[HD2 General] motoi_bogdan:
ok then.. it was built, it was tested and i'm already using it or at least.. trying to figure out a use for it :D

Anyway, if you're the camping type, if you feel geeky or wanna make a eco-friendly charger for the hd2 (or other usb charging enabled things) here's how to build one

You'll need the following materials:

- patience, this is a long post, try not to get bored while reading it :p

- some basic electronics skills, basic understanding of components, measurements and circuit troubleshooting (this is not a beginner project i guess, as hard as i tried to make it, if done improperly... well of course, you risk killing the poor phone in the process).

- some second hand laptop batteries, 3-4 would be ideal, they can be kind of old, but must be functional to some degree.

- a cheap usb charger used for cars, the one that plugs into the cigarette jack OR some good electronics skills to design a switching power supply. Guess most of people will go with the first option, in order to be more helpful i also designed this circuit using this option. You should buy the cheapest adapter, the cheaper the better. That's because the expensive ones have a feature that enables them to stop working if the voltage of the car battery drops to a certain point. That's supposed to be some sort of protection not to allow the car battery to discharge and thus preventing you to start your car. We don't want this protection, we want to be cheap asses, we want dirt cheap. However the adapter you want to buy must deliver 5V at around 1-1.5Amps minimum. 5Volts at 500mA is to little, it will simply kill itself when you start the thing once it's completed.

- a standard usb hub, any will do. This is if you want usb host functionality or use linux.

- a 5-10W solar panel. The bigger the better - it will allow to recharge the buffer battery (the one you'll be building) at a faster rate. This is the single most expensive part of this build. If you simply want an external battery for the hd2 you can skip the panel, if you want solar charging.. this is .. of course, a must.

- one fast rectifier diode or a schottky diode, you should buy it if you use a solar panel, the supplier of solar panel could also recommend one to use with that specific panel. We have to use one, this will make sure the current goes from the panel to the battery, not the other way around. :p

- bunch of wires, a multimeter and a soldering tool witch you're not afraid to use.

- a variable power supply, either it be a wall charger with variable output voltage, a laboratory power supply, or some charger that can output anywhere from 3.6 to 4.5 volts at anywhere from 300mA to 1A. Any combination will do. This is required only once in order to precharge the cells to a specific voltage.

- one switch or something similar in order to ... switch the thing on and off.

- spare time/understanding wife/coffee etc

1. Ok, first of all you need to get those laptop batteries open. Use some sharp tool, your karate skills or whatever necessary to crack those batteries open and expose the individual cells inside. Be careful not to damage the cells in the process, at least if you use some sharp tools. Once exposed, the cells will be linked to each other, you need to separate them by cutting the wires or metal bands that links them. Once done sort them out, if you have multiple batteries, sort the cells from each battery in a different case or basket or whatever you want, the idea is not to mix them.

Here's mine:

2. You now need to measure up individual cells with a multimeter. You're looking for the voltage rating of each cell. If you find cells with 0 volts, they're dead. If you place the multimeter in continuity testing mode and the 0 volts cells are showing continuity across their leads, yep.. they're really dead. Never use these. Good cells have anywhere from 1volt to 3-4volts.

Once you selected your good cells start forming a pack. The idea is simple. The more cells you put, the longer the thing will last. Standard laptop cells are rated to a minimum of 3.7Volts and 2400mAh. Each of them is almost double the capacity of the standard battery that comes with the HD2. However since you're going to use second-hand ones, they will have sign of usage, a smaller capacity then that of a new one. Still they will perform at least the same as hd2 battery in terms of battery life. Minimum configuration starts with 2 cells, i recommend 4 cells as a decent start but you can go and add more cells if you like. The more cells the longer battery life but at the expense of added weight. My choice was 6 cells. If you had 2 laptop batteries and each of them had 6 cells, you can make your pack on anywhere from 2 to 12 cells.

