It's nice out, and I'm done with finals, so I took out the RC car again and made a few improvements. Then I crashed it and undid some of those, but don't worry it's not too bad!
First, I changed the batteries. I had been using A123 26650 cells. These are LiFePO4 cells that are most abundantly found in 36V DeWalt drill batteries. In fact, since they are virtually unavailable to the general public, the cheapest way to get them is actually to take them out of drill batteries. I'm sure there are significantly more 36V DeWalt drills than there are batteries for them at this point. Each pack contains 10 cells, and can be found on eBay for roughly $100, so about $10 per cell. Wasteful...and still expensive.
A less expensive solution is lithium polymer. Sites like Hobby King sell these pouch-like batteries for more like $5 per cell, for an equivalent capacity. They're 3.7V nominal, instead of 3.3V, and they come in prismatic packs, so overall they have a higher energy density than LiFePO4. The 30-40C discharge versions are comparable in power density as well. The one downside is that they tend to burst into flames when abused...
A compromise, then. Hobby King also makes LiFePO4 prismatic batteries. (We call them HK123s now.)
For $70, you get 12 cells. That's just under $6 per cell. Now, these packs are decidedly hit-or-miss, based on the reviews. They might arrive with one or more cells completely dead. In fact, the one I ordered had two dead cells. :( But since I was going to cut it open anyway, I wasn't too upset. Taking the packs apart is a real pain, but I don't imagine it is significantly harder than taking apart a DeWalt battery. One snag I ran into was that the positive terminals are aluminum crimped to nickel tabs. I ripped off the nickel, not realizing its importance, and had to resort to aluminum soldering to connect the cells. (Yes, you can solder aluminum.)
Questionable soldering job.
I rearranged the cells into 6S1P, same as the old RC car pack. Putting it together is easier than putting together cylindrical cells, which almost makes up for the difficulty of taking it apart. All the balance and power leads come out of the same side, and it makes a nice compact package that is easy to heat shrink. Or, if you don't have any heat shrink, caution tape...
Notice the size difference.
The cells themselves are almost the same weight as A123 cells. (They should be, being the same chemistry and capacity.) So I only saved 122 grams (6% of the total car weight), mostly in packaging. But since they are prismatic, they stack nicely and take up much less volume. I measured their internal resistance to be about 13mΩ, which is a little higher than an A123 cell but not much. I fully expected it to be able to handle the 40A peak current on the car. Here's what it looks like on the car:
Step two in the RC car upgrade was implementing field weakening control. Setting up the full explanation of field weakening would take one or two whole posts, so here I will just get straight to the point. The car uses half of my 3ph Duo brushless ESC, which is a fully functional AC controller that treats everything as a vector in the rotor frame of reference. The voltages applied to the motor are balanced three-phase sine waves, which leaves only two degrees of freedom: magnitude and phase.
Under normal operating conditions, the controller sets the phase wherever it has to to place the current vector right between the magnets, using feedback control. This is the point of optimum torque production, the least amount of current for the most amount of torque, and varies with speed and load due to inductance. The magnitude then just sets the magnitude of torque.
In field weakening, the controller sets the phase ahead of the normal operating point. There are many ways to think about what this does. In one way of thinking, part of the current vector is now opposing the permanent magnets a little, weakening the field that the other part of the current vector interacts with to produce torque. This reduces the back EMF of the motor, allowing it to spin faster at a given voltage magnitude, but with less torque. It effectively changes the motor constant, almost like an overdrive gear. Hey now, where have I seen this before?
I wish I could provide a more thorough analysis of the field weakening capabilities of this controller, but in all honesty it gets a little messy. For one, the unit frequency that the controller uses to do all its calculations is the PWM frequency, which is 14.5kHz. With the motor now spinning at 45,000rpm, or 750Hz, the resolution of everything is shot. Furthermore, lag in the measurements starts to become significant and the rotor frame may not even be in the right place anymore. This controller was designed for direct-drive scooter motors that spin at 1,000rpm...so I'm happy if it doesn't go unstable at this speed.
Time for testing! Here's the baseline, a normal "full-throttle" acceleration with no field weakening:
It gets from 0-30mph in 1.9 seconds, pushing about 34A (peak) to the motor. The top speed is 34mph and the peak power output (to the ground, m*a*v) is about 200W. Notice that by the time the motor has hit top speed, the voltage phase is a full 42º ahead of where it would be at no load. This keeps the current right between the magnets, lined up with the back EMF, for optimum torque. At this phase angle, the power in to the terminals of the motor is about 330W. So from the motor terminals to the ground, a system efficiency of 61% at peak load.
Now with field weakening:
The acceleration profile up to 30mph is identical, taking about 1.9s. Then, at 30mph I hit the "boost" button. The controller tries to put some current on the axis that fights the magnets (negative d-axis). It does so by advancing the phase to 75º, sacrificing current on the q-axis (torque producing current) to get there. So, the acceleration slows, but it can get to a higher top speed of 44mph. Maybe you just need to see it:
That ant-looking thing is the car. The sound is probably the more useful measure. With this kind of motor, field weakening is fairly limited, but a 30% increase in top speed without changing anything other than software is still much appreciated. That is...until you hit a pebble at 42mph...
Unfortunately, I do not have video of the crash, but I do have the car's last transmission:
Clearly I had half a second to attempt to slow down and regain control. I probably needed another half second to do so, though, because it still hit the curb at 27mph. This time, something happened to the radio before the battery came out. (You can tell because the voltage never drops.)
Yep, antenna got ripped out.
That might be fixable with some solder. The brand new battery did indeed fly out as well, landing several feet away. Here's the aftermath:
But even the bashed-in cell is okay.
That's a big check in the durability box for the HK123's, if you ask me. The only thing that really broke was a tab on one of the cells. And it wasn't the aluminum solder joint; the tab just sheared off completely. Good thing I have four spare cells from the original pack.
The car itself was totally fine.
So in summary:
- HK123s are a viable and fairly rugged LiFePO4 solution, if you don't mind disassembling and reassembling a pack. More compact and less expensive than A123.
- Field weakening is cool, until you hit a rock.
- I should find a wider test location.