The Need for Speed

The other day I enjoyed my first trip in the HOV lane!  Here in AZ if you have a 100% alternative fuel vehicle you get access to the carpool lane even if it's just you in the car.  To make it better, it was a Friday afternoon during rush hour, so I got to zip past all the suckers in the regular lanes while they were all going 25.  Oh yeah and I've achieved a new max speed of 72 mph.

There's a guy at work who converted a Ford Fiesta to be electric about 20 years ago.  But due to his max speed and acceleration limitations, it was nicknamed the "Siesta."  Over the time it's taken to build my car, I've been teased with the need for a similar nickname for my car, but I've been able to show that's unnecessary.  And speaking of acceleration, I've been getting the itch for a little more recently.

WARNING: The rest of this post is getting a little nerdy (okay, a lot nerdy).  If you don't like math, this might be your cue to punch out early!  I've had a few new inquiries about how this works, so it seems worthwhile to go over all this.  I hope it helps some of you out there!

During the initial testing phase I settled on a few current settings that I've yet to change.  The controller has 3 parallel IGBTs rated for 400 amps each.  Without having to reprogram the controller, I can change the max current up to 833 amps in 8 increments.  I limited the controller to output a max of about 417 amps to all but eliminate the possibility that an imbalance in the load sharing could blow one up.  I also limited the battery current to 166 amps for no real reason.  Since the car works just fine like this, I didn't want to mess with things until I got the car registered to avoid a situation where I'd have to make that 48 mile trip to downtown Phoenix on a minimum amount of testing since if something broke, I probably wouldn't have it fixed until after the 30 temporary plate had expired.  Now I've got no more excuses.

According to the specs, going from 400 to 500 amps at the motor will increase the motor torque from 67 to 100 ft-lb.  That's a 50% increase in acceleration!

http://www.go-ev.com/PDFs/003_09_01_WarP_9A_Graph.pdf

Now, the way it works, just increasing the motor current will improve torque at low rpm, but once you reach a certain motor speed.  The controller adjusts the % of time the switches are turned on (duty cycle) to control the amount of current going to the motor.  But the motor requires a certain amount of voltage to go a specific speed.  According to the chart, at 72v and 400 amps the motor will spin at 2400 rpm, whereas at 500 amps it will only spin 2150 rpm.  Furthermore, at 72 volts and 400 amps to the motor, my 230 volt pack will be supplying approximately 125 amps.  At 500 amps to the motor, the batteries are supplying 155 amps.  The controller allows the motor to produce a constant torque (corresponding with the motor current) until the duty cycle is 100% or the max battery amps have been reached.  Since I've set the controller to limit battery current to 166 amps, the rpm at which the motor torque will drop off will be quite a bit lower when the motor current is bumped up.  To compensate, I'll probably need to increase the allowable battery current.

At 166 amps, the battery pack voltage sags to 215v and provides 35690 Watts.  At 400 motor amps, this is 90 volts.  To figure out the associated motor speed, you look at the chart and see 400 amps lines up with 2400 rpm at 72 volts.  The speed constant is supposed to be linear with voltage, so the max rpm I should currently get at 67 ft-lb torque is 90 / 72 x 2400 = 3000 rpm.  So this means that the motor torque will be constant up to 3000 rpm, and above that the torque will start to drop off.  In vehicle speed, that's 16 mph in 1st gear.  Do the same thing at 500 amps and you'll see the torque starts dropping off at about 2100 rpm or 11 mph.

In order to get the torque to be constant (flat) up to 3000 rpm again, I need to increase the max battery current.  Here's how I figure how much.  According to the chart, at 500 amps and 72 volts, the speed is about 2100 rpm.  3000 / 2100 x 72 = 103 volts.  103 volts x 500 amps = 51429 Watts.  51429 W / 207 volts = 248 amps (I had to do a little iteration to figure out 248 amps causes the pack voltage to drop to about 207 volts).  This is about a 4C peak discharge rate (248 amps / 60 Ah = 4.1C), which should be a piece of cake for these batteries since the datasheets have charts going up to 5C continuously.  But before I do that I'm going to check the torque on all the battery connections.  A loose connection turns into resistance, heat buildup, and a molten battery terminal that's more probably the higher your discharge current goes up.

