BMS round one

I received my first prototype board from the fab house this weekend, and soldered it together.
The circuit worked the way it was supposed to, so I have ordered 3 more "final prototypes" that will have a few small changes, as well as a more polished look, with solder mask and silk screen and a slick oval shape. Once those arrive, I will probably borrow a few batteries and test the whole system.

TPS Report

Throttle Position Sensor, that is. I need a way to communicate to the motor drive that I want the car to go. Traditionally, electric car builders have used something called a "Curtis potbox", which is a variable resistor in a box, controllable by a cable hooked to the accelerator pedal. Trouble is, this device is made for controlling forklifts, and so isn't really suited to the abuse of thousands of stop-and-go cycles in a car.

Luckily, (newer) cars already have something very similar built in. The throttle position sensor tells the engine management computer to adjust spark timing and fuel mixture according to how much go the driver wants. It should be easy enough to adapt it to my purposes. The best part is, I got this one on ebay for $5. I think it came from a Ford. I 'm gonna keep all the useless-but-cool gasoline-related accessories on the module.

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: