Building an EV Mini-Kart from a Kit and Racing It at Tsukuba: Ichiken's First CQ EV Mini-Kart Race
イチケン / ICHIKEN
Electronics YouTuber Ichiken entered the CQ EV Mini-Kart Race at Tsukuba Circuit in 2025, driving a homemade electric kart on the actual Tsukuba Circuit Course 2000. In this race, teams compete on how many laps they complete in a fixed time. The video follows the whole process: assembling the inverter and motor, building the chassis, tuning the kart around a limited battery, and racing it. Ichiken's stated goal as a first-time participant was modest. They wanted to finish the race. The account below covers how close they came and what they think went wrong.
How Ichiken Came to Enter
CQ Publishing, the publisher of the electronics magazine Transistor Gijutsu (Transistor Technology), invited Ichiken to take part in the EV mini-kart race. After hearing more about the event, they found it interesting and agreed.
Entering requires three things: a kart body, an inverter, and a motor. CQ Publishing sells the EV kart itself as a kit, and assembling the kit produces a vehicle that can enter the race. Ichiken treats the motor and the inverter as the heart of the kart, so they built those first.
A Two-Day Assembly Seminar
The motor and inverter are also kits, and you can assemble them yourself. Ichiken chose to attend a two-day assembly seminar run by CQ Publishing. Each day combined a classroom lecture with hands-on work. The lectures covered how motors work and how inverters control them. The practical sessions covered assembling the inverter and motor and then checking that the finished units worked.
Day 1: The Inverter
The inverter is the drive circuit that turns the motor. The kit's circuit board arrives with the surface-mount components already fitted. Participants solder the through-hole parts themselves, including ICs, resistors, capacitors, and power semiconductors. They also assemble extras such as the Hall sensor boards, which detect the motor's magnetic flux. The kit manual lays out the assembly procedure, and Ichiken says that following it exactly makes the work straightforward.
A microcontroller handles control, and the kit includes its code. Users can therefore adjust the motor control method and the current flowing through the motor themselves.
Ichiken points out that the race regulations barely restrict the inverter. Any system under 48 V is allowed. Entrants may use the kit as supplied, modify it, or skip it entirely and use their own inverter design. That leaves a lot of room to modify the controller, and Ichiken later changed some of the inverter's components.
At the end of day 1, the assembled inverter drove a motor on the first try, and the motor spun steadily. Ichiken then adjusted the advance angle by shifting the Hall sensor position until the current draw was as low as possible.
Day 2: Winding the Motor
The second day covered motors. The kart uses an outer-rotor motor. Ichiken says it is built almost the same way as the brushless motors in small drones: magnets sit in the outer rotor, and copper wire is wound around the stator in the center.
The rotor came first. Participants glue ferrite magnets inside the rotor's iron core. Each magnet has a south pole on one face and a north pole on the other, and getting the orientation wrong ruins the motor. Ichiken checked every magnet against a polarity indicator while placing it.
Next came the stator windings. Mass-produced motors are normally wound by machine, but in this kit each motor is wound by hand. The supplied wire is 1 mm thick, which takes real force to wind. Ichiken's tip is to hold the bobbin of wire with your foot, hold the stator in your hands, and keep moderate tension on the wire as you wind.
The number of turns used at the seminar was, in Ichiken's words, decided quite arbitrarily. They knew the motor would need rewinding before it went into a race kart.
How Much the Regulations Allow
The regulations require competitors to use the kit motor. Within that motor, however, they may change the wire gauge, winding method, connection method, core material, and permanent magnet material. Ichiken says this allows countless ways to build the motor. Winning depends on turning electrical energy into mechanical output as efficiently as possible, and they believe this is where much of the accumulated know-how in the event lies.
The finished motor, driven by the inverter from the day before, also worked on the first try, even though winding it had been hard. After the seminar, participants got to test-drive an EV kart kept in CQ Publishing's warehouse. Ichiken wanted to get a feel for how the kart handled and use it when building their own.