I've harped on this before, but I love stringed instruments so much I'll do it again!  A lot of people out there want a car with huge acceleration, so they get an 11 inch motor (more torque per amp than my 9 inch motor), a 1000 amp controller, and 100 volts worth of 180 Ah batteries.  You will get lots of torque this way, but not for very long, resulting in a disappointed EV owner who's scratching their head.  The same math I showed above applies here, except it gets worse with a bigger motor because the motor.

If you look at the datasheet for the Warp 11 motor, at 500 amps and 72 volts the motor is turning about 1300 rpm.  If the controller is at 100% duty cycle (100 volts at the motor and batteries), the motor will spin 100 / 72 x 1300 = 1800 rpm at 500 amps.  In my car, that's only 9 mph.  Making some assumptions your torque vs. speed will look something like this:
0~5 mph - max torque (0~1000 rpm)  actual torque at 1000 amps ???
9 mph - 165 ft-lb (1800 rpm)
12 mph - 75 ft-lb (2300 rpm)
15 mph - 50 ft-lb (2850 rpm)
20 mph - 24 ft-lb (3800 rpm)

I don't know what the torque of a Warp 11 is at 1000 amps, but you can see how quickly the torque drops off with this type of setup, requiring you to UP shift gears to increase your torque.  Even though the gear ratio is working against you, you need the lower RPM to increase the motor current (which is what gets you the torque).  The only way to beat this is to have a high battery pack voltage to keep your duty cycle from maxing out.  If you have this controller and battery option, you may be better off with a 9 inch motor (or smaller).
Warp 9 with 100v battery pack supply and 1000 amp controller:
0~13 mph - 200 ft-lb (0~2500 rpm) this torque is an estimate
16 mph - 175 ft-lb (3050 rpm)
20 mph - 47 ft-lb (3800 rpm)
31 mph - 17 ft-lb (6000 rpm)

Alright, ENOUGH boring stuff and back to the testing!  I hooked up the laptop and punched in "t-pos-gain 5" and my controller now maxes out to 521 amps to the motor (I'm sure you already knew that because t-pos-gain / 8 x 500^2 / 300 = max current).  Like I mentioned before, that roughly correlates with 100 ft-lb of torque out of the motor, where before I was getting about 67.  Man, does it make a difference!  I can also see how I need to boost the battery current limit.  In about 1 second you can feel the jerk, and for those of you who forgot your days back in Physics 1, the term "jerk" refers to a change in acceleration (wow, I'm feeling nerdier every second here).  So in about a second, my battery current limit is pegged at 166 amps and the acceleration quickly drops off, just as I'd predicted.

I've driven the car around like this for about 25 miles now and so far the controller hasn't blown up.  It's really easy to hit the battery current limit now, so I'm not getting full use out of the 521 amps to the motor.  Since this post is already way too long (man I love talking about math) I'll end this here.  And if you have any questions, please ask!

Burning Rubber

The other day I proved that my electric car has enough power stored up to spin the tires.  Unfortunately for me, it was a total surprise!  We'll start with a little back story.  Whenever I start moving the car, it tends to bounce forward and backward slightly.  In most geartrains, there is a certain amount of backlash and wind-up that is the cause of this.  At first it seemed a little odd to me, but I've convinced myself it's not because of the controller.  I'm fairly sure the reason a regular car doesn't do this is that the engine is always spinning, and small amounts of friction keep the whole geartrain loaded up.  In the electric car this isn't the case, so when the motor starts moving, it kind of bounces against the load of the vehicle since it momentarily spins up without any load and runs into a brick wall when all the "slack" is taken up.

Okay so enough back story.  The other day I put the car in reverse and am convinced I can keep this bouncing from happening if I ease onto the throttle slow enough.  I'm pushing on the pedal, a little further...a little further...then all of a sudden SKREECH as the tires tear loose! I quickly pull my foot off the pedal and the car stops a split second after it started.

Now at this point I'm worried that I already blew up the controller that took me 3 months to build a short 9 months ago.  The way these controllers work, the unfortunately most common failure mode of the switches is shorted, or full on.  If that happens, that means you get 100% battery power to the motor, which in my case would be 230 volts and likely over 1000 amps.  The switches are likely not rated for this kind of power and they blow up a few seconds after your car tears off into another car in the parking lot.  In my case, either the controller hardware protection, the feedback control system, or my cat-like reflexes limited it to a foot long stripe of rubber in my driveway.

