Of course, with pleasure. Having ready-to-use design + money to cover manufacturing is a pure joy - this way, if it turn out to be working, I have nothing against offering it as alternative type for everyone interested. It's just that I don't think designing something like that by *me* would be feasible, but if someone else have motivation and skill to do it - I'm all for it.
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As for 18650, stlpaul described it accurately. Being widely used, standardized, cheaper (as money You need to pay per mAh), easily available in plentora of types, etc, benefits of using them over standard "phone" flat batteries are gigantic. It's just that they're little harder to fit, due to cylindrical size. But, even having this restriction in mind, they still supply more mAh per square cm. (even considering a "loss" due to empty space between cylinders).

Only one drawback, is that You can't go lower than width of one single cell, i.e. it have a minimum thickness much bigger than flat battery. But, for people that already use Mugen covers for dual-batteries, it should be a problem - as long as it still fits into pocket I would happily trade few mm of additional thickness for having 6000 mAh + additional mod space + guarantee, that my batteries won't ever disappear like scuds + cheaper batteries
as opposed to:
3000 mAh and lack of all mentioned benefits.

/Estel

http://www.rorylofthouse.co.uk/wp-co...eryWidth-1.jpg
http://www.rorylofthouse.co.uk/wp-co...eryWidth-2.jpg

How about making n900 run on penlites? As you can see in above photos, the penlite has 2450 mAh vs 3000mAh but is much smaller. (I am sure you have thought about it and I am missing something).

And is it possible to get original N900 CAD drawings from Nokia?
Or do you have a scanned in digital drawing of N900?

So it seems the 18650 battery consist of an internal coiled flat-pack lithium-ion cell. Is it possible to dissemble this battery and make a flat battery out of it?

All of the talk about 6000mAh batteries and Mugen-sized covers is making me a bit scared about the future of this project.

Are you still planning on producing a regular, thin cover without any of the fancy ****?
I don't want a charging port.
I don't have a need for extra space to fit a SD card.
I don't require my phone to last a week on a single charge.

I greatly support (and am thankful for) your initiative and the creative ideas the community produces, but if the product is changed so that it no longer matches my needs, then I'll just stick to the stock body + backplate.

Roughly the size of a Galaxy Note?

The Wizard of Huz
As for penlites instead of 18650's, it's impossible, due to different chemistry - they're, most of the times, NiMH/NiCD, and we need LiIon (all it subtypes are ok).

If You or someone else know about AA LiIon's accu's offering decent capacity - or, generally, any type that would suit N900 better than 18650's, offering decent capacity and at least being half as widely available as 18650's, I'll gladly consider it as an option - 18650's pose quite big design problem due to it's size. IMO it's out-weight by pros, but maybe there is better alternative that I'm not aware of?

Also, it's not possible to disassemble 18650 and re-build it as flat battery - at least, I'm not aware of such possibilities. Maybe dr_frost_dk know something about it?

As for CAD drawings from Nokia - no way to achieve this. I don't have digital probe, so I'm just designing case by hand - knowing measurements took by hand, and sometimes, by trial and error That's why it takes time - with proper digital probe, I would probably be offering body replacements already

@fffa
No worries, of course I'll still make regular, flat covers. As for body replacement, it will be build a way, that it can be used with both flat and big cover (much like it's not with regular body, although I won't re-create original locking mechanism, probably). So, people like me would be able to have body replacement with big backcover and geeky mods, but it will be perfectly possible to use it with flat alu cover, without tripod mounting points etc.

/Estel

Can't dismantle a 18650.
Other battery 14650, same length but 2mm less wide, capacity wise they have less, about the same as scud, so again, only gain here will be with 2x18650 or a design that will accommodate 3x Scud's, but problem with the 3x Scud is price and availability.

2x 18650 can be bought for less then 10$, of course here im talking about 2000-2500mAh (pr battery), but even the 2x 3000mAh cells will often run less $ then 2x Scud's, witch also only carry 2x 1500mAh.

So if price is the point to consider then the 18650 are the way to go.

Other features here are to make a Direct port to the battery's like i have when i charge mine, and use power from my N900 to drive small AMP's and such, but having 6000mAh and something like 3C discharge gives you the ability to attach a DIY POWER Led array, we are talking about 60W output here, you can even run a laptop for a while from the N900 then (with a power converter -DC-DC).

So not only power/runtime gains, also a device that can have SERIOUS attachments.

I can try to ask the machine workshop at university is they can scan my device. I'll let you know.

This sounds great. I'd be interested if for no reason just to support the project spirit. Some thoughts:

1) I'm ok with either single or dual battery. I like the idea of 2 batteries in the same plane instead of stacked, to keep case thickness down.

2) I was a proponent of full sized SDXC but I think it's not worth it any more. 32gb microsdhc is dirt cheap and 64gb is affordable. Full size exists in 128gb but is it worth it? Dual micro slots is probably better than 1 full size if it comes to that. (But probably not feasible).

3) I do very much like the idea of 2mm charge port and hot swapping.

4) It's not worth messing with 18650's instead of flat cells (which aren't that expensive). Flat cells of high capacity are available. In fact it might be nice to have mounting option for add-on layer holding a 4000 mah flat cell like the one in the Open Pandora palmtop. It could be set up with a 5v step-up converter so it would charge the phone like an external charger, rather than going directly to battery contacts. But it would still be a solid piece, no external wires etc.

Please stay away from 14500's, they combine disadvantages of almost every format. One thing I've wanted for ages is a phone powered by commodity AA NiMH (not lithium) cells. Yes they're less efficient but they're cheap and easily available. Good AA's are about 1.2V 2500 mAH so two AA's have about 1.4x the energy of a BL-4J, I think. It could be possible for the external layer to use more AA's, maybe even 6 or so of them. I still have a 6AA holder for my ancient Motorola analog flip phone and it was a practical device.

5) I like the idea of threaded hole over the camera (27mm? 37mm?) to allow screw-in lens accessories available for digicams/camcorders (telephoto/wide angle converter, filters, etc). Might be interesting to also supply some kind of remote flash trigger.

6) One thing I wonder, will the metal case interfere with GPS reception? Same for Bluetooth, wifi, etc.?

Anyway, best wishes for the project.