emini

This is my third BMS backplane layout.

• It's too short (I forgot the bms overhangs it's edge connector)
• I'm going to try replacing J101 and J201 with double row headers and put them both on the same end
• At the expense of having all the heat at one end, I clustered all the MOSFETs near their connectors so the tracks don't get tangled
• I haven't attempted to bring the heat through to the back side while still preserving all those tracks running under the MOSTFETs, putting J101 at the other end ought to help
• This would do 10 cells, I'll actually print it 2 or 3 or more up to get management of 20 or 30 or more cells on one backplane

I used kicad to draw the schematic and then do the layout. The last time I did something like this was 10 years ago and the free tools were fairly primitive (and crippled) and I'm very impressed with kicad, it even has an autorouter!. The autorouter only deals with two layers at a time and needed a little help to finish this board, but I can tell you it's really satisfying to see most of the tracks appear automatically. There's even a version for windows!

Bob Simpson designed the EVD5 hardware to sit in the top of a box of A123 cells. You can see this design in action in the background of the photo above, the blocks of cells drop into boxes and the BMS circuit plugs into the green PCB down the side. Each layer of cells is connected together with a thin plate, and a tab on each of these plates sticks out and is simply soldered to the side PCB. The temperature sensors are also connected via the side PCB. The foreground shows what happens when this elegant layout doesn't quite fit -- you end up with a lot of wires. Bob has more electric trail bike construction photos and riding videos on his website.

To reduce the many wires and many fuses problem in my Thunder Sky battery, I am planning on (almost) only one wire to each cell. The BMS is set up for a kelvin connection which makes measuring the voltage easy. Without a kevlin connection, when I turn on the bypass current, the measured voltage will drop (because current is now flowing through relatively thin wires). This can be corrected in software as the voltage drop is purely resistive and won't change very quickly, if at all. We can see in the graph below that without a correction, the measured cell voltage drops 60mV when we bypass 500mA. With correction, this error is only 20mV. I used gnuplot to fit the line to the uncorrected data, and it appears to have given more weight to the low current end (where there are more points) as we have a larger error at higher currents. (I still need to investigate why I'm getting such quantised data)

Unfortunately, with one wire for each cell, adjacent cells share a wire and current bypassed in one cell will effect it's neighbours. Since the slaves do not talk to each other, the master will have to take care of this effect. Internally each cell's slave only uses the voltage reading to implement an antonymous "dumb" balancing. The master will coordinate normal BMS operation, so this won't be a great problem.

I managed to blow up both of my LJTick-RelayDriver boards. These are little boards that plug into a LabJack and allow it to drive relays. Since I wanted to drive a relay to turn the battery charger on and off, I had to investigate. The circuit is quite simple, a ULN2003AI Darlington Transistor array and 3 resistors. R1 provides a return path for the base current by and also connects the relay power supply ground to the LabJack's ground. LabJack's datasheet suggests it should be 10Ω, but on both my units it was open circuit. I guess what happened was I put the relay power across this resistor (perhaps by using the a supply sharing a ground with the computer and connecting it backwards) which would cause a great deal of current to flow. If it was 12V, you would expect about 14W to be dissipated in a tiny surface mount component. No wonder it went open circuit.

I replaced this resistor with a 120Ω 1/2W unit which will still be overloaded by a 12V fault, but will last a lot longer.

Normally I'd use my EVision to measure the charge current, but it needs more voltage than a few cells provide. Last year Vik gave me a few Alegro ACS754LCB-100 hall effect current sensors, so I dug them up and now I'm measuring the charge current. A hall effect sensor uses physics to measure the magnetic field created by a current passing along a wire -- this one is calibrated to produce 20mV on it's output per amp flowing in it's sense wire. I tried 3 sensors and only one of them matched the datasheet. After I'd soldered that one with my too-small soldering iron (which means the part was kept very hot for a long time) it wasn't all that close to specification either. It's response is still linear so I've just adjusted the mV/A value in my code. I'm measuring the signal with my LabJack U3. The code will be in svn soon.

A hall effect sensor is good because it isolates the sensed current from the signal wires. This way I can tolerate one isolation fault between my computer and the battery. If I used a regular resistive shunt, the battery would be connected to the computer and an unexpected second isolation fault could cause potentially damaging current to flow. The low insertion losses of the hall sensor also make the charger's voltage regulator more accurate. The disadvantage of hall sensors is they produce more noise in the output signal than a typical resistive shunt.

Below we see 3 cells being charged. The charge current starts out high, I adjust the voltage control down and we see the current drop as the cell voltage rises. Then I adjust it up a little. It becomes obvious that the green and blue cells are nearly full while the purple cell is not, so I connect a separate 3A supply to that cell only. This increases the total voltage and the charge current drops. The PFC30 voltage regulator seems to cut the charger off completely below about 300mA.

If you have to use compressed air, blow from the solder side through to the component side, not the other way around. Blowing spreads solder everywhere, and doing it from the component side spreads it all over the components. The solder sticks to the tip of a hot fluxed soldering iron, so it's easy to clean the splatter, if you can get to it with the iron. It took about an hour and half to clean the solder off the components shown above. It took about 2 minutes to do the same job on the other board where I blew from the solder side and and the splatter wasn't hidden in all the component nooks and crannies.