Tuesday, January 10, 2006

Rant: The EV Magic Box

As I look at all the stuff necessary to convert this car to electric power, I'm coming to realize that a lot of the existing parts available are sub-optimal. Like most things in a market economy, they are more of a result of who can make money making what than parts that coherently fit together as a whole. What would be ideal would be some kind of magic box that would integrate AC motor control, battery charging, a 12VDC power supply, and an optional fluid heater into one enclosure. AC Propulsion's AC-150 system comes close. It integrates a first-rate AC motor controller with a high-power charger (with excellent power factor) by recycling the magnetics and electronics of the drive system, and it integrates the DC/DC converter. It is also designed to integrate cleanly with a vehicle that wasn't originally designed to be electrically powered. But the system also has its shortcomings: it is far too expensive (at US$25,000) for an EV hobbyist, the enclosure is too bulky to integrate cleanly in many donor cars, and the minimum nominal battery voltage is too high for a lightly-modified vehicle. It seems to me, however, that these shortcomings can be addressed, primarily by reducing the power level. A hundred and fifty kilowatts may make for good publicity, but most people don't expect that kind of power from their vehicle, particularly given the phenomenal torque characteristics of an AC system. An "AC-75" would be a much more practical system for everyday use[1]. The second change I would make would be to water cooling (in fact, nearly everyone without Alan Cocconi's rumored chip on his shoulder would probably make the same change :). While you can argue that an air-cooled system is simpler, smaller, lighter and cheaper, a suitable heat-rejection system is already installed in nearly every conversion donor car, and water cooling makes the packaging of the motor and electronics much simpler. So I'd replace the two large heat sinks and fans with cold plates, some hose, and a small pump. Finally, the minimum 336 volts are simply too many. Depending on battery chemistry, it's either too much weight, too much cost, or both [2], although this may change as lithium battery chemistries mature (at 3.6 volts per cell and with their excellent weight characteristics, both the cell count and weight could be reasonable). If the minimum nominal voltage were roughly half that, it would be much more accessible for people using mid-priced batteries like Optimas. I don't know enough about their Reductive™ charging topology, but lowering the voltage to 180 or so should still allow charging from a 120V source. Alternatively one could go the route Toyota has gone with their newer hybrid systems and create a bi-directional DC/DC converter to raise the voltage that's practical for batteries to one that's practical for motor control. But the cost would probably be hard to justify at hobbyist EV volumes. Another place to save money would be on the RPM range, since most donor vehicles retain the original transmission. Shrinking the torque/speed range to something resembling that of an internal combustion engine could provide some opportunities for further cost savings. One side advantage of such a system is that there is exactly one termination point for the high-voltage wiring: the magic box. The only exception would be a resistance heater, but it may be possible to integrate that with the water cooling (with Peltier devices or a bigger cold plate enhancing the heat rejection of the electronics) so that a fluid heater could built into the unit. In the end, the cost for the combo would have to be about $5,000 at retail. Without a large market to recover the NRE costs, it's probably not economically feasible. But maybe some sort of open-source-style development process could bootstrap things, especially if it was a BSD-style license that allowed commercial interests to embrace and extend it. [1] I can understand why AC Propulsion is reluctant to produce such a beast: there's not a lot of money to be made in EV conversions, and at that power level they would be competing directly with DC systems that can advertise better performance. [2] If you're optimizing the cost and performance of the motor/drivetrain pair, this is probably pretty close to the optimum. But if you're optimizing for run-of-the-mill donor cars, this is at least twice as high as is practical.


Blogger Dan the Man said...

Hi Frank,

I just came across your blog. I am new to the EV world and interested in converting my Type 3 also.

Have you looked into the suspension issues involved with the torsion arms? I get the impression that the EV is heavy, that as a rule of thumb the batteries will weigh as much as the donor car (?!).

Do you think this means new torsion bars will be required to suspend the car?


9:15 AM  
Blogger Frank said...

I think "as much as the donor car" is overstating things a little bit. The rule of thumb I've heard (and this was for flooded batteries) is that the final weight should be about one-third batteries.

Since the non-battery electrical components typically weigh less than the ICE parts they're replacing, I'd plan on a final weight between one-and-a-quarter (with AGM batteries) and one-and-a-half (with 6V flooded batteries) of the donor car's curb weight.

As for how to correct the ride height, I'm looking into it (see next post).

6:54 PM  

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