emini

Revenge of the Electric Car is on at the Documentary Edge Festival in Auckland and Wellington. The director, Chris Paine will be speaking at the opening night (tomorrow! 26th April!) and on the 4th of May. I'm going along to the 4th of May. Unfortunately it doesn't look like he'll be attending the Wellington showings.

Ed came over and helped me make a temporary battery box for the rear seat. This lets me evaluate performance with 40 more cells, for a total of 76. Knowing how well the car performs at higher voltage with a larger battery lets me decide how hard I need to work to squeeze cells under the bonnet. The intention is still to retain 4 seats in the car, this box is just an experiment.

Building this battery box was harder than it sounds. The seat base is full of curves so the bottom of the box needs a strong base, and holding the cells down with their dangerous high voltage terminals very close to the edges is not trivial. I'll post a picture of the lid which was key to holding the cells down shortly. The box is made of wood which is more flammable than I would like, restraint will be achieved with ratchet tied downs attached to the seat belt anchors.

In other news, the new cable connecting the inverter to the motor has resolved my inverter instability. I'll blog more about this cable soon.

Previously, I reported that my Simovert motor controller was happy in the rear of the car with the motor in the front of the car. It turns out this is not the case. While going up and down the driveway did not uncover any problems, going up and down the road has. The inverter shuts down with a range of errors:

• GateDriveUnit
• LCA Err Latch
• Overcurrent
• VC_Over_Voltage

I am hoping that this is caused by a combination of poor motor phase cable routing and poor motor-inverter grounding. It turns out that the Siemens supplied motor cables have a shield which is grounded in the cable glands, effectively joining and extending the casing of the motor and the inverter to envelope the motor cables. Such a shield will do a lot to control noise radiated by the motor cables. My motor cables had none of this and worse the motor was not well grounded -- I'm told that non-trivial current can flow between the motor and inverter through the ground due to "induction effects". Regardless of how real this is, properly bonding the inverter ground to the motor ground seems reasonable.

I am solving this problem with a 50mm² 3 phase neutral screen cable and appropriate glands. The very stiff underground rated cable I have isn't really appropriate and in hindsight, I should have found a supplier of the right cable, but that story is for my next post.

In 2008 I bought a battery. At the time, I knew that I was buying a product with a shelf and cycle life (it wears out even if you aren't using it). I didn't expect the battery to sit around for as long as it has, but here we are 3.5 years later. I did a capacity test of the 36 cells installed in the car and found a usable capacity of 33.5Ah: The nameplate rating is 40Ah which is substantially more. There are several possible causes of this discrepancy:

• Calendar Life
• Cycle Life
• High current abuse
• High temperature abuse
• Differing test methodology
• Low capacity when delivered

Calendar life and differing test methodology will certainly have an effect. I am charging to 3.55V at 1A which is substantially lower than thunder sky's recommendation. My pack is moderately closely top balanced and at the end of discharge, the highest cell was just below 3.1V while the lowest cell was at about 2.8V at 10A discharge. This suggests the cells have varying capacity or are not properly balanced. I also tested 10 cells which have not been cycled or abused with high current or high temperature. The pack in the car has had perhaps 10 cycles, a lot of it discharging at 3C for 5 or 10 minutes at a time. I limited my maximum current to 200A or 5C and kept currents above 120A to short 5 or 10 second bursts. This is not too far from Thunder Sky's specs but does cause significant battery heating. In the summer I have measured over 40C at the battery terminals. The 10 cell pack that sat on the bench has not been subjected to high current or high temperature, probably never getting higher than 25C. This pack achieved 34.3Ah and showed less spread in capacity, with the hightest cell at 3.0V while the lowest was at 2.85V: That these two packs show very similar capacity suggests the high temperature and high current and cycling had little effect on the battery in the car. Cell voltages were measured with the EVD5 BMS, discharge current and Ah counter performed by my EVision and recorded from it's CAN bus by the BMS data logger. The car's battery was discharged with two domestic 240V heaters and a kettle (which nearly boiled dry). The small 10 cell battery was discharged with a steel wire on a wood form in a large bucket of water. The 10 cell battery shows a larger spread of voltages under load because i did nothing to prepare the cell terminals before putting making the connections, the terminal connection resistance was all over the place.

On Thursday, Mike Duke & his Waikato University team's electric car will join Bochum University's solar car on a trip from Auckland to Bluff.

They're starting at the domain in the morning, but it's not exactly clear when they are leaving. I'll be there to see them off, if I get wind of the departure time I'll update this post.