Building the Chassis, Late
After the seminar, Ichiken left the kart mostly untouched. About a month before the race they realized they were short on time and started preparing in a hurry. The chassis still had to be built, and none of the testing or tuning could happen until it was done. Ichiken thinks the schedule ended up quite tight.
The chassis is a steel frame with wheels and other parts bolted on. Ichiken compares it to a large radio-controlled car. Working alone and filming as they went, they needed about four to five hours. They estimate the job should normally take two to three. They also note that they worked on the floor, and a workbench would clearly have been more efficient.
With the mechanical part finished, Ichiken temporarily fitted the inverter and motor, since the wiring only needs to be neat by the time of the real run. The motor and other components worked without problems. The motor was still exactly as wound at the seminar, so it would need several rewinds to reach race-spec turn counts and connections.
Why a Test Run Was Needed
Ichiken expected that showing up to the race with the kart in this state would not end well, so they did a test run. Getting used to driving was one reason. The run also had to answer several engineering questions:
- whether the kart runs safely under loads close to race conditions
- whether the motor and inverter stay stable during long operation
- how long the battery lasts and how the kart accelerates
- how much to limit motor current, and how to settle the other performance settings
After the test run, Ichiken checked whether they could finalize the setup.
Setting Strategy: Stretching a Limited Battery
The race lasts 30 minutes. To count as finishing, a kart must complete at least 70% of the distance covered by the winner. A kart that runs slowly just to save energy will therefore not be classified as a finisher. That makes battery life, along with the inverter and motor settings, critical.
The battery rules are strict. Entrants may use up to two lead-acid batteries, each weighing under 3,000 g. With that fixed amount of energy, the kart has to use it as efficiently as possible.
Past results suggested that an average speed of about 30 km/h had been enough to finish. Ichiken therefore set up the inverter and motor to sustain a 30 km/h average. The motor drives the wheels through a chain, and they note that the gear ratio is also a very important setting.
To size the current, they used the battery's datasheet. It includes a graph of battery voltage (vertical axis) against discharge time (horizontal axis) at various discharge currents. Reading off the 30-minute point, they found the battery could discharge continuously for 30 minutes at about 13 A. The target became running at 30 km/h on 13 A of battery current.
The open question was whether 13 A could actually produce 30 km/h. To meet those conditions, Ichiken changed the motor winding and the gear ratio. With limited time, they built only a few motor prototypes but got close to the target.
Inverter Modifications
The main inverter change was replacing the switching MOSFETs with lower on-resistance parts. The kit MOSFETs have an on-resistance of 3.3 mΩ, and the replacements cut that to 1.1 mΩ. The gate drive circuit stayed as it was, though, and Ichiken warns that the new MOSFETs' larger gate charge may raise switching losses.
They considered shrinking the gate resistance as far as possible. Radiated noise was not a concern for this use, and a smaller resistance could reduce switching losses. They dropped the idea because the gate driver IC did not seem able to handle it.
The board had spare room, so they also added electrolytic capacitors they had on hand. Ichiken doesn't think this helps much. Their approach was simply to do whatever they could.
Last, they applied a moisture-proof conformal coating to the board and made a cover for it. The kart runs in a harsh environment where dust, moisture, and debris can land on the circuit, and these changes make it less likely that such contamination causes trouble.
Motor Modification: Thicker Wire to Cut Copper Loss
On the motor, Ichiken aimed to reduce copper loss. They replaced the kit's 1 mm wire with 1.4 mm wire while keeping the same number of turns. They picked that gauge because it was the thickest that would still fit in the stator slots with the same turn count.
With 1 mm wire, the winding resistance was about 100 mΩ. Going to 1.4 mm roughly doubles the cross-sectional area, which halves the resistance to about 50 mΩ and halves the copper loss. By Ichiken's calculation, that saves about 8 W at the 13 A average.
