Tuesday, June 15, 2010

Revenge of the Cap Kart

The Cap Kart; the most ambitious and the most unique vehicle in the fleet of the Edgerton Center Summer Engineering Workshop, an ad-hoc group that for the last three years has taken on projects "that you can ride." We built it to reduce our dependence on foreign natural resources go fast. The kart can't rightfully be called our flagship; that distinction belongs to a different vehicle. But it's the fastest and most powerful by far, and after sitting on the bench (literally) for a year, it's going to exact its revenge.

The kart gets its name from its defining feature, a capacitor of 110F/16V (yes, F) that stores braking energy and puts it at the driver's fingertips in the form of a boost button, similar to the push-to-pass KERS setup. It's made by Maxwell Technologies specifically for automotive applications, and it looks like this:

The "Cap" part of the Cap Kart actually worked more-or-less on the first try. In fact, the kart as a whole worked well enough to get in a few test drives, plus some good flywheel test data. But it was still a rough prototype. It was also very impractical to test, for a number of reasons: It was too heavy, too wide, and stored in a building with no ramp access. Moving it involved taking off all the batteries, going sideways through a door, up an elevator, out another door, and down a flight of steps.

Now that we have some new space with double-doors, wide hallways, a freight elevator, and loading dock access, many of these problems have gone away. In fact, it is feasible to take the kart out without ever lifting it now, which also means the batteries could stay on. Then again, of all the things on the kart, the most obvious target for improvement are the batteries. We started with three 79Ah deep cycle marine batteries (lead acid), each weighing a whopping 53lbs. That's 159lbs worth of batteries...or roughly half the weight of the kart without driver. And one of the three original batteries has since died. So, we're ready to move on.

...but don't worry, the old batteries have found a new purpose.

We talked about lithium-ion batteries back in 2008 when we made the Cap Kart, but at the time there were two big reasons why we opted not to use them. One: They were (and still are) expensive. The price has come down a good deal, even in just two years, but they are a big investment. Two: We did not have the experience working with lithium batteries to justify the investment. Lead acid batteries can take more abuse. Now, though, we're more confident in our ability to handle a low-voltage lithium-ion pack, so we're going to take it on.

Our cells of choice: the Thundersky TS-LFP40AHA. These are the same brand that the Mythbusters used on their electric kart. The cells range from about $40-80 each, depending on the source. We opted to get a kit, which includes pressure banding, bus bars, balancers, and a 15A charger. This was from Elite Power Solutions. They come in a nice thick cardboard box, shipped regular-old UPS (not freight or hazmat).


We never really had a range issue, so the minor hit in total battery capacity will not be a problem. However, the peak current output of this pack is probably a bit lower than what we've become accustomed to. I measured the internal resistance of these cells to be about 3.5mΩ, which agrees with The Internet. That puts the realistic current output capability at something like 120-200A (3-5C). Our motor/controller was quite happy (or unhappy) with 300A. It would be nice to match the two current ratings a bit better. (Though at low duty cycle it doesn't matter.) There may need to be some...buffering, of sorts. And there's always that capacitor thing, too. Getting the most bang for the buck has become one of our themes, and we're pretty good at it.

The very reasonable goal is to cut the battery weight by 2/3. In other words, all of the new batteries will weigh as much as one of the old ones. Since the batteries are half the weight of the kart, that decreases the kart weight by 1/3. The drivers weigh the same, though, so really it's more like 1/5. Decreasing mass by 20% leads to a 25% increase in acceleration (because math is funny like that), all else being equal. Hey now, wouldn't it be nice to have some better handling to go with that extra acceleration?

Here's the rear of the kart as-is. It's a live axle: the wheels and rear axle are one rigid body. This is great for traction...not so great for tight turning, especially on asphalt. One wheel has to slip, which can be made easier if it is lifted off the ground by a combination of weight distribution and chassis flex. But we added a ton of weight to the back and welded solid steel battery trays across the entire frame, so so much for that. Instead, we're going to add a differential. And hey, it gives us another chance to rip off Charles Guan.

The last target for improvement will be the control(ler). That is, both the physical and the software side. As much fun as it was to blow up TO-220 MOSFETs over and over and over, I think we're ready for something a little more serious. And to go with it, a new control algorithm that takes full advantage of the separately-excited DC motor in the field-weakening (again?) region. It should be able to more closely approximate the constant-torque, then constant-power curve for which electric motors are so well suited. 

Oh, and the best part is that I won't be writing the software this time! Nor will I be designing the differential. Nor will I be cleaning up the wiring. (Okay, maybe I will a little because I have OCD.) But yeah, my favorite project team is on it, so I get to mostly just chill. Oh and figure out where the heck we're gonna test this thing...

1 comment:

  1. Sweet! You're going to like the batteries. Up to 10C is possible in a burst. Can't wait to see what your team comes up with for the differential and the controller(s) ought to be cool (as I've come to expect!)