After a little while I determined everything was okay and I drove to work without another hiccup.  Now I'm asking myself why this happened and how do I avoid it in the future.  I actually had this happen one time before a month or so back, but I thought maybe it had to do with the bouncing.  I came up with another idea which turned out to be true, and here it is.

The controller gets a signal from a potentiometer that changes resistance as you push the pedal down.  The controller reads the resistance and equates that to a % throttle input.  There's a dead band at both ends of stroke to account for adjustments in the linkage and a few other things.  At the zero throttle position there is a limit switch.  I use that switch to turn a contactor (big switch) that feeds battery power to the controller on and off.  The purpose of doing this is that if something goes wrong, I can take my foot off the pedal and it will immediately cut off all power to the controller - exactly the scenario I just came across.  Many people eliminate this and would have to fumble around for a manual disconnect to stop the wheels from screeching, but my method is second nature to most drivers.

Now the controller takes that throttle position and uses it to command a certain amount of current to the motor.  If you set it up to control a max of 100 amps, then at 10% throttle position it will adjust the PWM duty cycle to get 10 amps.  The switches turn on and off 16000 times a second and PWM duty cycle is the percentage of time that the switches are turned on.  More crude controllers would make 10% throttle position drive 10% duty cycle, but that causes the car to drive very abnormal compared to a regular car.

So here's the problem.  Remember that limit switch?  Well, I have to push the pedal down a short ways before that switch clicks and completes the high voltage circuit to the controller.  Well, on these two occasions I had eased into the throttle so slowly that I uncovered a problem with how I'd set things up.  The deadband before the current starts to ramp up is not quite big enough, so if done just right, the controller starts trying to drive a certain current before the contactor closes.  My attempt to ease into the throttle really slowly allowed this to happen, and I took so long the duty cycle must have risen higher than it's ever gone.  When the contactor finally clicks, that duty cycle is set until the controller senses the current has gone too high and rolls it back.  Of course, by then the wheels are already in motion.  (ha ha ha...)  Like I said before, luckily there are enough safeties in my system that the burnout only lasted a split second instead of tearing me out into the cul de sac and possibly wrecking the motor or car.  Even more lucky, the IGBTs were tough enough to endure this event unscathed.  And thanks to the design of the controller, after a couple minutes of snooping around I was able to reprogram the end points so the dead band is now big enough to work the way it's supposed to.  Something I should have done when I first set it up!

In other news, I got my cycle analyst back from Canada and am working on putting it back in the car.  While I'm at it I decided to fill in a gaping hole in the dash where there used to be a speaker grill.  Here's the hole in the dash.

And here's the old grill and new cover that'll house some LCD screens and buttons.  Now I just need some flat black paint.

My Audience

Last week was a bit of a cliffhanger, so I think I need to finish (or at least update) that story.  You may remember I had a nicely charred connector going into the back of my fuse/relay box.  After a staring at it for a while I finally concluded that this wire was the 12v supply for my headlights.  Makes sense since the headlights don't seem to be working.  I clipped that wire off and plugged the connector back in and everything's working identically to how it worked before.  All I need now is the right bullet connector to put that wire back in and hopefully my headlights will work again.

In other news, the blog recently passed 10,000 views!  I had no idea it would turn out to be so popular.  The pace is getting ever faster too because in the last month alone I've had nearly 1,000 views.  It's also interesting to see where people are who are checking it out.  Some of the top hitters are United States, UK, and Germany, which all make sense since I know people in all those locations.  However, there are tons of people checking in from places I don't know people from like Latvia, Russia, Sweden, Australia, Ukraine, and the list goes on.  Even more fun than that, I get occasional emails from people with questions or comments, like my new penpal from the Ukraine who is converting his own Porsche 924!  If you've ever thought about it, please feel free to send me a message.  I love talking about my project, just ask my wife!  On to the latest progress...