Tsukuba Circuit is not flat, though, and the uphill sections push the current well above average. The average may be 13 A, but the current can reach 30 A at times and 100 A briefly. At 100 A, Ichiken calculates that the thicker wire cuts copper loss from 1,000 W to 500 W. They call it a very impactful modification.
Instrumentation
Ichiken added a voltmeter to show battery level while driving, and an ammeter to measure battery current. The ammeter is a Bluetooth clamp meter, and the idea was to monitor it from a smartphone, which they describe as the easiest solution.
Race Day
CQ Publishing managed the race, and the Japan Electric Vehicle Racing Association organized it. At the venue, Ichiken serviced the kart, briefly checked that it worked, and made a test run of about 20 m. Until the start, they kept the battery charged.
The Breaker Trips
The start went well. Then, near the top of the hill on the home straight, the breaker tripped. Ichiken switched it back on and accelerated again.
The kit breaker is rated at 50 A. A careful reading of the regulations, however, shows that competitors must use a 30 A breaker handed out on race day. Ichiken calls this an easy detail to miss because it forces subtle changes on the day. They had read the rules in advance and set the microcontroller to lower the current during acceleration, but the adjustment was evidently not enough. Throttle control turned out to be harder than expected. Whenever the breaker tripped, Ichiken reset it and got going again.
The phone showed large battery currents, at times over 100 A for brief moments. Because of how the breaker responds, very short overcurrents do not trip it. Ichiken reflects that they should have studied the breaker's trip characteristic curve properly. They think that pushing current during acceleration right up to the trip threshold, without crossing it, would let the kart run quite well.
Drafting Behind a Group
Partway through, a line of karts running nose to tail came by. It was a team from a Tsukuba automotive college, and Ichiken says they appeared to be using the slipstream to reduce aerodynamic drag. The regulations ban aerodynamic parts, and the driver sits fully exposed, so drag is significant. That makes drafting a workable strategy for a school entering several karts.
Ichiken decided to follow the group and cut their own drag. They kept some distance, because running too close could cause an accident. For much of the race after that they simply kept driving. Top speed was about 35 km/h, maybe a little more on downhills.
When the track opened up, Ichiken sometimes tried to pass the group, but full throttle would not raise their top speed. They suspect the battery voltage and the motor winding were not matched well. They gave up on passing and stayed behind the group.
Some faster karts passed them. These were Unlimited-class karts running lithium-ion batteries, which Ichiken says are considerably faster.
Running Out of Energy on the Final Climb
In the second half, the battery voltage fell sharply and the kart slowed, especially on the climbs. The hardest section is the uphill onto the home straight after the final corner. The kart gradually lost speed there and finally could not make the climb. Ichiken tried rocking their body to push the kart up, but it didn't work. They could not finish the last lap, and the race ended there for them.
The Result and What Ichiken Takes From It
Ichiken finished 11th. They consider that a good result for a first attempt, since finishing was the goal. They completed seven laps, with a best lap of 3 minutes 49 seconds. For comparison, they note that most cars racing at the circuit lap in about 60 seconds, which shows how slowly the kart was going. Their average speed was about 30 km/h, which matched the plan.
Power consumption, however, was higher than expected, and the battery ran out before the end. To last the full race, Ichiken sees two options: drive differently or make the motor more efficient. On driving, they suggest coasting more and accelerating more gently. They also admit they ran out of time to fully tune the inverter and motor control and did not make good use of the battery's energy, and they think those gaps explain the result.
Ichiken says they have already found some areas where the motor can be improved. Their goal for next year is to keep the kart running to the end and complete the race properly.
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Hello. This time, I participated in an EV mini-kart race held at Tsukuba Circuit. I would like to report on it. At the actual Tsukuba Circuit 2000, you ride in a homemade EV kart and compete for the number of laps. This video shows everything from making the EV kart to actually going to the race. But first, here is today's sponsor. Go ahead.
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First, I'll explain how I came to participate in EV kart racing. CQ Publishing, well known for Transistor Technology, invited me to participate in an EV mini-kart race. As I listened to the various details, it sounded interesting, so I decided to participate.
