Batteries...

With the bulk of the mechanical work done, it's time to start thinking about the electrics. I've chosen the Lithium Iron Phosphate batteries from Thunder Sky, which don't catch fire like some lithium batteries do when overcharged. Nevertheless, the batteries can be damaged if they are overcharged or overdischarged. In a long string of cells wired in series (I'm using 96) some cells are inevitably stronger than others, and will end up being overcharged. The string voltage remains constant, so this means that some cells get severely discharged (even negatively charged). Both of these destroy the affected cell, so I need a way to keep the voltage of each cell between 2.5 volts and 4.25 volts. There are several ways of doing this. Charging each cell individually, then monitoring all the cells for undervoltage during discharge is the best way, but constructing a charging system to individually charge 96 cells, then connect them all in series to the drive circuitry would be a difficult design challenge, as well as a wiring nightmare.

Instead, I'm taking a simpler approach. Each cell will have a shunt regulator across its terminals. A shunt regulator is a device that conducts no current until the voltage across it reaches a set value. When the voltage rises above the set value, the device begins to conduct just enough current to maintain the voltage at the set point. This takes care of the overvoltage protection. Undervoltage is a bit trickier. I think I've come up with a clever way to signal the controller to shut off when the voltage of any cell drops below the setpoint using a single circuit (instead of 94), but I need to develop it a bit first.

The idea of building 96 circuit boards is a bit daunting though. Each board will have ~10-15 components and cost about $5 for parts. It only adds about 5% to the cost of the battery system, so is well worth it, but spending my evenings hunched over a soldering table doesn't sound like fun. Maybe I will send it out for assembly... with 100 pieces, the prices should be pretty good. I just need to prototype one or two to get everything right.

Back on track

I finally finished machining the new hub, tearing apart the clutch to get the spline, welding the two together, and putting the motor back in. I fired up the motor, and everything worked fine. I enjoyed sitting in the car listening to the motor & transmission turning for a while...

Setback

I finished the subframe and got the motor properly aligned with the transmission (not so easy) and bolted everything down for a live test (without the driveshaft installed-- no driving through the shop wall). I rigged up a 300-volt DC power supply to run the motor drive, just to make sure it wouldn't complain about running off of batteries instead of its normal diet of 3-phase 240 Volt AC. It worked great. On to the powered test of the motor/transmission assembly. I ramped up the speed of the motor slowly, and everything was fine. I took things up a notch by quickly accelerating the motor, to simulate an actual load. Suddenly, the adaptor hub that I had spent a few evenings turning on the lathe twisted right in two. Oops. I had followed the design guidelines given by Lovejoy, the shaft locker supplier in machining the hub. The lock holding the transmission to the adaptor hub works by compressing a thin tubular section of the hub onto the shaft. I had my doubts that such a thin tube would be strong enough to carry the 150-220 ft-lbs (200-300 Nm) of torque Lovejoy advertised. It wasn't. Although, strictly, the shaft lock device was still securely in place, so I guess the book value was right. That's a little bit like building a ship in a bathtub, though...

Nevertheless, I shall continue! The next plan is to snag a splined hub from an old clutch disc, and attach that to the hub that is still locked on to the motor shaft, using bolts or perhaps my new bowhunting TIG welding skills.

It's a motor!

After quite a bit of sweating and head scratching, I got the motor into place.
Don't worry-- the C-clamps are temporary. The engine bay looks cavernous, but soon enough it will be crammed with batteries.

This style of design and fabrication is quite a change from my job, where everything gets designed using computer tools and precisely manufactured. This is more the "Orange County Choppers" school of metal work, only with a lot less swearing and throwing things. I sometimes wonder about those chopper shows on TV, whether they have a whole sweatshop full of 3D CAD guys sitting in a poorly lit room somewhere, swilling mountain dew and churning out chopper designs.

Subframe

To make the motor, transmission, and car more or less one unit, I need to build a subframe. This will also serve as a platform for the front battery boxes. It was difficult to measure everything outside the car, so I decided to install the transmission, block it up to where it should be, and build the frame to it. Here's the transmission in place with the first cross member:
This is my first stab at TIG welding, so the welds aren't going to win any beauty contests. I'm happy with the overall result, though. Next, the top half of the subframe:

Mr Motor, meet Mr Transmission

I have been out of town for the last couple of weeks, so I was excited to work on the car all weekend. The shaft locks arrived while I was gone, so I set to work attaching the motor to the transmission. I had one small setback. The motor is a metric 112M size, which means that it is 112 mm from the base to the center of the shaft. It also should mean that it has a 28mm diameter shaft. For some reason, though, this one had an 1 1/8" shaft. This was a problem because the shaft lock I bought is 28mm. No worries, though. 28mm is smaller than 1 1/8", so I just turned the shaft down on the lathe.
This was good, as it gave me an excuse to take the motor apart.

Next, I test fit everything, and ran the motor for the first time attached to the transmission


Everything ran smoothly, so time to start putting it all in the car!

Shaft locking

Since I finished pulling the engine, I have been working to source parts to couple the electric motor output shaft to the transmission input shaft. Instead of using a splined or keyed shaft, I will be using keyless shaft locks from Lovejoy, combined with a hub I will machine myself. The keyless shaft locks work by drawing two concentric wedge profile rings together with jack screws. As the rings come closer, they wedge against the outside of the shaft and the inside of the hub, to form a very tight fit. I will post some pictures of this once the parts arrive.