[HD2 General] motoi_bogdan:
Stage 1 completed :D

So i figured out how hd2 charges it's battery. It's pretty simple actually.

Charging is made in the same way for either wall charger or usb charging. Same pins are used. However, when the wall charger is used, charging current can be as high as 800-900mA. This will decrease to some 20-30mA when the battery will be almost 100% charged.

If you however connect the usb data pins to hd2 and to something (computer, hub etc) hd2 will change the charging mode to usb. This feature ensures that when connected to a pc, the phone will never try to get 8-900mA from a 500mA usb port :D pretty neat. Instead, maximum charge current is limited to about 400mA.

If you get access to the usb wires inside the cable you can trigger either mode by simply disconnecting or reconnecting the data pins (green+white on standard usb cables). This is useful because i can devise some means of redirecting the charge current to either external battery (let's say after a night of use for the hd2) or to the hd2 if .. for some reason it's internal battery is discharged.

A usb enabled pic microcontroller (say 18f2550) can be used to control the charging process of either internal or external battery by simulating a pc connection, thus enabling hd2 to switch to low current charge. However if i'm guessing right, a simple usb hub could also do the job as it also contains a microcontoller that "knows" the usb protocol. So because i will use an external powered hub for linux, I'll try to use it also for switching between charging modes. Besides is more easier this way and i (or others) will be spared the time needed to program a microcontroller, or fabricate a pcb for it and it's corresponding components. And.. in case of nuclear attack there won't be any radio shack or electronics store where you could buy microcontrollers :D

Stage 2 completed:

Well i got myself some time ago a cheap 10watt polycrystaline solar panel for use with a robot i'm building. It was about $50 here in Romania. I guess i'll be using this one, even if it's quite big (something like a 16 inch laptop). Anyway i could also power other things with it. Will do some tests, and if it's suited for the job, it will be used. It arrived completely assembled so it also spares me the trouble and time of connecting individual cells into to form a solar panel.

Update : completed, will use the spare 10watt panel, until a new one arrives.

Stage 3 completed:

Got some 5-6 old laptop batteries. Stripped them down and got some ~30 cells. I'll test them out and sort about 4-6 of them.

Stage 4 completed: I'm currently sorting out some cells. As far as design goes, the capacity of the "extended battery" will be 14400mAh (single charge, using no solar power). That could easily power a full 15 inch laptop for about 4 hours, so i guess hd2 will have no problems staying alive for one night until next day and the sunrise to kick in for the solar charging to take place :D

Stage 5+6 completed:

There won't be any linear voltage stabilizer design for the buffer between the battery and hd2. They are quite inefficient with 30-40% power lost in the form of heat. Switching mode stabilizers+converters will be the way for this design. Found a ready made voltage stabilizer (converter) that's suited for the job :) , Saved some good hours that would otherwise have been spent on designing one from available parts.

Stage 7: yep, i'm now building the damn thing. I've simplified the design as much as i could, while keeping it safe for the phone, i guess the project will be possible to make by anyone with basic soldering skills.

apr. 13 - update: construction delayed due to one cell failure (the difference in internal resistance between the cells was greater then i expected). Will now search for a replacement, recalibrate the battery pack and recharge. However I expect that the battery module should be ready by the end of this day or tomorrow.