setup, and safety precautions, we heli guys have to employ to use these things. No big deal to a hobbiest... totally unacceptable to a cell phone user.

LiPo simply is not a reasonable technology for cellphones. I agree Apple is taking a big gamble, perhaps mitigated to some degree in gaining control of the situation by not having the battery user-replaceable.

All that said, there are some bleeding-edge technologies that will yield higher energy densities in the near future. A4 is out there now, and provides a modest improvement in density. A4 could cram about 1800maH into the same size cell as our current battery. The reason we don't see theses batteries in use right now in cellphones if simply cost -- it's not worth about double the price for a mere 20% more capacity to most people.

Far more promising is a technology invented by a Stanford professor and currently in the research phase. It is a variation on LiIon yielding TEN TIMES the energy density of current LiIon cells, employing silicon nanowires as part of the chemistry as explained in this article:

http://news.stanford.edu/news/2008/january9/nanowire-010908.html

Since then, progress has been made in improving charge cycle life -- the biggest hurdle to practical commercial application, by switching to a silicon-carbon nanowire design:

http://thefutureofthings.com/news/9473/improving-lithium-ion-battery-life.html

This reduces capacity to 6x current LiIon cells, still incredible and hardly anything to whine about.

Cells with this technology will probably hit the streets in the next 5 years; combined with on-going improvements in device efficiency, the time is not far off when we won't be worrying about this anymore... our devices, whatever they may be in 2017 (phones? tablets? neural implants?) will last weeks on standby, days with heavy use.

I know this has been a long post. If you made it this far, I hope this has been interesting and useful for someone... if so for just one fellow member, it was worth writing up :D

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