To participate, you need an EV kart chassis, as well as an inverter and motor to move it. The EV kart itself is available as a kit from CQ Publishing. You can assemble it into a vehicle that can participate in races. Information about these kits is collected in the links in the description. Check it out if you're interested.
I'll get right to building. The motor and inverter are the heart of the EV kart, so I'll assemble those first. This is a kit product that I can assemble myself, but CQ Publishing holds an assembly seminar. I attended this seminar and assembled the motor and inverter.
This seminar lasts two days and consists of classroom lectures and practical training. In the classroom portion, the instructor gave lectures on the principles of motor operation and inverter control methods. The practical training covers assembling the inverter and motor and checking the operation of the assembled products. Here's how the assembly actually went.
Day one is the assembly of the inverter. What is an inverter? It's the drive circuit that turns the motor. Here is the circuit board. Surface-mount components are already installed. We solder through-hole IC components, resistors, capacitors, power semiconductors, and so on. We also assemble Hall sensor boards used to detect the motor's magnetic flux. We soldered them like this.
The assembly procedure is in the manual that comes with the kit. If you follow it exactly, it's easy to proceed. Control is handled by this microcontroller. The microcontroller code is also provided, so I can adjust the motor control method and the current flowing through the motor myself.
Looking at the EV kart racing regulations, there are almost no restrictions on the inverter portion. If the voltage is less than 48 V, you can use this assembly kit, modify the kit, or use your own inverter without using the kit at all. The controller can therefore be modified quite extensively. In the second half of the video, I also replace some inverter components and make slight modifications.
With the inverter assembled, I checked whether the motor would turn. It worked on the first try! The motor is spinning properly and stably. Here, we're adjusting the advance angle by adjusting the Hall sensor position to minimize the current. That completes day one of the seminar.
The second day of the seminar consists of a classroom lecture on motors and hands-on assembly practice. The motor used for the EV kart is an outer-rotor type, almost identical in construction to the brushless motors used for small drones. There are magnets in the outer rotor section, and copper wire is wound around the stator in the center.
First, we assemble the rotor section. Ferrite magnets are attached inside the rotor's iron core. The front and back are the south and north poles, so be careful not to make a mistake. If you get this wrong, it will no longer be usable as a motor. I checked the direction of the magnetic poles using the compass on the right side of the screen.
Next, copper wire is wound around the stator. In mass-produced motors, it's usually wound by machine, but this is a one-off, so we wind it by hand. The copper wire is 1 mm thick, so winding it by hand takes a lot of force. A useful technique is to hold the bobbin of copper wire with your foot and the stator in your hand, winding while applying moderate tension to the wire.
The number of motor winding turns here was chosen quite arbitrarily. When we install it in the actual EV kart, we'll have to rewind the motor again. The EV kart regulations require using this motor kit, but the wire thickness, winding method, wiring method, and materials of the core and permanent magnets can be changed. There are therefore countless ways to build the motor.
To win a race, you need to convert electrical energy into mechanical output, so you need to build a motor with the highest possible conversion efficiency. I think there's quite a bit of know-how involved in this area.
The motor was completed this way, and we also checked its operation. The inverter we used was the one I assembled myself the day before.
It's turning. Good.
Winding the motor was quite difficult, but it worked properly on the first try.
After the seminar, we were also allowed to test-drive an EV kart stored in CQ Publishing's warehouse. The idea was to get a real feel for it and make use of what we could learn in the kart we're building. At this point, the electrical assembly is complete. Next, we'll assemble the vehicle chassis.
The scene has changed. Since assembling the inverter and motor, I'd mostly left the EV kart alone. About a month before the race, I finally realized I didn't have much time and started preparing with some impatience. Without assembling the chassis, we can't do test runs, adjustments, and all that stuff. I think it was cutting it pretty close.
