Introduction: So You Want to Build a Solar Car...
My name is Will Jones and I am the former team captain of the Okemos Solar Racing Club from Okemos, Michigan, our team is located right by our state capital Lansing. I started my team during the fall of my junior year. Our adviser was a former mechanical engineer at Ford Motor Company who had transitioned careers to working as a doctor. He was very hands off, and let myself and our team make mistakes and truly build our car. Don't get me wrong he was incredibly valuable and we could not have raced without him, however he never made a single decision on the car, he gave his input but, what the team decided the decided ran. During our first competition, the track race in 2019, we finished 2nd in the classic division, won the telemetry award, the engineering award, won the second day of the race, finished 10th overall of the thirty-three teams competing, and we were the third team through scrutineering. We were extremely happy with the results, we only finished seven laps out of first in our division to a team who had been able to race the year prior and learn from that finish. I was the driving force behind our car and I wished that there was some kind of knowledge base out there that could have helped me from avoiding some of my dumb mistakes. Additionally, I wish I had known how much of an undertaking building a solar car really was before I started the process. So I created this document. If you have any questions, don’t hesitate to reach out. This document’s advice is primarily set up for first year teams planning on competing in the Solar Car Challenge in the Classic Division on the track. Keep this in mind as you’re reading because some advice might not apply to different races or different divisions. Additionally, everything here applied to the 2019 rules as you’re reading this, certain things might become out of date, always check the rules first.
Step 1: Investment
Undertaking building a solar car is a huge investment, are you ready to commit the next two years of your life to late nights and hustling your butt off in order to build a real life race car? First, one must ask themself, two important questions: Do I have the time and what is my technical know how? Budget at least 18 months from start to finish. In order to undertake this challenge, you don’t need to have a ton of technical know how but expect to learn a lot and make a lot of mistakes. When I started, I really had no technical expertise. I didn’t know how to weld, solder, or drive with a trailer. In order to build a solar car, expect to be able to learn fast and adapt quickly. At the end of the day, you need to be absolutely ruthless, this is your undertaking. To every problem there is a solution, sometimes you just need to hustle. A lot. In terms of time, expect to invest at least 4 hours a week, with your team, and another three on your own putting things together.
Step 2: Quick Technical Run-down
When hearing “solar car” it conjures images of a sleek futuristic vehicle. For high school vehicles, and truly for all solar cars, the reality is a little less exciting, a solar car combines a residential solar system, and an electric go kart. That’s really all there is to it. Here’s a quick rundown of the electrical system.
Panels: collects energy from the sun
Charge controller: Takes energy from panels and regulates it
Batteries: Act as medium between panels and motor, most cars generally have about 40 miles of range on battery power alone.
Motor controller: Controls motor
Motor: Spins tire
Step 3: A Quick Reference on Electrical Terms Important to Know for Solar Cars
Volts: Shows battery capacity
Amps: Amount of power car is using
Watts: Volts x amps
Watt hours: Number of watts used in an hour (generally used to measure capacity), shows how much energy was used
Watt hrs/mile: direct correlation to MPG used to determine the efficiency of the car
Amp hour (Ah): The amount of power (amps) a battery is rated for for example, a 10 Ah battery could take a 10 amp charge for one hour or a 5 amp load for two hours
A quick note when looking at capacity: Always use watt hours or kilowatt hours (1,000 watt hours is equal to one kilowatt hour) to determine the capacity of the system. Most batteries are rated in amp hours. A 12v 100Ah system has half the capacity of a 24v 100Ah system.
Step 4: The First Component Batteries
The Solar Car Challenge allows for up to five kilowatt hours of energy. The EPA says that one gallon of gas contains about 33.7kWh. Basically, the SCC rules allow for about ⅙ of a gallon of gas. Since we are required to use lead acid batteries, if a team uses the maximum amount allowed, this is about 300 pounds of batteries.
As lead acid batteries are discharged, they experience what is called voltage sag. As you drive and discharge the batteries their voltage will decrease. A fully charged lead acid battery sits at about 13 volts. While a fully discharged the battery sits around 10.5 volts. Imagine this scenario for a minute, you floor the accelerator and the voltage goes from 13.00v to 12.00v and continues to steadily decrease. After accelerating, you take your foot off the accelerator and coast to a stop. The voltage then bounces back up to 12.50 voltages. The total capacity removed from the system is .50 volts. What you just saw happen is called voltage sag. During the race, your batteries will be experiencing a combination of voltage sag and voltage lift. Charge from the panels and discharge to power the car forward.