This is a good time to check what's left to do (not that it'll ever be 100% done).
- I still have not finished installing and programming the BMS
- I need to install the DC-DC converter
- The charger could use some more optimization to boost output and efficiency
- There are a number of cosmetic things that would be nice to fix

For some reason I've been ignoring the BMS for a while.  Probably this is because it would be nice to have a solid day or two to install, debug, and calibrate the whole thing.  Even though it really shouldn't affect things, I feel like anything hooked up to the HV system shouldn't be in a state of debugging for an extended period of time.

Not having the DC-DC converter in has been inconvenient for a few reasons.  First, that would have avoided that first breakdown, and second, it's a pain to have to charge the 12v battery every night.  I actually started installing this just the other day.  I've got all the wires attached to the converter and mounted it, I just need to make the connections to the HV and 12v batteries now.  This is a Meanwell HRP-600 12v power supply.  It can put out up to 53 amps, and costs only $130, which is a steal compared to the DC-DC converters you see on EV websites that are "designed for EVs" that cost hundreds of dollars more.  Here it is with the wires hooked up and before I put it in the car.

The two little wires go to the HV battery +/- terminals, and the 10 gauge wires go to the battery terminals and the body.

Speaking of breakdowns, I also mentioned a while back that after the 12v battery died on me I still had problems after charging that battery.  I believe if figured out that problem.  In the process of troubleshooting, I wiggled the connector that plugs into the control board in the motor controller.  I'm pretty sure that either the 12v or ground pin on there was loose.  The wires are probably a little too short and were pulling on the connector.  "Soon" I will replace those wires with longer ones and recrimp the pins.  If the problem comes back I'll just solder them directly to the board.  For now it seems to be working after going back to the old wire.  I've had no problems in the last 50 or so miles.

The charger probably will get another look when I install the BMS because I'll have to reprogram it to accept a BMS input.  Until then I'll settle for 4.3 amp charging rate out of a standard outlet that doesn't seem to trip the breaker (unless I'm also trying to use my electric lawn mower).

As far as cosmetic work on the car, I replaced all the hardware for the sunroof.  Now when I take it off, the wind dam doesn't want to dangle in my face while I drive!  The next thing to do is get a new interior light that's dangling in front of the rear view mirror from its wires.  That and the lack of a driver's side sun visor and window crank knob are really annoying!

Well, I've taken up enough of your time for one post, plus I need to put the kids to bed!  Next time I'll tell you about all the problems I'm having with my cycle analyst.

Good News, Bad News Part 2

Good news everybody, the Porsche is finally registered!

The other good news is I can finally charge the car without having to open the hatch and the hood.  I installed this spiffy outlet where the gas tank fill spout used to be.  I also got a nice 12 gauge cable so I don't have to worry about overheating the wires anymore.

Now for the bad.  I also had my first breakdown!  I was driving to work and I suddenly lost power.  Luckily I had enough momentum to easily coast into a parking lot.  I realized the voltage on the battery was way low and quickly remembered that I haven't charged the auxiliary battery in about 80 miles!  I conclude that the power to the controller was so low that the brownout feature on the microcontroller shut it down before it started malfunctioning.  My kind and loving wife picked me and the battery up, I hooked it up to the charger, and took the jeep into work.

After work I pick up the battery and my wife drives me back to the Porsche.  I plop the battery in and away I go.  I'm about 7 miles from home and after 5 miles, the car starts shutting down again.  Now I'm not feeling so good because the voltage still reads okay.  I figure out that if I take my foot off the pedal, the controller seems to reset and I am able to limp home.

After thinking for a while, a friend comes over and holds the laptop for me while I drive.  The same problem keeps coming up, and luckily I get a clue from the RTD program.  When I lose power that the controller sends restart info to the RTD program, so it appears that the controller is momentarily losing power.  When it regains power it goes into "high pedal lockout" mode.  The controller will sense if you have your foot on the pedal when it starts up and won't allow the PWM signal to start until you take your foot off.  That explains why I can lift my foot off the pedal and keep going.

So now, what's causing this?  I've got a few theories and I try running a dedicated wire straight from the battery to the controller.  Luckily, this seems to work!  I drive 4 miles and have no problems compared to at least a dozen restarts in the previous 4 miles.