The chassis consists of a steel frame like this, with wheels and other parts attached to it. It's like a large radio-controlled car. We'll assemble it. This time, I was working alone and filming as I assembled it, so it took about four or five hours. In terms of the workload, I think it should take about two or three hours. Also, I'm working on the floor, but doing it on something like a workbench would clearly be more efficient.
Once the mechanical portion of the chassis is complete, we temporarily install the inverter and motor. We just need to secure the electrical wiring and everything else before actually driving. For now, it's a temporary assembly.
With everything installed this way, I confirmed that the motor and other equipment operated without problems. The motor hasn't changed since we assembled it at the seminar, so we'll rewind it several times to get the number of turns and wire connections suitable for racing.
Next is a test run. Entering the race in this condition without testing probably wouldn't end well, so we'll do a trial run. The purpose is, of course, to get used to driving, but also to check whether the vehicle can be driven safely without problems under loads close to race conditions, and whether the motor and inverter can continue operating stably for long periods. We also want to refine performance settings such as battery life, acceleration, and how much to limit the motor current.
For this race, our goal for now is to finish. Through the trial run, we checked what approach to take to finalize the settings.
Now let's talk about the settings. For this race, our goal for now is to finish. Looking at the regulations, the race lasts 30 minutes, and to be classified as a finisher, you must cover at least 70% of the distance covered by the winning vehicle. You therefore can't just drive slowly. You need some speed, or it won't count as a finish. Battery life and inverter and motor settings will be quite important.
The battery regulations are clearly defined. For lead-acid batteries, each must weigh less than 3,000 grams, and you can use up to two. We therefore need to use this finite energy as efficiently as possible.
We decided on this approach to the settings. Looking at past race results, an average speed of approximately 30 km/h seems sufficient to be classified as a finisher. We'll therefore set up the inverter and motor to travel at an average speed of 30 km/h. As an aside, the motor and wheels are connected by a chain, and the gear ratio is also a very important setting.
First, we'll look at the battery data sheet. This graph shows how long the battery can continue discharging at a given discharge current. The horizontal axis is discharge time, and the vertical axis is battery voltage. The race lasts 30 minutes, which is around here. If the battery current is about 13 A, it can discharge for 30 minutes. This means that if the battery current is 13 A while traveling at 30 km/h, it can run for 30 minutes.
The problem is whether it can actually reach 30 km/h while drawing 13 A. To run under these conditions, we change the motor winding and the gear ratio. Time was limited, so we could only make a few motor prototypes, but I managed to get close to this state.
Let's talk a little about the modifications, mainly to the motor and inverter. First, the inverter circuit: I replaced the MOSFETs, which are switching elements, with the aim of using MOSFETs with lower on-resistance. The MOSFETs included in the kit had an on-resistance of 3.3 mΩ. Replacing them with higher-performance ones reduced it to 1.1 mΩ.
However, the gate drive circuit remains unchanged, so the increased gate charge may increase switching losses. Since emitting noise and so on isn't a problem here, making the gate resistance as small as possible could also reduce switching losses. But the gate drive IC didn't seem able to withstand that, so I gave up on it.
There was also space to add electrolytic capacitors, so I installed some I had on hand. I don't think adding them has much benefit, but my approach is to do what I can for now.
Finally, I applied a moisture-resistant coating to the board and made a board cover. The operating environment is quite harsh, and dust, moisture, and other debris can adhere to the board and adversely affect the circuitry. These modifications reduce the likelihood of problems even if dust or other debris gets on it.
Next, we modified the motor. This time, we took the approach of reducing its copper losses. The copper wire originally included in the kit was 1 mm in diameter. We'll use 1.4 mm wire, keeping the same number of winding turns. We chose a thickness just at the limit of what could fit in the stator slots.
With 1 mm wire, the motor's winding resistance is about 100 mΩ. Increasing the diameter to 1.4 mm doubles the cross-sectional area, halving the winding resistance to 50 mΩ. This can halve the motor's copper losses. At an average current of 13 A, it saves about 8 W.
