This week we formally announce a project I’ve alluded to in the past – the conversion of a 2008 Cadillac Escalade EXT pickup truck to electric drive.
From the get go, we’re going to do this a little differently from what you might expect.
Our first choice is drive train and we’re going a little different direction. Our regenerative braking experiments were less than persuasive to me that this is an absolute requirement for an electric car. We would estimate the finished weight of this vehicle to curb at 7200-7400 lbs if we include sufficient battery power to move this vehicle to a useful range.
Moving that amount of mass is non-trivial. We’ve looked at a number of larger vehicle projects with 150 to 200 kw drive trains and I’m underwhelmed. The Tesla has a 200 kw system in 2800 lbs.
While we do not need to accelerate to 60 mph in 3.6 seconds, my sense is we need a Tim Allen approach with more raw power.
When it comes to raw torque, AC induction motors simply do not have the muscle per mass that a series DC motor has. You are “inducing” a current, and subsequently a field, in the rotor by proximity. It eliminates the brushes and commutator, but it simply does not produce the torque that a series DC motor does.
Permanent magnet motors are indeed interesting. The dirty little secret there is that the permanent magnets are not very permanent, and they become very non-permanent when exposed to heat. Heat is our pernicious and persistent problem area.
Series DC motors have the current flowing through both the stator and rotor. This gives each element a field of proportional power, and the torque developed is simply irresistible.
We’ve asked Jim Husted of Jimerico to do a special heavy duty build of two Netgain Warp 9’s on a single shaft, and mate it to our transmission.
The transmission is the second rather unusual choice. The Escalade has a fascinating transmission in it already – a 6 speed automatic termed a 6L80E coupled to an All Wheel Drive unit. The 6L80E is very strong, and has the unusual characteristic of having manual shift built into the tranmission controller, which is contained within the transmission. This allows you to manually shift this automatic transmission with a little up/down arrow button on the shift lever.
Unfortunately, it really needs the engine ECU to do all this, and to handle torque converter lockup. We’re removing the engine. We have very limited and not very successful experience fakijng signals from an ECU, and apparently NO ONE makes a stand alone controller for the 6L80E as yet.
We did hear quite a bit from our viewers on this and Tom ALvarey keyed us to the problems with the 6L80E. At his suggestion, we looked at the LENCO racing transmission series. In fact we contacted them and found them curiously uncooperative in this project.
Matthew Hauber, originally from Grand Junction Colorado was out in San Diego and had worked at a transmission rebuild shop for some time. He e-mailed us with some suggestions that became increasingly, and impressively detailed. They centered on a company from Ashland Mississippi called TCI and their 6X transmission. This is a 4L80E, a four speed GM transmission to which they have added a third control solenoid to activate overdrive clutches for the first two gears – effectively a six speed transmission. They also use some hardened parts and best of all, they have their own controller which is quite programmable. The net result is a transmission that almost bolts on in place of the 6L80E, has paddle shift manual control, and which can run entirely without an ECU including torque converter lockup.
This will give us enormous leverage, in fact torque multiplication, with a soft fluid drive that allows us to start the motors from 500 rpm instead of from stall. I think this is the way to initiate motion in a 7200 lb vehicle.
It still requires a lot of power. WE are going to try a pair of EVnetics Soliton1 controllers in this project – 1 controller per motor. No series/parallels switching. We are going to individually drive the two motors. This will let us idle on one motor at 500 rpm and go to both motors in tandem on acceleration. EVnetics has experimented with, and included an “idle” range for automatic transmissions and is the only controller maker we know of to do this. That’s not precisely correct actually. The Curtis 1238 does indeed have a “creep” mode that effectively does the same thing.
In this way, we can apply up to 1000 amps at up to 192v to EACH motor. We’re actually going to do a pack voltage somewhere around 180v. So with two controllers and two motors, we could in theory generate 360kW into this system. Netgain 9;s are probably not going to do that very long. But if we do NOT start them at stall, but rather from 500 rpm, we buffer it with the slip in the torque converter, and rather quickly get them up to our lockup RPM of 1900, I THINK they’ll do it for the 4 or 5 seconds necessary to get this vehicle firmly in motion. At that point, the power requirements should drop dramatically to continue acceleration.
That in turn requires a power pack that can deliver 180v at 2000 amps. We have not made final battery selection at this point, but if we DID parallel two strings of 52 CALB 180AH cells, they are ostensibly capable of 4C continuous or 1440 amps and of course 12C briefly.
To my mind, frankly, this sounds like the perfect storm – a superb opportunity to actually cause a lithium ion iron phosphate battery to explode. And as we all know by now, I DO like melting down batteries. But I prefer to do it in a controlled shop environment, not on the road.
So we are currently constructing a capacitor module of 72 Maxwell 3000F Ultracaps for a total pack of 42.66 farads and 194.4 volts. We are probably NOT going to encumber this with the byzantine trail of boost circuits so much the darling of the cap crowd. Rather, we are just going to parallel this pack with the cells. In this way, they will buffer our batteries, and should be able to produce the 2000 amps for about 4.5 seconds.
I’ve studied about six different papers on the use of capacitors to extend the LIFE of batteries, and I’m convinced that this buffer pack will extend the life of our battery pack to somewhere between 3 and 4 times their normal life span. And we’ll essentially negate the inherent voltage sag, in this case about 15-20% pack voltage, for a brief period.
The result should be dramatic improvement in acceleration, and dramatic improvement in battery life.
Unfortunately, it probably will NOT result in any extension in range. This is why capacitors never quite make the cut. This pack will cost just over $10,000 to construct, and will weigh over 100 lbs, and will be three feet long and 20 inches wide. The Elescalade will be the first vehicle we’ve done large enough to even carry a cap bank. For the same weight and cost, we could do another 40 miles of range. In this case, we’ll forego that in favor of life extension of our 60kWh battery pack, and to feed our Solitons to their maximum.
Yes, I know the Solitons can be configured to do 1400 amps. We won’t. The 1000 amp limit is there to protect the 1400 amp rated IGBTs. We WILL have a pretty serious liquid cooling system for the controllers, and indeed I’m planning a kind of monster blower system for the motors. But we want this system to last for a half million miles, and keeping that pad is an important element to do that.
I love the Escalade. And many Anmerican drivers just feel safer and more comfortable in a larger car. Our mission is to prove you can have all that, give up nothing, and still eliminate the gasoline and the emissions – and drive TOTALLY electric.
Could we go further with a hybrid? We don’t do hybrids. We do electric cars. It’s a line we don’t cross. No compromises. If you find yourself at a gasoline station pumping gas into your car, you are NOT driving an electric car. Period. That’s your first clue.
We think this will be a fun project. We think we’ll learn a lot. And we’re going to share it all, including the mistakes, quite publicly. Please do join us.