Having already modified my model B Rasberry Pi, I decided to make an even more custom version of my battery board. The big advantage of using the model B Raspberry pi is the low power consumption compared to the model A – about 30% less. The biggest setback using the model B is only having a single USB port to work with. To make up for the lack of the additional USB port I decided to incorporate a small hub with my second board.
Sourcing a USB Hub
Doing some shopping on ebay I found a couple of small cheap USB hubs with what looked to be a small form factor that I could incorporate into the battery board. After waiting a couple of weeks I dissected the candidates and found they were both single sided boards – which was good for my double sided tape construction technique. The disappointing part of the cheap USB hubs I found on ebay was the lack of any sign of a crystal oscillator to control timing or capacitors for power protection. With the exception of a couple small ceramic chip capacitors most of the USB hub was run from a single epoxy encapsulated chip.
When I built the first V1 board I added 220uF caps across the usb power pins. I thought this would be a good idea as I had read a number of USB hub chip datasheets that recommended this in their example circuits.
I tested the hub board that I had selected for my V1 battery board and found that it appeared to work normally. However, doing some file transfer tests I found that the cheap USB hub was doing a file transfer at about 1/3 the speed of a slightly more expensive hub I had just laying around. I was pretty disappointed by this since one of my intended applications was to stream video to my Android devices. So I promply ripped out the cheap USB hub and found a slightly more expensive version (with a real crystal and electrolytic caps) to replace it. This slightly more expensive hub was on sale for $6 US. The new USB hub board didn’t fit as well as the cheap non-crystal one but I managed to get it mounted in the space I had available.
Battery functions for the V1 battery board were handled in much the same way as my V0 battery board. I used a ebay sourced boost converter for the voltage step-up and a MCP73831 chip to handle the charging. However, this time I built my own circuit for the MCP73831 instead of buying one online.
To build the MCP73831 based charge circuit for my V1 battery board i used a SOT23 breakout board to make the connections easier. The entire circuit used only 6 components so I built it directly on the battery board. In the picture with the faster USB hub you can see where I have soldered in the MCP7381 mounted to a breakout board (circuit was incomplete at that point). Another feature you can see in the photo with the new hub is a switch that I added to let me manually switch off power and data lines to one of the USB ports. The idea there was to have a USB WiFi dongle used in that port and giving the user the option of switching it off without having to remove it from the board.
More Mods to the Pi Board
I had already removed the RCA jack and the stereo jack when building my V0 battery board. To make room for all the extra hardware that I had added to the battery board I removed more of the standard hardware. First to be removed were the header pins. The second item that I pulled off was one of the two cable connectors. I kept the ribbon cable connector nearest the USB port since I could see using it for the camera option. I also removed the USB port so that I could hard wire the new USB hub directly to the board.
To make the final assembly a little more compact I also removed the SD card slot and hard wired a micro SD card adapter made by Adafruit Industries. Hard wiring the adapter shaved off about 1mm from the overall height of the board.
Putting Things Together
To increase the area available to mount the battery, I decided to make some custom board mounts out of acrylic. The custom mounts would use double sided tape to bond them to the bottom surface of the battery board and provide threaded 4-40 screw holes so that the Raspberry Pi board could be mounted directly to the battery board. Eliminating the need for screws on the top surface of the battery board would give a nice flat surface to mount the battery. The mounts were cut using a CNC mini mill that I put together a few years back.
Below is a picture of the V1 battery board and the modifed Raspberry Pi board ready to be mounted together. You can see the two acrylic mounting blocks glued to the battery board. Notice that USB hub wires are soldered directly between the two boards and the DC power is on a flying connection.
Like the V0 battery board the Li-ion battery was held in place with a piece of card stock sandwiched between two pieces of double stick tape.
Instead of building a case I decided to use shrink wrap. I used the type of shrink tube that is normally used for making large battery packs. To cover the ends, some plastic sheet taken from some cheap dollar plastic folders modified with a hole punch and scissors. The end pieces are held in place with some scotch tape before the shrink wrap is applied. A little work with a sharp knife provides holes for the slide switches and LEDs.