There's a relay I put in to power all the stuff I added (controller, pumps, fans, etc.) and it turns on with the ignition.  I've had problems from the beginning that the instruments seem to have a bad ground or something.  A fresh 12.5 volt battery only reads about 11 volts.  The headlight motor doesn't work either (forums tell me that anything less than 12 volts will cause this to not work) and when the headlights go on, the voltage drops even more.  In fact, it seems like turning on the blinkers would cause the controller to cut out.  I'm thinking the voltage gets low enough that the relay loses power and flips off momentarily.

I've known I need to fix this for a while, and it seems like now is the time.  I checked all the ground connections, and those seem to be good.  I look at the fuse and relay box and find a couple of loose wires!  Unfortunately, it is not obvious at all where these go, but it doesn't appear that they have anything to do with grounding or the headlights.  I snoop around for a while, and suddenly I see this!

Gah!  I'm pretty sure this is one of the 12v power supply lines going into the fuse and relay box.  This must have been loose for a while and the poor conductance built up heat and eventually fried this connection.  I have no idea how long it's been like this, and to my knowledge I've never messed with these connections.  I'm not sure how I'm going to fix this but I'll have to reroute power to the fuse box.  Hopefully this is the problem!

Let's end on some good news.  I've racked up 235 miles so far, so I've almost saved an entire tank of gas!  I can feel the savings rolling in!

Officially Electric

I finally achieved one of the major goals today.  Here's my story!

In order to register your vehicle, you have to have your car emissions tested.  Unfortunately, not having a gas cap or any of the emissions equipment the car was originally equipped with, I can't pass this test even though I'd pass the intent of the test.  So I basically have to have the emissions test waived.  To do that, I have to get an "Alternative Fuel Certificate" issued for my car.  To get that, it must be inspected, and unfortunately, that cannot be done any any old MVD office.  In fact there are only two offices in Arizona, the closest of which is 21 miles from my house.  I guess it could be worse if I had to go to the Tucson office.  My design of the car was for a 50 mile range, so that doesn't leave a lot of room, especially when you consider you can't test much until you get the car registered!

Luckily, there is a 30 day temporary registration option that I got back in mid June.  I've racked up 137 miles of testing since then, the longest trip being about 25 miles.  One thing I learned is my range is theoretically greater than I was planning.  The car is more efficient, ranging from 200-240 Whr/mile depending on how many lights I hit, and I was designing around 275-300.  Another way to look at it is I get a little more than 1 mile per 1 Ah on the batteries, and since they're 60 Ah batteries, I should get 60+ miles.  Of course I haven't really tested this yet, but it makes me feel a little better.  Also lucky, my friend, Joe, lives about 6 miles from the inspection office and he offered to let me charge a couple hours before I made it rest of the way home.

With my 30 temp plate expiring on Sunday, I took the Porsche on the journey this morning.  The most direct way to get there is on the highway, so I decided to add that to my list of new things to do for the day.  The car had no problems speeding up and keeping up with traffic.  I even got up to 70 mph at one point!  I pulled into the inspection area after consuming 19.5 AHr.  Three guys hop out of a little hut and come check out the car.  98% of the questions they ask have nothing to do with the paperwork they needed to do, and one guy even pulled out a camera for a few pictures!  One of them said they hadn't seen an electric conversion in over a year, which surprised me.  I'm guessing the electric vehicle tax incentive that went away 18 months ago has something to do with it.  5 minutes later I'm back on the road with my embossed certificate!

6 more miles to Joe's house and I stop to add about 14 Ah back into the batteries.  Then I'm headed back home.  About 2/3 of the way home a sickening feeling enters my stomach as a terrible melted plastic scent fills the car.  I feel slightly better when I see a truck pulling onto the shoulder with his hazards on, and much better when I pass him and the smell goes away.  10 minutes later I'm safely home with 48 miles under my belt for the day.  I figure I'm about 30 Ah down, so it'll take about 6 hours to top the batteries off again.

Now I just need to make a stop at the MVD (yes another one) to get the registration taken care of.  I'm hoping to get one of those alternative fuel license plates with a custom number.  I'll let you know what I decide on.  18 months, and the car is finally legal!

More Driving Stats

I've had the car registered for almost 2 weeks now and have racked up 90 miles of driving. I had some clients from Boeing in for a review at work this week and I got to impress them with the car.

