Before I knew it, Summer had turned to Fall, then Winter was upon us. Next thing you knew it was Y2K, and what do you know? The world didn't come to an end! I was glad the world didn't come to an end, because I hadn't finished the EV. Be a shame for the world to come to an end, and not finish this project.
What a project it's turned out to be! If I'd known a year and a half ago I'd still be working on it... Anyway, it's great fun, though admittedly expensive. I made a New Year's resolution to get the car on the road this year, so the trick is figuring out how to finish the car without going deeper into debt.
If I don't get the car done soon, it's in danger of becoming like Quincy's yatch. Do you remember the old TV show "Quincy" with Jack Klugman? When he had free time, he was always working on this yatch he had in dry dock. Throughout the whole series, he was always working on the yatch, and it was always in dry dock. They never showed him with the finished boat on the water. I definately want to get this car out of "dry dock" and on the road.
Above you'll see a picture of the radiator battery box. What's cool about it, is there are batteries in it! I put 4 used 6 volt golf cart batteries in the rack for a total of 24 volts. Since I didn't have the cable or crimping tool, I used automotive starter cables to connect the batteries. I charge the batteries two at a time using a 12 volt automotive charger.
Here you see the main contactor. Since I'm working with an ON/OFF system (no controller) and low battery voltage, I use only one contactor. The finished product will have two. To provide 12 volts for the car's lights and accessories, I use two of the four batteries. In the finished product you wouldn't want your battery pack in common with the cars 12 volt wiring, but for now, since I'm running at low voltages I do it.
Here you see my shunt, which I borrowed from EVAmerica (at this point I need to pay him for it!). The leads from the shunt go to a prototype board with some Op Amps to amplify the signal and then feed it into an A/D converter. A microcontroller reads the A/D converter and scales the value into Amps, and sends it via an RS-232 interface to a laptop PC. This is the beginning of my SOC monitor. I could just buy an E-meter, but this is too much fun for me to skip.
Below you'll see some actual data from my circuit. The data shown below are the samples taken when the main contactor was engaged, starting the motor from a dead stop. Data on the left are amps. Data on the right are volts.
Amps,Volts
00,24.8
00,24.6
249,20.1
249,21.6
191,22.1
116,22.5
92,22.5
85,22.7
70,22.8
60,22.8
41,22.8
You can see that when the contactor first engaged, the current spiked to 249 amps. Actually it probably was higher. 249 amps was full scale for my initial design. I didn't think the current would get higher than 250 amps with only a 24 volt pack. You'll notice the voltage read 24.8 volts with no load. When the current peaked the voltage sagged to 20.1 volts. As the motor rev'd up current dropped all the way down to 41 amps while pack voltage rose to 22.8 volts.
I also have a name for my car: it's the "Runsonly." One of my students joked that he had a friend who had a car like my EV, and called it the "Runsonly," because it "runs only" when I push it! He figures my EV will run only when I push it, too. So in the same American spirit that took the term "Yankee doodle" and turned it from a term of disparagement to a term of endearment, I christen my EV the "Runsonly."
Radiator Battery Box Finished
Designing the Firewall Battery Box
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