The Graphs is Always Greener….

This week we return to our 1957 Porsche Speedster Replica, the electric rebuild we call REDUX as it is the second version of the same car USING the same car. Recall we had upgraded this vehicle to 57 CALB 180 Ah prismatic LiFePo4 cells, a new version of Netgain’s Warp 9 series DC drive motor, and a new to the market Soliton1 Controller by EVnetics.

After a broad misadventure with our first Soliton1 installation, Sebastien Bourgouis came to examine the problem and replaced the controller with a new one. It indeed seemed stronger, first blowing a Stage II Kennedy clutch and on replacement of that, blowing a 400A fuse we had forgotten entirely.

To find out just how much was gained in a numeric sense, we returned to Slingblade Performance in Anna Illinois for another round of dynomometer testing on their Dynojet dynomometer.

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The results were interesting from a number of angles. Though young, the Soliton1 proves notably easy to configure, and now confirms a muscular presentation in the arena of the spinning of the tires. We hit about 156 peak horsepower at around 3200 rpm, and a clutch tearing 277 lbs of torque in the lower regions of the rpm band.

We had developed a bit of mistrust of the Soliton1’s data log. It was clearly reading 150 rpm off on our test bench, and as it turns out, it’s initial difficulties spreading the mustard were also entirely a measurement issue – not ours – but rather internally. It thought it WAS doing 1000 amps when our other best indicators were showing something in the 720 to 750 amp range. That’s a noticeable variance that needed to be investigated and adressed.

As it turns out, the measurement itself WAS the problem. But the new Soliton1 that Mr. Bourgois so graciously not only provided, but personally delivered, measured amperes with good attention to detail. All comparisons with the other test equipment we had available looked essentially identical.

Surprisingly, the RPM problem went away as well. We assume the same calibration procedural issues had caused the RPM variance as well. And at that point, the Soliton1 log file function became seriously more interesting. Once we had some confidence the values recorded were real world, the value of this data logging capability quickly became apparent.

Essentially, the major gain was the ability to correlate voltage and current information with the Dynojet data files’s horsepower information using RPM as the link as both machines record RPM.

This allows us a heroically finer level of graphing detail, as we can graph each mph of the run instead of every 10 miles per hour, and better we can correlate motor amps, battery volts, and more at this order of magnitude more frequent rate.

The leftmost column of the Soliton1 log is simply a timestamp and it indicates that the Soliton1 confesses its’ secrets each 20 milliseconds. This is a useful thing, we actually then have a clock on our car runs.

The first column to the right, let’s call it column two, is somewhat less useful. It features the percentage of available CPU cycles that are being used by the program. We have to assume the EVnetics software guru cares.

Column three indicates current in amperes. But it is’t real. This is actually the level of current, after your throttle calibration, deadzone, and mapping are all taken into account, that the Soliton1 thinks is commanded by the accelerator input. Picture this as a request for power.

Column four provides a readout of actual motor current in amperes. This value, an important measure of the power in the system, is not nearly as controversial as many of our viewers believe. And I guess I’ll have to explain that in more detail below.

Column five lists the duty cycle as a percentage. If you assume the entire period of time for one cycle at 8 khz, performance mode, or 14khz in quiet mode, is 100%, then duty cycle is that portion where the output waveform is positive.

Column six is the voltage of the battery pack. This will vary somewhat with load and indeed we saw the pack voltage sag as much as 22% during our runs.

Column seven is the internal temperature of the Soliton1 in degrees Centigrade. Almost all controllers monitor their internal temperature and when it reaches some higher value, the controller normally LIMITS the output current in an effort to reduce temperature.

Column eight is Revolutions Per Minute or RPM. This requires an external input from a magnetic or optical pickup reading the motor shaft rotation. This is important for our purposes because it allows us to correlate these Soliton1 values with the Dynojet values by finding the same RPM.

Columns 9-11 show the input voltage to IN1 through IN3, the general inputs to the Soliton1 that you can use for various things such as connecting the motor temp switch, etc.

Column 12 is the voltage of your 12v auxiliary system.

And finally the Soliton1 records a STATUS message indicating if it is running normally, is limited by slew rate, is limited by temperature, etc.

This wealth and granularity of information allowed us to graph in a resolution we’ve not been able to achieve before. We can take the horsepower output at each MPH increment and then use the RPM recorded by the Dynojet to lookup the same moment in time in the Soliton1 log. At that point, we can note battery voltage, motor amperes, temperature, and much more.

We published the graphs and put them on camera. The video wasn’t up a couple of hours before we received a note from Steve West of New Zealand that we had it all hosed up.

Indeed, we had used motor amps and battery pack voltage to calculate kilowatts – kind of mixing apples and oranges. And of course we used kilowatts to calculate eHP by dividing by 746. And we used eHP to calculate efficiency by comparing it to the HP output on the Dynojet. In this way, we can compare how much power we are putting into the system from the battery pack, to how much power we are getting OUT on they Dynojet to see our losses.

And this brings us right back to the motor current vs battery current that many of you believe is so very important. For the purposes of determining peak power output of the controller OR the entire drive train, it really really isn’t. But for this last calculation I threw in, the difference is significant.

The relationship between the two currents is largely a function of duty cycle. And it is quite proportional.

So to put out 500 motor amps with a 20% duty cycle, we need but 100 battery amps. Battery amperes will never exceed motor amps, as at 100% duty cycle the battery is basically connected to the motor. But motor amps can quite exceed battery amps.

The corrected graphs are depicted below. This grandly changes the calculation of KW. More importantly, if perhaps less grandly, it also changes our efficiency curves.

But while doing it, I might as well graph duty cycle and motor and battery amps separately. This leads to a bit of a busy graph at this point, probably beyond the level of useful. But it does show the relationship between battery and motor amperes.

We originally simply wanted to measure battery current to find our peak power output. A whole little army of people jumped up to try to “splain me” the relationship. I was quite familiar with it. The point was it doesn’t matter. If you stomp on the accelerator pedal, they are one and the same very quickly, and we will certainly see full power, and full current, out of any controller very soon. And we were not getting it originally. We were getting something like 720 amps.

You will note that the motor and battery current are quite different right up to the point of peak power, where the duty cycle first reaches 100%. From there on, they are precisely the same. That point is also our peak power point as RPM also builds with duty cycle and current. From that point on, we are starting to be reduced in current and power by the counter electromotive force generated by the motor itself. Notably, you can move this point further right along the RPM scale by INCREASING your pack voltage. And of course, you move it left and down by REDUCING your pack voltage.

There are a couple of things that jump out here. Most of you do NOT have a dynomometer and I would find it astounding if anyone went to the trouble we did to manually extract several hundred data point values out of the Soliton log and type them into an excel graph. Assuming your controller is working properly ITSELF, you really can accelerate your car at maximum speed, and read battery amps and voltage to get a pretty good idea of how much power you are applying. Volts x Amps = kilowatts/746 eq eHP. Multiply that by 85% and that will be pretty close to what you will get on a dynomometer.

Without having a “go by” it is VERY difficult to tell if you are getting full power seat of the pants without such calculation. One of the reasons we have spent so much effort on this is that we have found NUMEROUS instances of people with perfectly operational electric vehicles, that ran WELL. But they were experiencing a FRACTION of the power available in their system – which was artificially limiting their power output.

This one even got US as it was an unusual situation where the controller itself was at fault. The most common problems in achieving full power, in our experience:

1. Battery voltage limits. Note that our pack voltage sags as much as 22% during a full acceleration. Most controllers have a well intended and largely useless function to limit current to protect a low battery pack voltage situation. This is intended as a poor man’s “limp mode” and it almost never performs any useful function. But assume we had set this value to 160v. With a 192v pack this would seem logical. The controller would increase power until the pack sagged to 160v and at that point it would limit current. It is microprocessor controlled and can do this at a very fast rate. The result is such a smooth limitation of current, that it will hold your pack voltage at almost EXACTLY 160v, and vary the output current to do so. The result would have been MUCH less power than we see here in sagging to 147.

2. Temperature Limits. This is another very tricky one. First, again the controller can limit current in a very timely fashion to maintain it’s temperature limit. You will notice that temperature rises to the limit, but never really exceeds it. This is because the controller is making constant adjustments to output current to maintain this temperature. Again, you won’t really know you are current limited. Further, temperature in a controller is really quite a tricky thing. At these high current levels, it can change DRAMATICALLY and INSTANTLY. And so you will see a temperature spike ONLY when requiring maximum power. Let off the accelerator and temperature falls quickly. You won’t even know this is happening.

Indeed, in our fourth gear run we can see that late in the run we WERE actually temp limited. Fortunately, it was so late in the run that we were not making much current anyway so it had little effect. But understand, we have a Toyota Prius pump squirting 20 liters per minute of glycol through the Soliton1 and thence through a pretty good sized heat exchanger. True, the car wasn’t actually moving and so there was little air flow, but the point is that we intentionally OVERBUILD our cooling systems.

One of the myths of EV’s is that they do not have the failures of components like ICE cars. Unfortunately, they have regular component failures and the most common is the controller. You can not only achieve maximum power, but GROSSLY extend the life of your controller if you keep it cool. There is no heat sink to large. We have developed a profound preference for liquid cooling of all controllers. And in the event of a controller such as the Curtis 1238, we actually manufactured a liquid chill plate to mount it to so we could have liquid cooling. And we entirely avoid the cheesy little kits most of the controller guys suggest. We use automotive pumps of at least 20 liters per minute, expansion tank fillers, high efficiency exchangers and quality hoses and fittings. It is always a thousand dollar expense. But it avoids power limitations and more importantly extends the life of the controller.

Finally, temperature is local. There is rarely a lot of heat energy to deal with, but it is very localized. The “case” temperature of your controller isn’t the issue. It is at the IGBT itself. It drops about a volt and a half and at 1000 amps that’s 1500 watts. Good internal design is important, but you cannot overcool this device. In fact, we suspect if you could cool it to about -400F, the volt and a half might go away.

3. Throttle input. The controller’s primary job is to convert your throttle input to motor control. If you have the maximum throttle input set at 5v, but your throttle really can only provide 2.8v full pedal, you’re going to be disappointed in your car. The car is fine. The configuration of throttle input is not fine. The controller basically maps the available power across this throttle input range. And 2.8 v is a little past half power. If you set the throttle max value in the controller to 2.7, then you will get full power BEFORE reaching the end of the pedal. Some controllers have byzantine configurations for this with different throttle TYPES and a MAP to alter the curve of applied power across the throttle input. You must become familiar with those configuration items.

Those appear to be the three most common power limiting mistakes. This has been the only event we’ve ever had where the controller itself was a problem. But if you have one of the Soliton1’s, you might want to check. If they do not have now, they will undoubtedly have soon a procedure for you to follow to determine if you have a calibration issue I’m sure. But you might check with EVnetics to see what it is and check it. Ours sure got better when it was replaced. But again, the pernicious thing about this problem was the controller operated PERFECTLY in all respects and the car drove VERY NICELY and without actually measuring the current, we would have NEVER known how much power we were missing, if we had not measured.

Jack Rickard