Now here’s where it gets interesting, if you discharge the batteries at a faster rate (more amps), their capacity is decreased. Here, you can see in this chart the amp hour rating of the batteries used in our vehicle, The Lead Sled. As we discharge the batteries more aggressively, their capacity is reduced. At a 1000watt amp discharge, a 2kWh battery pack would last significantly less than half the time a 4kWh system would. This is called battery efficiency. Advanced cars which are able to run lithium ion batteries have efficiency near 100%. Now the problem with using a full 5kwh’s of lead acid batteries-- is it is quite heavy. Estimate for every kWh 60lbs. Four 12v 100ah batteries weigh about 280 pounds. Additionally, over the course of the competition, we found it will be impossible to fully charge our batteries meaning our battery pack was too large. Because of this, I would recommend you go slightly slightly smaller to around 70ah. Doing this will save about 100 pounds of battery weight for really no loss of energy storage. The best classic car ever on the track, the Iron Lions in 2017, used 69Ah batteries.
You need deep cycle batteries. These batteries are specifically set up for this purpose and if not installed on your solar car would likely have ended up on a golf cart. If you do not run deep cycle batteries it will destroy the batteries very quickly. Deep cycle batteries have thicker lead plates meant for deeper discharge cycles. Also, make sure once you purchase your batteries you keep them stored at 100% on the charger always. Never should you put your car or batteries away (other than during the competition) not on the charger. The success of your competition is dependent on the health of your batteries. Storing them discharged can lead to sulfation on the plates which then leads to decreased capacity and performance. Use sealed batteries, unsealed batteries can give off hydrogen and require the addition of distilled water. Ideally you will buy or have donated absorbed glass mat batteries with 70Ah rating. You can buy batteries with those specifications from a ton of different suppliers. We had our batteries donated from local battery seller Complete Battery Source (Thank you!). This is actually something easy to have donated, go talk to all your local battery stores. Batteries like these are quite common, if you do pay for them expect to pay about $1,000 for all four.
Never buy used batteries, or run used batteries other than solely for testing purposes. As batteries age their capacity is reduced and you never know what you’re paying for. When storing the batteries, purchase a battery maintainer, the batteries should always be stored at a full charge, buy a battery maintainer. If they aren’t stored properly, their life will be significantly reduced, additionally, their efficiency will be significantly reduced. A word of caution, batteries are dangerous and should be treated like gasoline. If they are shorted they can do serious damage. Always use extreme care around the batteries. In 2019, teams ran into trouble because the lugs on their batteries were not connected properly. Caution is key.
Step 5: The Panels
The SCC requires the module efficiency be less than 19%. Additionally, teams have to fit as many possible panels within a 1.8m by 5m box. Most teams will have around a 1500 watt system. On our car we had five 295 solar world panels with an efficiency of 17.95%. They measured 1.68m by 1.001m this meant we had to offset the panels as shown in the picture above in order to get them to meet the rules and have them fit inside the 5m box.
This worked okay, however it caused some problems when the sun was low in the sky with casting shadows onto the panels. Additionally, each of our panels weighed around 40 pounds. When selecting your panels, try and find something that is lightweight, and just barely below that 19% efficiency cut off. Some teams have reported success using SBM panels which are much lighter, and can be more efficient. I would recommend just looking around and trying to find something efficient with good low light characteristics. We were able to buy all five of our panels for $975 shipped from a local solar panel installer. (Thank you Mid-Michigan Solar King!) Some teams will build their own arrays from individual cells. This is a time consuming process, however, you can end up saving some money. We did not go this route because it is extremely difficult to encapsulate cells especially with the resources provided to a highschool team. If this is an avenue you wish to pursue I would recommend reaching out to the Staten Island or RAHS team and asking them how they did it.