I seem to have successfully moved my inverter to the rear of the car. This will allow me to put a much larger number of cells in the front of the car and improve the weight distribution (I'm currently rear heavy). I found connectors to extend the motor's encoder cable and Ed made one up using some special cable he had left over from a job. I replaced the terminal cover on the inverter with a mounting plate for the new Gigavac GX14 contactors (replacing the large Schaltbau 162 units that wouldn't really fit anywhere), precharge circuit and the EVision shunt. This has a number of drawbacks,

• quite a lot of effort is required to inspect the terminals under the cover
• the positive battery cable connection is partially obscured below the boot opening and is close to the body, working on it is a little scary
• a rear end crash could cause major problems
• I haven't worked out how to form a cover

I may yet move this contactors plate above the inverter or to the right of it.

Pictured is my current test implementation, there are a number of things that need fixing,

• there are some very thin copper bus bars that need to replaced with thicker bars
• the jumpers between the contactors and the inverter are much too thin
• the fuse should be much closer to the battery
• one of the phase cables is completely unsuitable (about 10mm2 total from all three conductors of a household electric oven supply cable (including the oven's on-off switch!)
• the low voltage wires need some tidying up
• putting everything on top of an insulator and standing some insulation between the various components might be a good idea
• I need to make a cover

This worked well for going up and down the driveway, well enough to make me feel confident the inverter will live in the rear of the car without being effected by noise escaping from the phase cables.

I did have one mishap while testing. I know you have to keep the loop area small in the phase cables (this is a clever way of saying "keep the phase cables close together"!) and Ed repeated this several times while I was fabricating, but when it came time to actually put it together, well, I forgot. The inverter was most unhappy to have one phase cable go over, one go under and one go around the side (the latter two going out the boot aperture). It refused to run the motor smoothly with no load and then it blew the 30A test fuse! I re-directed the cables to all go over the inverter and through the hole in the front of the boot and everything was smooth and happy. My testing so far is limited by the thin wires in both the battery and motor circuits but I think (and hope) I put enough current through the phase cables to prove it will work. The encoder cable runs down the side of the car while the phase cables run down the middle. Shortly additional battery cables will also run down the middle, hopefully this won't introduce problems.

This Sunday 8th May is the EV Builders Expo 2011 at TAPAC, 12 - 6 pm. I'll be there (possibly even with my car, I'm working through mounting the inverter in the boot). This should be a good show, my friend Phil explaining how he will go racing in his his electric Saker sports car, F40 Motorsport designed my adapter plate so it will be good to catch up with them, and the Formula E electric go karting guys are good value too.

Theo, the organiser is making an EV documentary which I'm keen to see (not least to find out how our interview went) and he's building an electric Toyota Sera. This is a fund raising event and there is a $20 cost which goes towards finishing the Sera.

There is an electric truck for sale on trademe at the moment. It appears to be an industrial 3 phase motor with a 336V lead acid battery. 20km range and 45km/h top speed. The vendor isn't supplying too many other details, but it looks to be a bargain at the reserve price. If it was closer to home I'd go have a look, but Whakatane is a bit far.

Last time I talked about the call for feedback on the LVV Standard 75‐00(00) Electric and Hybrid Vehicles Draft #5. You can read my feedback below, unfortunately it doesn't make a lot of sense without the questions so you might want to have them too.

