Electronics Tutorial 1: Overview of the Electrical System
Gerry Chen
Written: Jun. 26, 2018
Last Updated: Jun. 26, 2018
The first step in designing an electric vehicle's electrical system is to hash out a general flow of energy and to size the most important bits appropriately. To illustrate how to do this, I'll walk through our vehicle's design and explain how to calculate some of the important numbers.
Motor
The motor is the primary load and, because everything else has to be able to give the motor what it needs, we should start here. To be perfectly honest, I don't know enough about motors to give much input here, but I'll ask Patrick to chip in. I'll work under the assumption that whatever motor you choose, you know approximately how much power it needs and the voltage it likes to run at.
Battery/Power Source
The next logical step is to choose a battery, power source, or other energy storage unit which is appropriately sized. From past competitions, we know that we generally draw an average of 50W on a bad day, or ~25W on a good day. Assume we want to be able to drive for an hour at this rate, we'd want something like a (1hr)*(50W) = 50Wh battery. Our motor is rated for 24V nominal, so we'd be looking at something like 24V and 2Ah. Thus, we run on a 3300mAh 7S lipo battery. We also have to check that the battery is capable of supplying enough instantaneous/peak current. Lipos generally have very good discharge characteristics and rated current is well above what we need.
Depending on the energy demands of your system, you may use multiple batteries in series or parallel. Keep in mind that any time batteries are combined in series, special care must be taken for cell balancing. Care must also be taken to avoid over discharge (under voltage), over current, over voltage, and over temperature conditions. Some battery packs have protection circuitry built in, while others don't. Make sure to do research on the specific battery type you choose to operate safely.
If using a power source such as a hydrogen fuel cell, make sure it can provide the power you need at the voltage and current you want. For example, if you know that your vehicle will need 500W during short bursts to accelerate and your motor needs at least 24V or else it won't operate, you will probably want to consider a 1000W-2000W fuel cell @24V nominal, or 750W fuel cell @ 36V nominal so that it won't dip below 24V. Power sources drop in voltage as you ask for more power, so keep this in mind when selecting power sources and try to obtain I-V curves from the manufacturer when possible. Find the point on the I-V which corresponds to the power you need at peak (P = IV) and check that the voltage is adequate.
Hybrid Energy Storage
I'll take a moment to discuss energy storage for hybrid systems as well, since this was a large factor which contributed to our success at Sonoma.
In a hybrid system, a more energy dense (i.e. hydrogen) but slower generating (i.e. fuel cell) power source is combined with a less energy dense (i.e. battery) but high power/dynamic power source. To elaborate, one of the reasons gasoline is so ubiquitous in vehicles despite the relatively low efficiency of an ICE is due to the high energy density of gasoline and the ability of gasoline engines to generate lots of power on a moment's notice. In contrast, fuel sources like hydrogen are much more slow to convert to useful energy. A car which goes 10mph all the time is not very useful compared to one that can easily transition from 0mph to 60mph depending on what the driver needs. The natural solution is to have a high-power, dynamic device working together with the hydrogen in order to produce lots of power when needed, but maintain the energy density and efficiency of hydrogen otherwise. In our system, we use supercapacitors as our high-power energy store due to (1) its very good charge/discharge characteristics (doesn't lose much energy if you charge/discharge it super fast, peak current is very very high) (2) its long lifetime in terms of charge/discharge cycles, and (3) it being allowed by the Shell Eco-Marathon.
In the hybrid system, we continuously use the fuel cell to charge the supercapacitors at a rate of ~50W (average power consumption over the course of the race). The supercapacitors then supply all instantaneous battery to the motor, which typically consumes a couple hundred watts for 5-10 seconds a few times per lap. To size the supercapacitors, we need to know what voltages are acceptable and approximately what our power consumption will look like during the race (in the case of a practical application, a general idea of its typical use case). We know that one lap takes around 3-4 minutes, and we'll be generating an average of 50W. We think we want enough energy to go around half of the track (basically a battery that only lasts 2 minutes), which makes around (50W)*(120s) = 6000J We also know that our motor should always be in the range of 33V-40V for acceptable operation (this differs from our battery-electric car which used a different, 24V motor). Energy stored in a capacitor is given by E = (1/2) * C * V^2. If the capacitor's voltage is only ranging from 33V-40V, then useable energy is actually
E = (1/2) * C * (40V)^2 - (1/2) * C * (33V)^2 = C * (255 V^2) = 6000J
Solving for C, we find that we need ~24F of capacitance. The most common types of supercapacitors, ELDC, are almost always rated to 2.7V, so we'll need to connect 40V/2.7V = 15 capacitors in series (n capacitors C in series have capacitance C/n), which means each capacitor has to be ~24*15 = 360F. We ended up going for 400F capacitors to stay on the safe side.
Motor Driver
Depending on the type of motor you have, your motor driver may be trivially simple or grotesquely complex. In most cases, motor drivers can be purchased from reputable vendors or from open-source projects. In the case of the Shell Eco-Marathon, it is required that teams build their own motor drivers.
If you purchase a motor driver, make sure it can handle the motor. This seems obvious, but make sure the voltage and current ratings are suitable for the motor and energy supply. Erring on the side of caution, I would suggest getting a motor driver whose current rating is at least double what you expect at peak power draw and whose voltage has either a recommended voltage matching your application or a max voltage 50% higher than what you expect to see. Motors are incredibly "noisy" devices and can wreak havoc on devices which are not properly sized/protected. Also make sure the motor controller you choose can reach the speeds/torque and has the features (i.e. serial communication, arduino compatible, etc) that you want. If you have the resources, buy more than one to prevent setbacks.
If you design your own motor, again, choose components which are well larger than what you need and make sure to incorporate plenty of protection circuitry. Consider checking that there exist abundant MOSFETs and driver ICs that can handle your needs (should be no issues, but just to make sure). We'll go over designing a motor controller in a future tutorial, but just get an idea of how beefy you'll have to make it.
Other Circuitry
Decide generally what you want and make sure there exist ways to do it. For example, we need to convert the fuel cell's voltage from 20V to 33-40V in order to charge the supercapacitors. We found that a boost converter should work fine. We also want bluetooth to relay telemetry data to the driver's phone - is that possible? Yes, but we'll need a logic level voltage line and microcontroller. etc. Generally this stuff shouldn't be too important because you can generally find a way to make it work, but particularly higher power parts such as the converter are important to consider now.
Putting it all Together
Finally, you should sketch out a diagram of what is going where, what voltages/powers will be being used, what devices will be connected, how communication will occur, etc. The image below is an example that we used for our competition documentation, but you be believe there were countless whiteboard drawings hashing out these ideas before formally documenting them into this diagram.
Summary
In short, make sure that you choose devices which are compatible with eachother and sized correctly, particularly for the higher power components. Although you can make just about any logic-level device work with sufficient finagling, realizing your battery is too small for your motor a week before competition is a terrible place to be in. Make sure you have the big stuff figured out before working out details. These will also dictate many of the design constraints in later electronic design stages.
As always, let us know if you have any questions or want advice via this form or by emailing us at dukeecomarathon@gmail.com.