Volts, Amps and kWh’s
This is another subject that is discussed at length on all of the internet EV sites. I am an electrical engineer but found myself having a hard time understanding this, because in my mind Volts create Amps (or the other way around) and the two are inter-dependent with a very defined ratio, while on all of the expert-sites they are able to somehow manipulate both independent of one other. The only way I can wrap my brain around this is to more or less let go of what school taught me and follow what looks like the Basic Rule of EV conversion electricity.
That basic rule seems to be that Volts drive your top-speed, Amps drive your torque, and kWh’s drive your range. From there, let the controller be the magical thing that figures out whether the Volts or the Amps rule. All I need to be concerned with is that the controller takes the kWh’s and sends the right number of them to the motor to match the throttle-pedal.
In my application the primary design-consideration is range, then cruising speed & overall efficiency, and super-acceleration is a distant last. Running that through the Basic Rule says that I should
- Have a 90 kWh battery capacity. I chose old-style Tesla cells, they have a rated power of 3kWh each, so I need to fit 30 modules
- Have a voltage as high as possible that still fits the controller/motor. I am using the Azure motor(s) and controller(s), so I have to stay under 350 Volts for the traction battery
Each Tesla-battery is 57 Volts (nominal). To get as close to 350 Volts as possible I would need 6 of them in series. If I had 7 I would reach 399 Volts so that’s too much, and putting 5 in series would only get me 285 Volts so I’d be leaving 65 Volts “on the table”. With 6 batteries in series I achieve 342 Volts, as close to 350 Volts as practically achievable, so that point has been accomplished.
Having a total of 30 batteries and wiring 6 in series means I get a clean 5 in parallel so here too the math works out OK. If both motors go all-out they use 170 kW which is just under 500 Amps (at 350V). Those 5 batteries in parallel need to team up & produce that, so they need to be able to support ~100Amps each in peak discharge. In normal driving conditions it should be < 50 Amps. All of that is well within spec for these Tesla cells.
Placement of the batteries
Back in the day when this project was still more of an engineering study and less of a practical project, the idea of having to put 30 Tesla batteries in a Cadillac land-yacht sounded like a “what could possibly go wrong” kind of project. Surely that car is so big… Well, turns out that it is and yet it isn’t.
The car has an X-frame, and the big open spaces between the front mufflers and the rear axle are where the passengers get their foot-wells. There really is nowhere between the front wheels and the rear wheels to do an under-mount of anything. That means these batteries need to go in the back where the original secondary mufflers and the gas tank are, or they have to go under the hood.
So, that’s what I ended up with. I have a split battery pack with 12 cells in the back and 18 cells up front (in 2 layers, and I have 4/0 gauge wires running the length of the car along the X-frame to keep it all connected.
The charge-port goes where the original gas cap was (center rear) and the chargers are going into the rear fenders, so I have built a small module to hook the charging set-up into the major power lines, and it also contains the sensors for pack-voltage and pack-current for the central “brain” of the car. That module (and the brain) sit under the package.
In the front, underneath the battery boxes, I have built a master disconnect / fuse box. The master switch can be reached from below the car without any disassembly etc, but replacing the master fuses would mean removing the two battery boxes. That is less-than-easy just because of the weight and the lethal voltages, but it can be done and it should not happen too often unless I totally goofed things up…
Placement of the motor drives
To drive the dual Siemens Azure motors, I am using two of the matching Siemens Azure drives. They’re called the DMOC 645. There are (quite a few) other drives out there for these motors, and most of those are probably simpler to use because they come with actual vendor-support. The Siemens drives are out of the Transit-van bankruptcy, and they are what they are. They will handle are fairly high input-voltage (340V) and can drive the motors at capacity. They are built as nicely as the motors, they are water-cooled, and with the amount of power going through these units I think that is pretty much required.
The bigger issue (for me) with these drives is that they are BIG. I just filled up all rectangular space under the hood with 18 batteries, the motors are in their place and fit by 1/2″ either way, I can just-just fit the steering-shaft, this project is really tight already and now I need to house 2 large motor-controllers. The solution I came up with is to house them in the original 12-V battery space and sticking out inside the fenders. That means I have to make a small modification to the original fenders (bending over a metal rib), I am very much not proud of that, but it’s non-destructive / can be un-done so I guess it’s OK? It does make the whole setup look good from the front….
Battery monitoring module
This thing sits in the main positive traction-pack cable that goes from the rear battery to the service box in the front. It uses a Hall-type current sensor to measure Amps going into / coming out of the traction battery, and it has a Voltage-transducer that a) translates the 350V traction voltage into something the on-board computer can read and b) it isolates that voltage from that computer so it doesn’t blow up. I drew this up in CAD, and when I built it the model actually looked like the finished product:
The wires are in place but not crimped yet because while the placement is about right, it’s not final-final and I can’t really cut an extra inch onto the cable if it’s somehow too short. So, I’m leaving it dangling for the moment but the setup is looking pretty convincing.
Service switch/fuse box
This thing sits underneath the front battery stack. It’s actually super-awkward, but that’s where I had the space available and it keeps the cables out of the way / gives the cables a space all their own where they are protected.
The motor-drive manual is very explicit that one should NOT ever disconnect the traction battery. It uses a double safety where you (dis)connect the 12V power to the electronics instead, and once the electronics are plugged in there is an external “enable” switch. I think this way of doing things a) is more elegant and b) makes more sense than the typical EV-conversion setup with pre-charge resistors and contactors, so I (for now) don’t have those. The plan is to drive & charge the car multiple times a week so the tiny leakage from having the high-voltage connected to the drive will be negligible as far as range/drive-readiness go. I did include a pre-charge button (per side) for when I first fire this system up, it’s a just-in-case, but after that I do not intend to ever mess with the main disconnect switch unless there’s major maintenance going on.
I drew up a ton of versions of this thing in CAD because finding a box for this that was just the right size (space is limited yet I have to fit a lot of stuff inside) but I think the end-result came out OK:
Motor drive / DMOC modification
As I said before, the Siemens/Azure drives are big, and getting them to exist in the car forced some odd choices. In the end I got them to fit behind the head-lights / in the original 12V-battery space, but not by much.
Unfortunately, the original connectors for motor-feedback and drive-control are on (what in this case is) the top of the unit, and plugging in anything there would add “tall-ness” and meant the hood won’t go down anymore. It’s that tight. So, I’ve had to modify the DMOC’s to get the control wiring to work/fit.
The short version here is that the two original connectors got removed from their printed circuit boards. The connectors are what hold the circuit-boards in place, plus not having the connectors means there are now a couple large holes in the casing. I’ve 3D printed a couple plastic spacers/brackets that fill the holes, and to mount the circuit boards back (mostly) where they were. I installed a new all-in-1 connector in the side of the controller (thank goodness there is a little bit of empty space inside the box) and wired that in to the PCB’s. It came out looking OK, and once the control wiring of the project is done we’ll know if it all works…
An after (L) <> before (R) shot looks like this