Introduction: Gas Beating Electric Race Kart
Invented in the 50's and race tuned over the last 60+ years, the kart is the training ground of F1 drivers and the simplest and purest driving experience on 4 wheels. What could make it better?
Arguably the weakest link is the noisy engine that needs regular and expensive rebuilds to keep it competitive. Why not keep the finely tuned frame, wheels and steering and replace the motor with something modern.
To improve on the experience and excitement of the original, the electric go kart will need 20+HP, a light weight battery pack and a minimum 20 minute ride time between charges. Here's how we did it....
Step 1: Choosing an Electric Drive
A rotax 125cc gas powered kart engine puts out 21kW (29hp) at 11,500rpm and 21Nm at 8750rpm, weighs 24kg (60lb) and costs around $3,500 rrp. To keep the gas karts honest we need an electric motor and battery pack that is not too much heavier to keep the handling sharp and a power output of at least about 15kW. With a much higher torque output and a wider torque band, an electric motor with a slightly lower power output will beat gas in pretty much every situation except top speed.
A popular high output, smallish electric motor is the Motenergy (previously Mars) brushless motors. We thought they were about the best electric equivalent to Rotax, so we selected the ME1114 which will deliver over 20kW (27hp) on 78V, a peak torque of 98Nm at 0rpm, weighs 16kg (22lb) and costs around $900. A cheaper option is the ME1117 with 20kW peak weighing 10kg and costing about $600.
You can get high power RC model motors with euiqalent power ratings that are lighter and cheaper, but the Motenergy motors will reliably run all day in a small electric vehicle, where as the RC model motors need to be derated for gokart use and are more prone to overheating and failures.
As well as the motor, you still need a controller and a battery pack. We're using a brushless PMAC (permanent magnet alternating current) motor which means instead of brushes, you need an electronic controller to turn the coils in the motor on and off. These give greater efficiency, power, control and reliability in a very compact package, but they do require a more expensive motor controller which is likely to be at least as expensive as the motor. In this case we are using a Sevcon size 2 controller which is reliable european brand and is fully programable which retails for about $700.
So for about half the price of a new Rotax motor, ($1600) we can buy a new electric motor and top spec controller with a similar power output to the Rotax.
Step 2: Battery Selection
The battery pack for the kart comprises 24 Headway cells, weighs 12kg (26lb), has a nominal voltage of 78V and a peak current output of 300A
The battery pack needs to be matched to the electric motor to ensure we can get the maximum output from the motor, we don't damage the battery by drawing too higher current and provides a reasonable range.
To keep the performance in line with gas karts, lead batteries, although being cheaper are not going to cut it. Apart from affecting the handling with their weight, they will also blunt the power output of the motor. This is because lead acid batteries are not only much heavier for their storage capacity compared to lithium, but they also have much lower peak current outputs, this means that we will need to size a much larger battery pack size so that it can handle peak current bursts. And lets face it, we don't carry around lead acid batteries in our phones. Lithium batteries are the obvious choice. I will leave to other websites or instructables if you want further info on battery types.
There are a few different lithium battery formats available including pouch cells (common in RC models), large format prismatic (usually used in car conversions), 18650's (this is the AA of lithium cells and is found inside the 18v Makita Li-on battery and other cordless tools or headway cells. Pouch cells can be ok, but are more prone to damage and it's harder to be sure of the quality. They can be the cheapest option, but probably the riskiest. The large format cells are not suited unless you are racing enduros as they do not have as high peak discharge so you will end up with a pack that has much greater range (or kWh) than you need for a the required peak current capacity. 18650's and Headways have similar power density and are both suitable, but the headway cells are a lot larger so you will need a lot less of them, making for a simpler battery pack.
There are a couple of different size Headway cells. We selected the 15Ah 40152 cells which are 3.2V, 15Ah capacity and can deliver a 300A pulse. At $30 each the 72V battery pack (to suit the Motenergy motor) requires 24 cells and costs $720. The battery pack also requires battery management and a charger, so the total pack cost was around $1000.
All up the battery pack weighs 12kg or 26lb (480g per cell)
Step 3: Building the Battery Pack
78 Volts DC from this battery is unlikely to harm you if you touch it with your finger but don't connect them to your jewellery, pacemaker or little brother. These cells will deliver huge currents if you short them out so be very careful not to accidentally short them. They will melt tools, cause nasty burns and can self destruct. Have a system in place to ensure you do not drop a tool or a tool across terminals of the pack and insulate your tools just in case.
Building a lithium battery pack is not difficult if you head the above warnings, go about it systematically and purchase a suitable battery management system. The battery management (BMS) can be daunting but you need it to avoid damaging your battery pack and to prevent the risk of a battery meltdown. The BMS monitors and limits charging currents, peak current draws from the battery pack and may also monitor the temperature of the pack, thus protecting your cells and leaving you to race your kart as hard as possible without worrying about your cells and we would recommend BMS as essential.
