Entries by carrott

There is room for another row of cells in front of the two shown, but this row will have to be slightly lower since the front of the car curves down. There is enough height in the middle to put cells above the motor where it protrudes through the plywood, higher than the rest.

It seems improved Weight distribution can be had by putting the inverter in the boot and most of the battery in the front. This does introduce longer motor cable runs which will reduce efficiency but it is looking worth it -- with the inverter up front there isn't much room for cells and it's 70kg lighter on the front axle than when I started. I'll be mocking up the front to see exactly how many cells fit without the inverter before I commit to this move.

The contactors supplied with my Siemens electric drive system were large Schaltbau C160 series contactors with arc chutes and blowout magnets in an even larger box. This was quite inconvenient in the mini as the only place they would was in the boot. I'm planning on putting at least some cells in the front of the car, which would require some clever packaging to get the contactors to at the front. So I replaced them with two Gigavac GX14 sealed contactors. At the moment I've mounted them on a plastic base in same place the old contactor box lived. When I'm done, they will likely be split up, I'm expecting one in the boot at the negative end of the battery and and one at the front of the car at the positive end.

In the foreground you can see the precharge circuit, two small relays and 8 PTC resistors allow the controller's capacitors to charge in an orderly fashion before the main contactors close. If this isn't done the capacitors charge very suddenly. This huge inrush current is hard on the capacitors, battery and contactors. Such situations may even cause the contactors to weld closed. You can see this inrush on a small scale when you plug your laptop or cell phone charger into the mains. Sometimes there is a pop and (in a darkened room) quite a large flash as the circuit connects (it only happens sometimes because the AC might be in a low voltage part of the cycle when you connect, or it might be at a high voltage). Check out the ends of the pins, you'll see pitting and scorch marks on most laptop power cords. Charging an electric vehicle's capacitors involves perhaps 100 to 1000 times more energy than a laptop so there is a soft start circuit to avoid the bang and the flash.

I left the wires long and curly because I'll be moving the contactors. When they're in their final location, I'll cut the wires just right. Unless I keep them curly (never underestimate the aesthetics of curly wires).

I'll post a side-by-side comparison of the contactors some time. The GX14 appears to be safer in an accident.

I don't have enough room for a proper press brake, and it seems that a shop press brake adapter for my shop press isn't available in New Zealand, but what I have found, is a good way to break your vice. The Vice Brake is sold as being capable of bending 2mm mild steel, and while this might be true for small pieces, you vice will struggle to do a long bend in 2mm.

Enter the vice brake shop press. This works surprisingly well, with the vice brake seemingly very stable and not at all threatening to explode out the side.

Of course, even a small 10 tonne shop press is much more powerful than a vice. I used the back, less visible, end of the die to bend a small piece of steel and didn't supervice it sufficiently. This damaged the die, but it still copes with bending 3mm steel.

The other end of the die is undamaged.

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.

Not as serious as a reprap tantrum, my RS485 bus is not behaving very well. If I terminate it properly, it doesn't work at all, and if I don't terminate it at all, then it works, but charging at more than 10A causes a lot of extra characters to appear. Termination has two functions, first, it absorbs the energy in each character as it hits the end of the bus. Without terminators the pulses bounce off the end of the bus and travel back the way they came, causing interference. The second function of the terminators is to hold the bus in a relatively low impedance state, so any stray energy (say that created by the electric and magnetic fields generated by the charger) that gets onto the bus is absorbed without causing spurious characters.

Without terminators, I'm seeing extra characters, but with a 2 metre bus, I don't have problems with reflections.

So why doesn't it work with terminators? It turns out the transmit enable circuit cannot predict the future. With no cells transmitting, the bus floats, and both wires sit at 0 volts. When a cell enables it's transmitter, one wire rises to about 4V and the other to about 1V. This is how the bus should look when we send 0xFF:

And here is how it looks when the EVD5 sends 0x55:

Note the difference in the first transition, the the inverting line should go from high to low, but it goes from 0 to low. This is because the transmitter is enabled during the start bit, rather than a sensible time before the start bit.