But Tsukuba Circuit isn't flat; there are uphill sections too. Although the average is 13 A, the current can reach 30 A in some cases, and momentarily 100 A. Assuming 100 A, thickening the winding reduces copper losses from 1,000 W to 500 W, making this a very impactful modification.
Other modifications include adding a voltmeter to check the battery level while driving, and an ammeter to measure the battery current while running. The ammeter is a clamp type with Bluetooth, monitored on a smartphone. It's the easiest solution.
Here we are on race day. The race is managed by CQ Publishing and organized by the Japan Electric Vehicle Racing Association. This is footage of us at the site servicing the vehicle. After a brief operation check, I did test runs of about 20 meters. Now we just need to keep the battery charged until the race starts.
And so the race begins. We've started. We're off to a good start.
After this, around the top of the hill on the home straight, the breaker tripped! I turned it back on to accelerate. The breaker included in the kit is rated at 50 A, but a closer look at the regulations shows that the race breaker distributed on race day is 30 A. That means subtle changes to the settings are needed on the day. This is something that can catch you out if you don't read the regulations.
I had read the regulations beforehand and adjusted the microcontroller settings to reduce current during acceleration. Still, it seems the adjustment wasn't sufficient, making the throttle harder to control. For a while, I repositioned myself so I could immediately reset the breaker if it tripped.
The battery current monitored on the phone is quite high, sometimes exceeding 100 A momentarily. However, because of the breaker's operating characteristics, a momentary overcurrent won't trip it. Ideally, you should carefully read the breaker's operating characteristic chart and, during acceleration, apply current just short of the point where it trips. I think that would let it run quite well.
After running for a while, a group appeared driving in a line like this. This is a team from Tsukuba College of Automotive Technology. They seem to be using the slipstream to reduce aerodynamic drag. The EV kart regulations prohibit attaching aerodynamic parts, so this kind of strategy is possible if you enter multiple teams. The driver is exposed, so aerodynamic drag is quite large.
Since we were there, I decided to follow this group to reduce aerodynamic drag. Getting too close could cause an accident, though, so we'll keep some distance. After that, it's just a matter of continuing to drive. The maximum speed is about 35 km/h, perhaps a little faster downhill.
I stayed with the group ahead and sometimes tried to overtake when the course was open. But even at full throttle, I couldn't increase the maximum speed any further. I think it's because the battery voltage and motor winding aren't matched. So I decided to stay behind them instead of overtaking.
Sometimes fast karts pass us. Those are karts from the Unlimited Division with lithium-ion batteries. The different batteries make them considerably faster.
We're now in the second half of the race. The battery voltage is getting much lower, and the speed is dropping too. Uphill slopes are quite tough. Coming out of the final corner onto the home straight, this uphill section is the hardest part. I'm not sure I can make the climb.
It gradually slowed down like this. In the end, I couldn't make the climb. I tried rocking my body to use the momentum to climb, but it was no good.
In the end, I couldn't finish the last lap. It ended as it was.
Here are the results. He finished the race in 11th place. I think that's pretty good for a first-time participant. I had set a goal of finishing the race. The number of laps was seven. The best time is 3 minutes and 49 seconds.
In a car that's going to race, it seems that most of the lap times are around 60 seconds. Compared with this, I'm not sure how slow you're running. I think you know what I mean. The average speed was 30 km/h, so it's rather on target.
It consumed more power than expected. Finally, the battery ran out, leaving me unable to run. To run to the end, you need to change the way you run and the way you run. The only way seems to be to increase the efficiency of the motor.
As for how to run, for example, more periods of coasting. It's somewhere around there. Slowing down acceleration may also help.
In this case, I didn't have time to fully refine the inverter and motor control. Battery energy was also not being used effectively. This has led to this result.
Are there any areas that can be improved on the motors? I'm finding some of them. Next year, I will finish the race and finish it. I will try to keep it running.
So thank you for watching the video until the end!
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