Tsk tsk Simon, not using bi-color or tri-color LEDs yet? :-)
BTW, I saw your post to Maemo-Developers@ ... the conversation hadn't come up before to my knowledge, hopefully one of the Nokia folks will be able to answer it definitively.
I was going to but the way my circuit is I couldn't use a common cathode LED! After lurking a bit reading the Maemo dev list I realised that there are Nokia guys there. Someone hopefully has something to say.
"Standard sized" 9VDC Alkaline batteries (a battery of six AAAA cells typically) is rated for ~500mAh nominally from what I've been able to determine. 9VDC Lithium are probably in the ~800mAh range.
Rechargeables are in the ~120mAh to 250mAh range, unless you go to a larger form-factor.
The big problem is that as you crest over C/20 (sustained load at 1/20th of rated capacity) and head towards C/10 depletion rates, things like internal resistance and resulting voltage-drop / heat become painful.
I guess bhima idea is the best for portability and a reasonable battery life then. Does anyone have a link for a schematics?
Wikipedia has good info on this (as always).
Google also comes up with this that looks interesting : some schemes will even step up OR step down (like going from 4 fully charge AAs to almost depleted ones), but it's rather more complicated :-) http://www.maxim-ic.com/appnotes.cfm/appnote_number/661
I guess bhima idea is the best for portability and a reasonable battery life then. Does anyone have a link for a schematics?
I will be posting schematics as soon as I have built a prototype. There are still some questions about precise values for the parts I need. For example, "best results" will be obtained if I use two capacitors which cost $2.50 each, but the spec sheet also mentions that capacitors that cost $0.30 each could provide "acceptable" results.
What I'm hoping to do is provide both working schematics and ready-made devices for people who don't want to built the devices. The efficiency of the conversion and the quality of the power do depend on the parts you use, so it's not as easy as going to Radio Shack and getting whatever capacitors they happen to have.
I STILL haven't put up the circuit for my little USB box yet sorry. There is something wrong with it. It seems to go flat after only a day or so and that's with it not being used. When I measured it there was a drain of about 9mA from the battery which should be rated at 900mAh. I should be getting a lot longer life than that I think. Well, theoretically 900 hours right? I know it's not quite that straight forward especially with the high self discharge rate of NiMH but that still seems too short. Mind you until someone comes up with some fix for that repeating keyboard problem I don't really have much use for the thing anyway.
A load of C/100 (9mA on a 900mAh battery) should give you at least the rated output voltage to somwhere past the 60% (remaining) mark, or 40hrs. C/100 is a very small load, so internal resistance should have negligible impact.
Theoretical run time (100%) of 100h (900mAh / 9mA).
I don't know if Bhima's solution will come down to less than 20$, but this looks really tiny (much more compact than anything homemade anyway :-).
That's just about what I'm going to build - though my single board will have a USB connector attached to it. I'm hoping to keep the price at around $20, but on Monday or Tuesday when I finish my first prototype, I'll be more clear on the cost of the parts I need.
It'll probably be only a bit bigger than that device - I'll be buying printed circuit boards by the square inch, so I want it small
I'm currently working on two different approaches:
BYOB == Bring Your Own Batteries. Small device that takes one or two AA's, and you just stick in whatever ones you want.
The other approach would be more fully sealed, perhaps looking like a fat cable. It would have an internal rechargeable, and would probably be charged over USB from another computer.
I made some research on the subject at wikipedia, thanks fpp. Also, browsing Maxim's site I found this: http://pdfserv.maxim-ic.com/en/ds/MAX1674-MAX1676.pdf
It looks like the perfect regulator for the job, it will work with inputs from 1.1V and up. Maybe one of these is used in the tekgear product, the specs seem similar. It needs a few support components, but it makes for a very compact design. The only thing I couldn't find out is how to select +5V or +3.3V, does anyone know?
