Oklahoma was hit by the biggest snow storm in state history this Christmas. Since we were snowed in and couldn't visit relatives, I found myself with some unexpected time on my hands. So I built my own Android phone.
I bought a Neo Freerunner last year, but some design flaws kept it from being usable to me as an everyday phone: pitiful battery life, audio problems, and no single Linux distribution stood out as the one best choice to use. But the battery was the worst problem.
My previous experiments had shown that if you remove the battery monitor circuit board from the original battery:
...you can transfer it to a larger Li-Ion battery, like this one from a portable DVD player, and it will work. Surprisingly the BQ27000 chip on the smart battery board is able to learn the larger battery capacity.
Of course, I didn't let the old battery go to waste either...
The prototype battery mod worked but I needed to give the phone a bigger enclosure so the battery wouldn't have to be on the outside where it might be damaged.
Having proved the concept, I exchanged the junk DVD player battery for a brand new 6 AH Li-Ion battery from Sparkfun, and put it all into the case from a 2.5 inch portable hard drive. (To put that in perspective, the original battery was only 1100 mAH.)
Rather than try to cut the hard drive case to perfectly match the Freerunner circuit board, I simply reused the original phone faceplate since it already had the proper board mounting locations. I cut an oval hole in the hard drive case (free hand with a dremel), and then used JB Weld to join the two pieces and to even out the gaps between them. I put the JB Weld on sloppy but used a wet paper towel to wash away the excess, leaving a nice even seam all the way around. (Turns out JB Weld is water soluble - who knew!)
By the way that silver squiggle shape on the front is the cellular antenna. I ran out of room on the inside, so I put the antenna on the outside! And putting the antenna next to my cheek is a good way to extend a sarcastic "bite me" to all those idiots who argue from a position of ignorance that cell phone signals cause cancer without knowing the first thing about the physics of radio signals.
Although from the faceplate it may appear as though an entire Freerunner has been simply embedded into the new case, behind the faceplate there is nothing left of the original Freerunner case, and some components have been moved and some improvements made as I tried to rectify my main complaints about the original phone.
Here it is charging at a total rate of nearly 2 amps using both the fast charge board and the Freerunner's internal charging circuitry. No apparent harm comes of running them both at once, and the extra charger doesn't confuse the battery gas gauge chip either.
The BQ27000 learns the battery capacity by observing a complete battery discharge cycle. Unfortunately software will shut down the phone when it thinks the battery is empty, so we'll never actually reach a full discharge of the new battery to recalibrate the gas gauge! Luckily I built a battery discharge device a while back, which is really just a mess of enormous low-ohm power resistors strapped to a massive heatsink with a fan on top. And a fancy digital temperature readout so I know whether I'm actually frying my power resistors. This discharges this battery at a rate of 1 amp, which is a bit too fast, but with a 6 AH battery I could be waiting all weekend for it to discharge at "normal" current draws.
The documentation for the BQ27000 gas gauge chip mentions that it doesn't revise its capacity estimate downward more than 1/8th of the total at a time. It doesn't say anything about how the estimate gets revised upward, but experimentation with the test battery from the DVD player indicates that it caps the upward change at 1/4th of the previous estimate each time. The test battery reached 2.2 AH, so I can only expect the BQ27000 to revise the estimate up by about 0.55 AH on this cycle. It's probably not worth the trouble to work it up to the full 6 AH reading, since the process takes 12 hours and I'll be slightly damaging the battery every time I completely discharge it! Maybe when the battery is deteriorated in a few years, I'll calibrate it to whatever reduced capacity is still in it.
Not really visible in the pictures, but I removed the speakerphone (which was useless in its old location because its proximity to the microphone made it impossible to actually use speakerphone without getting a loud feedback squeal) and connected it in place of the earpiece. I thought that, because it was bigger, it would be louder than the earpiece, but instead it is much softer, nearly inaudible. I think that's because the earpiece measures as high impedance while the speakerphone speaker is low impedance, so there isn't an efficient transfer of power. Tomorrow I'll cannibalize the audio amplifier IC from that same portable DVD player I got the test battery from, and use it to fabricate a tiny audio amplifier board to drive the earpiece & match the impedances. (I can design a circuit in my head and build it on a new board as fast as I can take the components off the board I'm cannibalizing.)
That will also provide me a physical volume control so I won't have to rely on the goofy software volume settings that never seem to work right on the Freerunner. Hmmm... Maybe I'll make it stereo while I'm at it. The Neo 1971 (prototype to the Freerunner) had stereo speakers in it, but not the Freerunner. The audio amplifier chip in the DVD player is stereo, and I can pull a stereo signal from the headphone jack. Interesting...
So... Android right? Remember at the top I said that my original goal was to address some specific flaws with the Freerunner, one of which being lack of a definite distribution to stick with. I didn't actually like Android when I first tried it (still not sure about it) but it seems to be where the ball is rolling. When I began to think I wanted a Motorola Droid, and when blogs and podcasts started going crazy over the (latest) rumored Google phone, I had to stop and remember, wait a minute, I already HAVE hardware capable of running Android. It's just been sitting on my desk in pieces waiting for me to put it together.
Installing Android was a breeze; easier than any other install I've done on the Freerunner. You just untar a file onto an SD card, and then booting off the SD card prepares Android on the Freerunner. No messing around with the flashing the phone over USB, and no messing around with bootstrapping the system on the phone itself like Debian does. Just pop in the SD card and it just works. Not much else to it - except wifi doesn't work. Why does wifi never work for me on Linux? Oh well...