Let's say you choose 6 cells (always an even number). If 6 is you choice, you will need to divide that number by 2. So you get 3. You need 3 working cells from the same laptop battery to form a pair. Go back to the place you kept the cells and select 3 cells, NEVER mix cells from different batteries. These 3 cells you have (first pair) will need to be linked in parallel connection. The negative ( - ) of each cell is linked to the negative of the other and the positive to the other 2 cell's positive. We'll get there, but at a future step. Now we need the second pair of 3 cells. Again look in your cell basket and try to find 3 more cells from the same battery. This battery may be different then that first one you selected cells for the first pair. Yet again, the 3 cells must come from a single battery, no mixes. And.... again, these 3 cells must be placed in parallel. So if we already imagine them connected, we would have 2 pairs, each of them with 3 cells linked in parallel. The 2 pairs must be linked in series, so the minus of one pair will go to the plus (positive pins) of the other. The unconnected pins of each pairs (one minus one plus) will be used for voltage supply - you'll get the combined voltage of the cell pack here. If i were to draw this things for you to better understand.. it would look something like this.

green is one cell pair, orange the other. The black things are wires. This are the connections for a 6 cell pack. If you have 8 cells, you will make pairs from 4 cells (2 pairs). If you have 4 cells - the pairs will have 2 cells. A charged cell will have something like 4 volts. A pair made up of several cells in parallel will still have 4 volts across it's leads, but the overall current capacity of the pair is increased by the number of cells it contains. So if you have 3 cells each with 4v and 2200mAh, the pair will have 4 volts but with 6600mAh. If you place 2 pairs in series like on that drawing, you increase the voltage of the group by the number of pairs you add while still having the same current capacity. So if you get 2 pairs of 4 volts and 6600mAh, you will have 1 group, 8 volts and still 6600mAh. That's the total output of your pack. Because hd2 needs 5V (not 8 !!) we need something to decrease the voltage from 8 to 5 volts. That's why we need that car usb charger. It normally uses the 12V available at the cigarette jack to output 5v your phone can use. Cheaper ones, can use 8 volts, or 7 volts (lower voltages) because they don't have a circuit to prevent deep discharging the car battery like expensive one have. We need one without this circuit, because our battery pack only outputs 8V. So the car usb charger will take the 8 volts at it's input and give us 5volts at output.

3. before linking cells to each other, you need to charge them to the same voltage. Use a charger/power supply etc. I used a lab. variable power supply, if i had none, my weapon of choice would be a nokia standard wall charger (or another brand), older ones, i would cut off it's jack, expose the wires and connect them to my cells, it outputs 3.7 volts, enough to charge each cell. So charge each cell to about 3.7 volts. You will need to connect the multimeter in parallel to the cell and monitor the charging process. When a cell reaches 3.7 V disconnect it and charge another one, until all of them have 3.7 V. After this, leave the cells for one day. Next day you will be measuring each cell again. If one of them drops charge by it's own and you find.. let's say 3 V, you got a defective one, back to step 1&2 and select other cell pairs. If all cells still have the same aprox. level (somewhere around 3.5 to 3.7 volts) you're good to go.

Here's one cell linked to my voltage supply.

4. start thinking of either a case of something to contain your build. I used copper plated pcb (from electronics stores, radio shack etc). I will be connecting my cells to this thing, kind of like a pcb assembly. You may use some plastic housing and connect the cells with wires and secure them with some glue. If you have experience working with pcb, etching the copper layer and such things, feel free to try using pcb.

Here's my blank pcb for this job - i've already cut it to required dimensions. It's the orange metallic thing in the center. Beside it you can see my hub and the usb car adapter i will use.

5. disassemble (i repeat disassemble.. no more karate skills) your hub (if you're going to use one) and your car usb adapter. My usb adapter looks something like this.

The hub interior will look different, we'll get on that on a later step. Anyway, speaking of the usb car adapter, i'll be needing that small pcb with the components, so i'll remove it from there. The board contains the switching mode voltage converter, yummy yummy, i want that. It basically has 1 chip that generates a pulse signal that is feed to the input of a power transistor which pulses the input voltage across a coil. By autoinduction the coil produces another current, other components rectify and filter it so the second smaller current, produced by the coil, it's basically what powers on the devices connected at the output. In simple terms .. that's how it works. Again.. we need this, don't break it :p

Mine has a funny oval shaped form, so i'll be cutting my pcb in order to insert it inside.

There are 2 wires coming out of the small board inside. That's were the cigarette jack was connected. We will connect our cell pack to that, so you might wanna remember their position. The red one will be the positive one, black being negative. In a cigarette jack, the center pin is always positive, so if your wires have other colors, the one that's linked to the center pin will be the positive one.

6. Look for a way to place the cells inside your casing or on your pcb. Since i will do a pcb with them, i'm trying to find a possible placement for them.

this was one way, but i figured it was easier for me to simply place each pair on a line and form 2 single lines of cells instead of 3. Once done, i begun drawing the pcb with some paint marker. I will then etch the pcb, so only the paint covered areas will remain.

here's the pcb after etching, i'm connecting various wires to complete the cell pack circuit before connecting the cell themselves.