To build the pack, all you need to do is to connect each battery in series, i.e. positive to negative with a suitable connector. Our battery supplier, EV Power provided terminal lugs and plastic clips with the batteries to connect the cells together and the batteries come with screw terminals, so the first step in assembling the pack is as simple as clipping and screwing 24 cells together. There were a few decisions to make such as what was the best way to package the cells, a task reminiscent of using lego and finding the best way to package the power bricks together and fit them on the kart.
After connecting the cells together, the battery management will need a connection to each cell. The instructions for this will come with the battery management board that you are using. The BMS board from EV Power that we were using required one wire be connected to each positive cell in the correct order, so we started from one end and connected each wire in turn to the next positive terminal in a methodical fashion to avoid confusion and mistakes or ending up with a large tangle of wires.
The photographs show a BMS cell connector that was used in the pack for this kart. There is also an example wiring diagram for BMS (for a smaller capacity system than we used on the kart) showing connections to cells, charger and motor.
Step 4: The Controller
We are using a brushless motor so we require a suitable brushless motor controller. The controller takes 78 volts power from the battery pack, a 5v input from a throttle and inputs from position sensors in the motor (often referred to as hall sensors) and delivers power directly to the coils in the motor, rapidly switching the coils as the motor rotates.
The controller selected for the kart is a Sevcon Size 2 controller. Sevcon make a compact and reliable controller that is highly programable and the size 2 controller can be configured to 78V and will deliver up to 180A. We are running the motor within the 14kW capacity of the controller and a fair bit less than its 20kW+ peak that the motor will produce.
The Sevcon controller was sourced from EV Power along with the battery and motor and cost just under $700.
Step 5: The Chassis
This project is about building an electric drive train and fitting it to a Kart.
There are heaps of good second hand gokarts out there for under $1000, cheaper without an engine. We are using a sprint kart which originally ran a 100cc Yamaha J class engine for this project, but any style of Kart is going to go just as well with an electric drive train.
Step 6: Putting It Together
Putting it together just involves mounting the electric components on the kart chassis; motor, battery, controller and wiring it all up.
The electric motor has a face mount where as a standard kart frame has a foot mount for the motor. To mount the motor a 90degree angle bracket was made out of 8mm aluminium plate. One side bolts to the motor, the other bolts to the motor mount on the frame.
A small auto timing belt pulley (toothed belt) was fitted to the motor and a large pulley was fitted to the rear axel bolted up to the standard gokart sprocket carrier. It would be possible just to keep the regular kart rear sprocket and fit a chain sprocket to the motor, but chains are more noisy than a belt and we didn't want the quiet buzz of the electric motor drowned out by chain clatter.
The cells were assembled in two rows sitting vertically and a clear acrylic box was made to fit them in. The battery box was just thin enough to fit in the space between the kart seat and the side pod and was bolted to the kart frame using aluminium brackets front and rear. It is taller than the side pod so it sits higher and can be easily seen in the kart.
The controller bolted to the frame of the kart just in front of the motor and a small box in front of the controller houses an on/off switch, forward and revers switch an contractors. The controller has a power feed in from the battery pack, inputs from the battery management (BMS) which can turn the power off if the batteries run too low (or any other battery fault is detected), input from the throttle and sensor inputs from the motor. Three heavy gauge power wires connected directly to the motor.
Probably the most complex part is building the battery pack. Bolting the cells together is easy, but there are 24 BMC cell connectors that must be installed across each cell. Its not that difficult, but you do need to be very careful to follow the wiring diagram correctly or you will damage your BMS.
The throttle cable that normally goes to the carburettor in gas kart is attached to a throttle pot or electronic throttle (see photo). The electronic throttle has a +5V input from the controller and puts out a 0-5V signal back to the controller.
Big Red Button
The big red button is attached directly to the battery pack and can isolate the pack in the event of anything going wrong.
The a small on/off switch in the kart doesn't directly switch on the power to the controller and motor. A small switch operates a contactor (large amperage relay) which switches on the power. The contactor enables low voltage switching in the BMS to turn off the power if required to protect the batteries.
Step 7: Cost Summary
A frequent question on Instructables is what does it cost, so I've summarised the costs below.
Second hand race kart - varies a lot but you should be able to pick one up for under $1000 without an engine.
Motenergy motor $900
Sevcon size 2 controller $700
24 headway cells $720
Contactors, switches and wiring $150
Step 8: The Future of Karting
Racing Karts can be expensive to run with frequent engine rebuilds to stay competitive. With the electric karts other than the obvious no need for fuel and oil, the motor requires virtually no maintenance.
The battery pack installed on this Kart will be good for racing for around 45 minutes and the lithium cells will be good for up to 1000 cycles. It can be recharged in 30 minutes and if you absolutely can't wait 30 minutes for a recharge, a second battery (worth about $1000) can be easily swapped over.
Some controllers such as the Sevcon have the ability to program or tune the motor, but it is not the same as a gas kart which needs precise tuning of carburettors, exhaust, air intake, valves and timing to stay competitive. Tuning an electric motor involves plugging the controller into your computer and fine tweaking the torque delivery curve, throttle and regen mapping, so you spend more time having fun in an electric kart.
Karting, the Future is Electric.