One last important thing, the panels need to be able to tilt. In the morning, during the two hour break in the afternoon, and in the evening teams will tilt their panels. This is because it allows the panels to be more perpendicular to the sun, therefore, they will generate significantly more energy. How a team accomplishes this is up to them, however, make sure the panel mounting and tilting mechanism is easy to use and robust because the cars will experience serious wind gusts. Our car did not have the ability to tilt so we used a jack to try and at least get a little tilt going. It is not the end of the world if your panels cannot tilt, however, if you’re designing your panel mounts you may as well make them tilt the first time designing them. This is one of the keys things on my team’s list they would like to modify to help make the car more competitive next year.
Step 6: The Charge Controller
The charge controller takes energy from the panels and turn it into usable energy for the batteries. All five of our panels were in series, under direct sunlight into the charge controller they would give about 150 volts. The charge controller then converts that energy down to around 48 volts in order to charge our batteries. The charge controller is an incredibly important component of the cars. Do not skimp out on a cheap charge controller. We used the Midnite Solar Classic 250 model. Use this model. It retails for $1100, if you are friendly, you can probably talk them into a 50% discount like we did. We had absolutely no problems with our Midnite. Most teams run a Midnite as well. Buy a Midnite.
Step 7: Motor/Controller
Many teams run a ME0909 motor, it retails for about $430 and another $400 or so for the controller. This is a brushed motor and not super efficient. Also, likely (unless you have a super special controller) you will not have regenerative braking, reverse, or cruise control. We ran a Golden Motor HPM3000B this is a Chinese brushless motor and highly efficient. We could not recommend this motor more. We had absolutely no problems with it and it was undoubtedly one of the reasons we ended up second in the country and won the engineering award. The motor is incredibly efficient and easy to tune using their software. We paid about $830 shipped for the motor controller, motor, and programming cable.
One problem we encountered was originally, we had a 12 tooth sprocket on the motor and a 72 tooth sprocket on the wheel, giving us a 6:1 gear ratio. Our motor did not like this and did not operate very efficiently using this set up. So we installed a complicated jackshaft giving us a much better 12:1 gear ratio. It is key that whatever motor you run, the motor is run where it is most efficient. If you end up running a Golden, you need to make sure that it can spin where it is most efficient.
Step 8: Disconnects
Teams are required to have disconnects on the motor and between their charge controller and batteries. Every team basically uses the same kind, here’s a link to them: You can buy them for $50 a piece on Amazon. Just run these, it makes your life a lot easier.
Step 9: Wiring
Wiring is relatively simple, here are a couple of things to keep in mind. For your main powertrain wires, the wires that go from the batteries to the motor controller, you need to use thick wires. These wires carry lots of amperage and the thicker they are, the less resistance or wasted energy you will encounter. I would recommend using at a minimum 1awg wire. For the wires that go from the array to the batteries, I would recommend at a minimum 6awg. Use this handy chart to decide what works best for you.
This is one thing where overkill does not hurt, the added weight (which is relatively minimal) will surely be offset by the increased efficiency. For whichever wires your team buys. Find fine stranded, if looking online EVWest.com offers a decent price on 1/0 awg wire. Fine stranded wire is incredibly important because it makes it much easier to deal with and bend. On our first car, we did not use finely stranded wire and it made it incredibly difficult to bend and shape the wire to our desired length. Another thing to keep in mind when designing your car, the less length the wires have to travel, the better. The longer the wire, the greater your loss. In order to connect all your wires, you have to solder or crimp on copper lugs.
Here is a picture of what copper lugs look like, they are easy to buy online. Find a video on how to connect them to wires it’s pretty straightforward, you will need a blowtorch. These are super important because teams need a way to connect their wires to disconnects, batteries, controllers, etc. Once you attach the lugs to your wires, make sure to heat shrink over the connection in order to prevent from accidental shortages. Additionally, all of your high-voltage connections, deemed by the Solar Car Challenge by over 36 volts need to be properly identified and insulated. In order to insulate our motor controller, we placed a plastic container over it. On our disconnect connections, we wrapped the wires in foam and then secured the foam with zip-ties. Inside of our battery box (yes, you have to insulate inside of your battery box!) we used caps meant specifically for that purpose.