Efficiency tends to be in the range of 210-240 Wh/mile depending on how many times I have to stop. I've got RTD explorer working and tracked the temperature of the controller. On a 103 °F day it got up to a peak of 129 °F at the end of an acceleration from 0-50 mph and quickly drops down to 124 °F. If I coast to a stop at a light, it's down to 117-120 °F within a few seconds and seems to stabilize there. So I'm guessing the temperature of the coolant in the system stabilizes to around 117 °F or less (<15 above ambient?) since it's no different after 11 miles of driving.

I've been experimenting with charging. I don't have a heavy duty extension cord yet, so I have to string two regular (probably 16 awg) extension cords together. Originally I had an old computer cable pigtailed to the inlet wires in the loop too. That was bad because that wire was 18 awg and it got hot! All the internal wiring is 10 AWG in anticipation of wanting to charge at 20 amps or higher. If I charge the car at 5.0 amps, the clamp meter reads 14.5 or so Amps rms in from the house 120v line, and a 15 amp circuit breaker would eventually trip. So I've either had to drop down to 4.0 amps or run a third extension cord through my garage door and plug into the 20 amp outlet in the laundry room. Not great for charging when it's 118 °F outside. At 4 amps it takes about 5.5 hours to charge 22 miles (around 21 Ah). At 5.3 amps it's down to around 3 hrs 50 min. When I get a 240v 30 amp outlet installed and am brave enough to try it, I should be able to charge it in about an hour if the charger doesn't get too hot.

In the meantime I've been working on the BMS. The head board is complete and one of the slaves is complete. I've been modifying the code Joe sent me and think it's about ready to test. Unfortunately I completely missed a bunch of connectors in my order and have to wait until July 2nd for Mouser to get them here, so I can't do any testing until then.

So with no car work that needs to be done, I'm focusing on a few things I've been neglecting around the house. My wife is really happy that I cleaned out the garage and she can park in there again! I also tried my hand at cleaning the air conditioner to try and make it more efficient. The bozos who installed the unit in my attic put it in backwards so the access panel faces into a wall! I've had problems with it not draining properly because there's a bunch of sludge built up in the drain pan that I've never been able to clean out. Well, we've got a ton of family coming over on Sunday and it's supposed to be around 118 °F out so it needs all the help it can get so I bite the bullet, got up at 5:30 this morning and head up in the attic. After nailing down some boards I can lay down on and an hour and a half of cursing I managed to get the access panel off. Another hour and I've vacuumed all the sludge out and sprayed down the evaporator coils with some cleaner. Hopefully it was worth the effort. It's currently 115 °F outside and 76 °F inside. So far so good but the forecast is for 122 today.

I've only got two more weeks on my temporary permit to test out the car. I'd love to have the BMS functioning by the end of that before I take the 40 mile trip to the MVD, so hopefully I'll have enough time over the 4th of July weekend to make that happen.

Stay cool!

Free Parking

Today was a momentous occasion for me, I finally drove all the way in to work and back!  My employer has  a solar array in the parking lot and they lease shaded parking spaces under there.  But there are a few fortunate people who get free spots for 100% alternative fuel vehicles, carpooling, and hybrid vehicles (in that priority).  I pulled into my spot this morning even though I didn't have a permit yet (they've had one set aside for me for about two months now, but wouldn't give me the permit until I started driving the car) and within 3 hours I already had my first parking ticket!  Luckily there's no substance to the ticket, and I now have my permit so I can park legally from now on.

Here are the stats from today's drive.
According to Google it's 11.2 miles each way, so 22.4 miles total.
Total energy consumed: 4792 Wh
Energy per mile: 214 Wh/mile
Theoretical max capacity of the battery pack = 3.3x70x60 = 13.68 kW-hr
% Battery consumed: 35%
Max battery amps: 162 A
Minimum battery voltage: 216 V

Unfortunately, RTD explorer apparently experienced a user error and I didn't get any data from the trip other than when it first started up.  Mostly I was wondering how hot the controller would get.  It started out at 104 °F and finished hot to the touch, but not so hot you couldn't keep your hand on it.  So maybe 120 °F at least on the outside?  Not too bad.  Now I get to sweat through 4 hours of charging.  Speaking of sweating, pretty soon I'm gonna need to get the AC working!