• Q1 Yes.
• Q3 Yes, a plug such as an anderson disconnect should be allowed in place of a switch and such disconnects should be allowed in a battery compartment when non-venting batteries are used.
• Q5 Yes, Over current devices should be allowed within the battery compartment(s) Why would they be forbidden within the battery compartment? 2.2(1) (c) allows flame proof switches and relays within a battery compartment, fuses should not emit flames if they are of good quality and used within their ratings. 2.2(5) NOTE (d) recommends fuses near the electrical middle of the battery, but 2.2(5) (b) forbids fuses within the battery compartment, making this recommendation harder to follow in some cases.
• Q9 A manual ground fault detection system should be mandatory, with an automatic system preferred. I have not had time to find a suitable automatic system so would hesitate to mandate an automatic system.
• Q10 I agree that a simple single potentiometer design is not safe enough. This is a problem as some existing and in-progress conversions likely only use a single potentiometer throttle. I believe a single potentiometer with broken wire detection and a "pedal up" switch inhibiting the traction system is sufficient, with a dual potentiometer preferred. Would a single hall effect sensor be sufficient? My own vehicle has a single potentiometer with broken wire detection, which I am uncomfortable about. Is this sufficient? I will likely add a pedal up inhibitor, but the details of doing this are not trivial due to the design of my control electronics.
• Q12 No.
• Q13 Yes, with an explanation that a proper pre-charge circuit will protect the main contactors
• Q14 No, I think it sufficient to require cables be marked with the voltage ratings by the manufacturer or documentation provided to require this.
• Q15 Yes, but not to the same level as the FIA. The FIA's "insulation resistance to ground" test is equivalent to the ground fault detection requirement in 2.4(1)(f). 2.4(1) NOTE 3 suggests that a ground fault is permissible so long as it is "safe". The FIA's 250k or 500k Ohm requirement is not possible with a brushed motor or flooded lead acid batteries. A maximum ground fault current should be allowed, so higher voltage systems must maintain a larger resistance to the body. For example a 150V system with a 5k Ohm ground fault will have a maximum ground fault current of 30mA, while a 300V system with the same ground fault will have a maximum ground fault current of 60mA. I am unsure what is achievable in a flooded lead acid battery vehicle with a brushed DC motor without super-human maintenance. Note well that testing the insulation resistance should not be done with a multimeter set to the resistance scale -- a low resistance ground fault will likely destroy the meter.
• Q16 Yes. Live parts should be covered by strong materials.
• Q17 Yes, but 5 seconds is too fast and will cause the discharge resistor to run hot while the vehicle is in use. 1 Minute would be safe. Siemens placed a warning to wait 1 minute before servicing on my controller.
• Q18 No. This requirement has several parts. Most obviously inappropriate is the requirement that the battery cell manufacturer attest to the safety of the battery. Most lithium battery packs are assembled by the owner, so the manufacturer is unlikely to be available for comment on the battery pack. Note the preceding wording requires the cell manufacturer manufacture the cells, but does not comment on assembling the battery pack. A requirement that the cell manufacturer assemble the battery, or that the cell manufacturer attest to the safety of a customer assembled battery would prevent most or all electric conversions using lithium batteries. Requiring the battery management system monitor the temperature of the battery is a good thing, but it's not clear how many of the commercially available battery management systems do this. Requiring batteries that are prone to thermal runaway to retain the manufacturer's monitoring and safety system is a good idea but doesn't say anything about such cells that are supplied without any monitoring and safety system. The Battery Management System issue is highly contentious at the moment. It is clear that a bad Battery Management System is worse than no battery management system for Lithium Iron Phosphate based batteries. Mandating that there be a battery management system may or may not cause more problems than it solves with this chemistry.
• Q19 No. This is already covered in 2.4(1) (a). I don't believe any additional instrumentation is required to warn of danger during maintenance since the battery box already has a high voltage warning on it, and turning the car off does not make the battery safe.
• Q20 Yes, you already do in 2.2(1) (b).

In addition to answers to the numbered questions, I have the following comments:

• The Note in 2.2(2) suggests typical operating temperatures of -10 to 40 degrees C. This is incorrect. Most electric motors operate happily with internal temperatures well over 100 degrees C.
• 2.4(1) (c) requests a "handbrake on" indication to the driver. Why is this required? For cars that did not have this indicator as original equipment, this is a fairly complex requirement. Is the presence of a handbrake lever next to the driver in the "handbrake on" position sufficient to satisfy this requirement?
• The last sentence in 2.4(1) NOTE 4 should read "This should be done between the body and several electrical positions within the battery, for example at the most negative battery, the most positive battery somewhere in the middle of the battery. If the body is non-conductive, then ground fault detection testing should conducted between the battery and all metal structural members near the battery." We want to detect a ground between any potential within the battery and the body (which, if it's all-metal, is always at the same potential). My wording around non-conductive body issues is somewhat clumsy, sorry.
• Section 5 "Ground fault detector" definition is wrong. We are not looking for an "imbalance in current between the energised conductor and the return neutral conductor since:
  1. there is no such thing as a neutral conductor in an electric vehicle
  2. if current flows in the ground fault, we already have a potential fire
  What we are looking for is a connection between the battery and the body of the vehicle. No current flows through the first ground fault. I would word this section as "means a device that detects a connection between the battery and the body of the vehicle. Such a connection will allow a current to flow through the body of a person who is simultaneously touches the body of the vehicle and part of the battery during maintenance. A second connection between the body and different part of the battery will allow current to flow through the body, potentially causing a fire or damaging the battery."

In New Zealand, modified vehicles need to be signed off or "certified" by an engineer. In the 1990s an effort was made to create rules governing the certification of Electric Vehicles but it was never completed and the rules are now quite out of date. The Low Volume Vehicle Technical Association have been updating these rules over the last year or so, addressing a number of shortcomings with the old draft. They are now publicly requesting feedback.

I've already had a hand in the new draft document and I'll be posting my feedback here (and to the LVVTA) in the next few days. If you are building an electric vehicle or thinking of doing so, you should read the draft and submit feedback by the end of the month (8 days!).