Step 10: Fuses
Teams at a minimum are required to have a fuse between their batteries and their motor controller. The fuse must be rated for no more than 125% of expected peak current draw and it must be rated “fast blow” or “very fast blow”. We bought ours from Mouser Electronics. We paid $7, just make sure it meets all these guidelines and is DC rated. Breakers will not work, teams must use fuses. Now as a safety feature, we did use a breaker between our array and charge controller. This is a good idea because in the event your charge controller shorts, it could be incredibly bad for your batteries. However, it is not required. Below is our fuse datasheet, teams must provide when submitting their registration documentation. Our expected peak energy draw is 80 amps and we are using a 100amp rated fuse.
Step 11: An Introduction to Frame Design
The solar car challenge requires that cars roll bars are at least 5cm in diameter and 1.0 mm chromoly steel, 1.5 mm carbon steel, 3.2 mm aluminum. The roll cage must just be 1.9cm in diameter. Teams first thought is of course, is to use aluminum. The problem with aluminum is that it is extremely difficult to weld. It has to be tig welded which is much more difficult to do than mig welding. Our team got lucky and someone volunteered to weld our frame for free, however, he was only comfortable using mild steel. To be perfectly honest, it did not really matter that much. Most teams we were competing against also used mild or chromoly steel tubing. Using aluminium is an advantage but not as large as you might think. Our car beat several teams using aluminum frames. If you can use aluminum, absolutely use it! But if steel is what makes the most sense for your team, then use steel. If your car was competing in the advanced division then I would highly recommend using aluminium at all costs.
Step 12: Front and Rear Wheels
Designing the frame is an interesting task, we bought our suspension first then designed our frame around the suspension. In order to determine what kind of suspension we needed we first had to figure out how much our car would weigh. Basically, all the cars in the classic division weigh around 1,000 pounds. Using this basic rule of thumb teams can pick out their suspension. We used the front “suspension” off of a 6 passenger bicycle called a Surrey Quadricycle. The rear is off of a 1983 Kawasaki KZ750 LTD
Most teams will run a motorcycle swing arm in the rear, this makes life a lot easier because if you go to a motorcycle scrapyard you can buy a swingarm for relatively inexpensively, that includes the rear wheel, rotors, pads, shocks, master cylinder, etc. We bought our swing arm with all of this for $300, if we were to have bought all of those parts individually it probably would have cost five times that. To be honest, if I were to build another car tomorrow for the classic division, I would build my own swing arm. However, for a first time team it’s a lot easier to just buy one already made and more importantly, less expensive.
On the front, we called up Buena Vista Surrey, a company that sells the six passenger bike shown above and we basically bought the whole front end. In order to mount it to our frame, we had blocks machined so essentially the whole front end after a lot of design work just bolts right in. You can see that the bikes are unsprung. We did not have a front suspension. Our car does not ride great. A front suspension is a lot of added complications. For a track race you absolutely to not need it. It is a waste. On the road, I cannot say for the competition whether it is necessary or not. Based on our experience testing on the road it is not necessary, but would be welcomed. If possible avoid golf cart parts. The problem with golf carts are they are designed to not do damage to the lawn they are driving on and therefore have super high rolling resistance.
Avoid four wheels, if you have four wheels it means either one wheel is powered or it requires a differential which is just thrown away energy. If you’re looking at World Solar Challenge cars for inspiration note that the World Solar Challenge regulations require four wheels, likely many cars would only run three if allowed. There are very few Solar Car Challenge cars that have four wheels.
Step 13: Motor Mount
Onto our swingarm, we welded a motor mount out of 1/4 inch plate onto our swingarm and bolted on our motor. One thing that teams will get into trouble with is running the original sprocket that came with the motor. The chain meant to run originally on the motorcycle is way overkill for a solar car. Teams are running 2-8hp compared to 80-120hp. Buy a larger sprocket meant for smaller chain. #40 chain or maybe even #35 will work just fine. We had to have our sprocket machined so it would bolt to our rear wheel. No big deal. Make sure whatever you run you have a sprocket that will fit your motor. We would have preferred to run #35 but we ended up running #40 because that is the only sprocket we could get to fit to our motor.
