I've been fairly quiet recently, mostly because I fell off the blog wagon after going to Samoa on an olpc jaunt (see our report for details).
I have been making slow progress on the EVD5 BMS. I fixed no less than 5 bad joints in my backplane arrangement (I specified the wrong size hole and ended up drilling them bigger and soldering the top and bottom, but not so well) and have the BMS behaving well on my 36 cell battery.
I haven't implemented the necessary code to deal with voltage drop in the wires while shunting because I'm going to get rid of the wires. The wires are a disaster from a safety and construction point of view. Constructing the wires out of ribbon cable with soldered in fuses takes a very long time and I've already had to replace two blown ones (don't know the cause). My earlier backplane design packs too many boards too close together and makes me nervous.
Removing the scary difficult wire while keeping the 5 cell EVD5 BMS is possible, if you make a backplane like Tritium's IQCell system. The squiggly bits in the circuit board will allow the cells to move as the car jiggles down the road. The problem with this is that the EVD5 has an awful lot of wires to each cell -- temperature, sense, shunt+ and shunt- and I'm having trouble getting them all through the squiggles. It's worse if I want to mount the shunt transistors away from the main EVD5 board. You'll note that Tritium's hardware is a lot simpler than the EVD5, this comes from more R&D, when Bob designed the hardware (in 2007!) it was much less clear what was needed, so he designed it to be very flexible.
Its fair to say that the EVD5 isn't really suitable for prismatic cells and while this is true, it isn't a really a fair criticism as it was never designed for this purpose. It was designed for Bob's particular cylindrical cell construction.
I've put together a schematic with only 3 wires to each cell and none of them carrying current by putting the shunt transistor at the cell and only passing it's gate back to the central EVD5 board. To control current, I abandon the current sensing system in the EVD5 and add a series resistor. I haven't yet done the layout and seen if this works better -- thinner and one fewer wires isn't all that compelling.
If you've been watching the commit log, you'll see I've made a fair amount of progress with the software, with a bunch of small improvements to the laptop based monitor and better use of the LEDs on the EVD5 board.
I also found another hardware bug, the reset line on U10, the 555 timer in the RS485 section is floating. This is a FET based part and it doesn't take very many electrons to reset the timer and stop communications. With this line tied high, I don't have problems with the software addressing any more.