I tried enabling the bus with a very short pulse (arrowed) before sending a character (note inverting is on the bottom trace instead of the top like above):

This almost worked, but I couldn't avoid framing errors -- the receiver sees the pulse and gets confused. I need to wait long enough for the receiver to assume the start pulse was not the start bit of some random character, but the transmit enable circuit times out and disables the bus before this happens.

There are two possible solutions, capacitive termination, and increasing the baud rate. More on this later.

Recently I visited Sydney and met up with some of the local EV community.

I saw Nathan's workshop at Convert UR Car and he was kind enough to show me a Toyota Yaris conversion in progress and even let me drive Sparky, his lead acid powered '92 Hyundai Excel. I can now say I've handled a germanium transistor, as he had to replace part of the control for his army surplus generator.

I also met up with Anthony Wood (who helped assemble my Mini in January) and Michael Day (who's converting an MR2 with an MES-DEA motor) for lunch and had a ride in his Anthony's (yet to be converted) Cortina.

Jaron Ware & Mark Taylor showed me their Formula V race car which was converted to electric some time ago. When I turned up they were installing an open source DC motor controller, a variation of Paul & Sabrina's design.

Mark also had a very tidy flooded electric van conversion:

Jaron told me stories about his trip to Lake Gardener to crew for the Catavolt team. Kearon de Clouet did 177kph on a Modified Partial Streamlined Electric Motorcycle for a new world record. Besides getting the record, they learnt a lot on this trip, it turns out you should transport your gear 2000km in a covered trailer, especially when the last half day is on a dirt road and it's raining. The dirt was especially prepared by the Australian climate to get into everything. Also, you should get a big trailer and make a mobile workshop, so you can charge and perform maintenance while waiting in the queue for the next run -- you spend most of the day queuing and a mobile workshop would let them get more runs.

Paul Tuffery from the Video Guys came to Motukea from Christchurch and Bill let him drill a hole in the KillaCycle!. These are the best videos I've ever seen of the KillaCycle. In some ways they are even better than being there in person, as he has each run from 3 different angles in glorious HD video. See the KillaCycle channel on Vimeo for 5 more videos!. Make sure the HD button is off if your computer can't handle the large files.

Killacycle. Standing 1/4 mile. 4th run. Motueka NZ. 6/2/10. from Paul Tuffery. The Video Guys on Vimeo.

Athol Williams made a house to keep the KillaCycle dry while travelling on the back of his little Stud Welding NZ truck. We really can't thank Athol enough, he's the New Zealand & Australian record holder on his 6.1s nitromethane powered bike. He took care of an aweful lot of logistics on the KillaCycle tour, and it wouldn't have been the same without him. During the first part of the tour, the KillaCycle travelled in his mobile race workshop, and he provided racing advice and support at Meremere and Nelson. It was a great weight off my shoulders to have his years of drag racing experience, as I could simply be pit crew while Athol took care of the track side with Bill.

We had to wait after a V8 powered mini took out some timing gear in an unscheduled bit of off-road adventure. He managed to get it back on the track, and was loose all day.

Scotty

entertained

with

a

burnout

Not everything went smoothly, with the KillaCycle refusing to switch the motors from series to parallel. The bike starts in series, putting up to 2000A through both motors, but in series, the voltage across the motors adds, so when they are turning fast, we run out of battery voltage to push the current through them. Scotty pushes a button to switch the motors to parallel, where the current divides and we get up to 1000A through each motor, but the voltage doesn't add, so the battery is able to do this even when the motors are turning fast. Unfortunately, the shift only worked on the first easy run and while testing in the pits, it wouldn't work on the track at full power. Without shifting, we only managed a best time of 8.81s in quarter mile.

We did do a quick turnaround for the last run of the day, doing two runs within 8 minutes. We didn't do anything to the bike, we just waited behind two other cars before we could go again. The KillaCycle can do about 14 passes, including riding back to the start line, without charging. Bill tells me that when they do showboat back-to-back runs, the second run is often quicker than the first, but there is a good chance of damaging a motor as they get too hot. This time there was no damage.

Nelson is going well, we've done 2 runs with the KillaCycle, and are about to do a third. The first was a slow 10.551s @ 115MPH where we found the track is very slippery out of the hole. In the second run we had a ground fault in the motors and the series-parallel shift didn't work, so 9.594 @ 136.37MPH was slower than expected.

We hope to do an 8s pass next.