This image here shows the motor mount welded onto the swing arm, an idler sprocket for tension, and then the wheelside sprocket, which we had machined. So it would bolt onto the existing mounts for the previous sprocket. Look at your motors power curves and do some math. For our motor we needed our motor to spin well over 3,000rpm for it to operate most efficiently. You can see below the final setup we ended up installing, there is now an intermediate shaft and two chains giving us a final ratio of 12:1. Another thing teams will get into trouble with is their motor is not mounted onto their swingarm and they have serious chain tension issues. Mount your motor onto your swingarm, not your frame. It will save you a lot of headaches.
Step 14: More Frame Information
Once you have your front and rear suspension you can finish the design of your frame. Important things to keep in mind. The driver needs to be able to see ten degrees looking up. Make sure you have a way to mount your panels. The distance between your front two wheels should be more than the half distance between the front and rear wheels for roll over safety. Make sure you include your crush zones. We used 3/8’s rod for our crush zones. This worked really well. It’s super light and relatively inexpensive. You will catch other teams running seriously thick tubing for their crush zone. This is a waste.
Step 15: A Word on Nuts and Bolts
Our car is a mashup of standard (English) and metric. Never ever mix English and metric, the threads will get destroyed. When looking at bolts, everything has four measurements the length, thickness, type of alloy, and the thread count. In english units things are generally standardized as either coarse or fine, fine threads have more threads per inch, coarse have less threads per inch, if you have to replace anything made sure you get the right kind. Additionally, for all nuts and bolts there are different grades (for English) Grade 8 is required for certain stuff on all cars the seat and seatbelt. They are easily identifiable by their color.
Here, you can see the Grade 8’ers third on the left. Pro tip, anything Grade 8, the goldish color is english. One last thing, any bolt on the car that is critical, (drive line, suspension, brakes, harness, etc.) needs to be nylock, double nutted, or otherwise secured. The judges are very careful about this and if you get all the way down there and don’t have the right stuff it will be incredibly hard to fix. We found a local company called Mid-State nut and bolt located in Lansing. They have everything, and have been a huge help when looking for an odd size.
Step 16: The Importance of Knowing What Your Car Can Do
You need a way to measure your efficiency and how much energy you’re using. You need to install one of these.
This is an ammeter, from it you can see your live energy consumption and how many watt hours you have consumed. Using this you can find your current battery capacity. One important metric the OSRC uses is our wh/mile this is how much energy we use per mile. During the race we peaked at about 68wh/mile figuring we collected about 8.5kwh or 8,500wh per day this meant we could run about 125 miles a day and not use any of our starting battery capacity this also assumes a battery efficiency of 100% which is not totally true. As you make modifications to your car, your wh/mile will be an important tell on how the change impacts your car.
Step 17: How the Race Is Set Up
Teams will have three days to get through the six scrutineering stations. Everything but electrical and mechanical you need to have already practiced before even coming down to Texas and should easily pass. Look through the scrutineering book on the Solar Car Challenge website. A note on scrutineering, make sure you know the rules like the back of your hand. It is highly likely you might know the rules better than a judge flagging you for a problem, be prepared to consult the rule book. Also, try and move through the stations as fast as possible, then fix the problems in the night when it’s not possible to be going through the stations. Scrutineering is not that bad, just be prepared.
Once you pass, the racing starts. Teams will be able to race from 9:00-12:00 and then from 2:00-5:00. If your car can average 20mph likely you will be competing for a national championship. The key to being competitive is to run reliability and run a consistent pace throughout all four days. Running too slow or too fast is just wasted energy.
Quick list of important things to have at race:
Multiple copies of scrapbook
Multiple copies of registration
Proof of Insurance
Flashing light for car
Two flags for flaggers
At least three orange cones
Tarp to help cover sun
Soldering iron and accompanying items
Spare motor controller
Spare brake pads and fluid
Fix a flat
Tools to change tire
Tire patch kit
Paper towel to clean panels
Lots of tools
Tie down straps
Safety vests for at least 5 team members
Step 18: The Importance of Testing
You need to get as many miles as possible on the car. More miles the better. The more we tested our car, the more familiar with it we were and we fixed things that occured. Testing you do have to be a little careful. A lot of our miles were done illegally on the roads, we never had a problem but if we did get into one it could have been incredibly bad. We had over 200 test miles, and we wished we had done more. One of the most important testing things we did was we contacted our local police academy and they have a test track. Over the course of the day, we were able to put about 100 miles on the car. This helped alleviate a lot of fears about the long term reliability of the car. One thing I wished we had done was to play with the gear ratios and see how that impacted our fuel economy.