If you're using some sort of case, it's time to begin assembling your cells together. Use the solder gun or whatever you have for soldering to attach some wires to the each cell leads. Li-ion and heat aren't good friends, be as quick as possible when soldering, you don't want to heat up the cell too much. If it starts to make any strange noise, hiss or is venting anything from it... run away, don't touch it, don't throw it.. simply leave it and run away. Of course, this is a very rare scenario.. but take your safety when working with high reactivity materials like li-ion cells.

You want to arrange the cells in that paint draw up in the post. 2 pairs, linked in series. First solder wires to make one pair, then the other, then connect the pairs to each other. If you use a case, use some insulator to cover the solder points and to avoid some accidental short circuits in the future.

Back to my pcb solution, here's my assembly.

7. If you want to use an usb hub, you can try to salvage some usb port from an old pc or laptop's motherboard. This way you could avoid using a permanently attached wire to the device you're building in order to have the hub connected to the phone. I found an old laptop motherboard with an intact usb jack.

i remove the usb port from it and soldered on my board.

8. Next you need to connect the car usb charger's pcb to the battery pack you assembled. Basically the 2 wires from the charger must be linked with the 2 wires from the battery pack. Insert a switch on the cable in order to be able to turn on and off the whole thing. In my case, i'll now connect the car charger's pcb to my pcb, in the portion i've cut.

9. now you need to modify your hub to be able to power on hd2 during usb hosting mode. There's a link in the linux section (ubuntu for hd2) about this, you may want to read that also. I basically soldered a wire across each hub's usb port positive pin (the 4 usb jacks) and the input usb jack. The ground connection is the same for all jacks. So all jacks including the one used for connecting to a usb host device (pc) have the power pins linked together. Those 2 power pins must also be linked to the output of the usb car charger so that when you power up the thing using the switch, the charger also powers up the usb hub. After you solder all the wires, also insulate the soldering points and secure the hub in your casing along with the battery pack and usb car charger's pcb.

Here's mine, it was soldered on my pcb.

as you can see, there are some couple of wires coming out from it. Those need to be connected to the phone for me to have usb host functionality. So i'll connect these wires to the usb port i've mounted at step 7 so i can use a standard usb - microusb cable to link this thing to the phone. If you want to make it simpler, cut out a usb - microusb cable and directly solder the wires onto the hub's pcb as shown in the guide on the ubuntu linux thread for hd2.

In my case, i use that port i salvaged, as i said before.

10. assemble the whole thing and carefull inspect the connections. The order of this will be - battery pack - linked to the car usb charger - that's linked to the hub power pins (for each usb port). You'll then have one usb port for use when requiring usb host functionality, 4 usb ports for connecting all sort of usb slave devices, and one usb port (the one that it's soldered to the car usb charger) for use when you want to simply charge your device normally.

Here's my build. I've also placed a fuse between the battery and the usb charger, so that in case of malfunction it breaks the circuit. The fuse holding pin is the black thing at the opposite side of the usb hub. Near the pcb, you can see the fuse and it's cap.

11. Check again all connections. when ready, press the switch and bring the thing to life. Use the multimeter and check all usb ports voltage. You shoudn't have more then 5.5Volts and no less then 4.5Volts. If you do, then you did something wrong, turn off, disassemble and recheck. If you did it right, you'll get a voltage inside the above interval. Inspect the device once again and make sure all things are safely placed and secured inside. Try plugging some cheap usb devices you may have, a mouse, usb flashlight, another hub etc. If they receive power and all it's ok you may try to connect the phone.

Voila.. usb charging from the ghetto style external battery.

And here's a small video of preliminary testing (i haven't yet tested the usb hosting capability but i have no reason to think it will not work). At this time i didn't placed any switch on the board so i switch on and off the thing by placing the fuse inside the holder or removing it :p

http://www.youtube.com/watch?v=yf6kRpNNqkw

Next step... maybe some of you may think.. well how does this thing recharge when the batteries are depleted. At this stage the battery pack is recharged by connecting a 8.4 voltage supply across the battery pack leads (wires) but the next logical step will be adding the solar panel to the build and securing this pcb to the back of the panel. Then.. further testing. I'll be keeping the panel and that rectifier diode handy. This is still work in progress. :D