The most important thing you can do to be competitive is to test your car and find what is weak about it and then fix it. During the first hour of racing at least five cars broke down. In the first hour! If your car cannot run for an hour without problems, you cannot be competitive. During the race it’s also important to make the starting order and to have efficient pit stops this is pretty straightforward, just talk it over with the team when you’re in Texas.
Step 19: Kids Build the Car
On our team, the kids researched the car, the kids designed the car, the kids built the car. If something went wrong, we did not turn to our advisor for help. We called all the strategy during the racing, in fact our real advisor had to work during the racing and was a thousand miles away in Michigan. We made a lot of dumb mistakes but, in the end it helped us be more competitive because we knew the car better than anyone else. It also made scrutineering a lot easier, and I think it helped in our winning of the engineering award because we were the ones who knew the car not some adult.
Step 20: Body Panels
We just cut coroplast and had a local sign shop do the artwork, doesn’t it look good? In order to secure the body panels, we first glued in a washer, poked a hole and then zip tied it our car. This is strong and light and looks pretty good. Most teams either ran coroplast or nothing at all. That’s what I would run it just makes life a little easier.
Step 21: Transportation to the Race
Your car will somehow have to be transported to the race. During the race you will also be required to have a trailer with you in the event your vehicle breaks down. Depending on how far you are from the race it can be extremely expensive to take the car down to race. We had to rent a 20ft enclosed car trailer and a F250 to pull the trailer with. During cross country years, depending on how far you are coming from, you could require two trailers. An enclosed trailer to transport the car down to Texas, then an open trailer to transport the car during periods of the race when teams are required to trailer their vehicles. We rented our truck from Enterprise Truck Rental, all in all we ended up paying a little over $1,500 for the truck and then $250 for our enclosed trailer. No one ever said solar racing was inexpensive.
Step 22: Fundraising
My least favorite topic surrounding solar cars. These darn cars are quite expensive. Budget at an absolute minimum $10,000 for your car. We fundraised from a wide variety of sources recyclable can drives, food fundraisers at local restaurants, we set a $100 fundraising minimum for each member of our team, many local businesses helped out. It is an absolute grind. Money doesn’t guarantee a competitive car but it sure helps. Teams also need to budget $1,500 for the registration fee, which is a lot of money. If teams pay cash and face value for everything here is a basic rundown of very loosely estimated costs.
Panels: $2000 Charge controller: $1,200 Batteries: $1,200 Motor controller/motor: $1,000 Rear suspension/wheels: $1000 Front suspension/wheels/tires: $1000 Steel for frame: $1,000 Various little things: $1000 Unexpected costs: $1000 Registration: $1500 Spares needed to race: $1500 Total: $13,400 (keep in mind, no team should pay face value or cash for most components)
Step 23: Final Thoughts
If you have any questions don’t hesitate to reach out to Dr. Marks, the race staff, other teams, or myself. These cars are pretty simple at the end of the day. Make sure safety is the number one priority and prepare yourself for lots of long nights. Building a solar car is one of the coolest projects any high school can undertake. Very few kids or high schools can claim that they competed in the Solar Car Challenge. The entire race staff and other teams are extremely friendly. The Solar Car Challenge is one of the coolest events you will ever earn the opportunity to compete in. Good luck, and enjoy the process if you have any questions, feel free to reach out to me email@example.com or our team firstname.lastname@example.org.
Step 24: Chapter 24- One Last Thing
I’ve included a link to our 2019 registration document, our team scrapbook, and our team presentation. The registration shows the documentation for all kinds of parts and is a good guideline for what teams need to include in their registration document. You will notice at the end there are lots of “random” pictures with documentation for various parts of our car. This is so if challenged on the authenticity of any part of our car we could show them the documentation backing up our claim. Additionally, you will find the half waiver for our roll bar provided by Dr. Marks. The team scrapbook was a last minute effort and could have been done better, but it passed and that is all that matters. I would consider this the bare minimum needed for a scrapbook. Our presentation, at first might appear to just be a bunch of photos, you will notice in the speaker notes a lot of notes. What we did was display the image then have each member of our team walk the judges through the stages of building our car.