EV OTK build

Hi all, I’m the other Ghost Racing guy, Matt. You might be familiar with Derek he’s way more active online than I am. However I decided on kind of a whim I wanted to try my hand at building an EV kart that has a shot at being a legitimate race class. With that in mind I worked on designing/building an EV package that would drop into a standard kart chassis with no chassis modification. Here’s my breakdown

Goals:
Ka100 speed
15mins of track time at race speeds
battery in place of fuel tank
no heavier than Ka100 package

Powertrain:
Battery: 72V (20s8p) Molicel P42A custom build by myself
Motor: 45Nm 15Kw
Controller: up to 200A continous, 400 burst

Weights:
Battery: 28lbs
Controller with mount: 9lbs
Motor with mount: 32lbs






7 Likes

Wow that is nice. Do you really think it will get 15 minutes of decent speed run time with that small of a battery? How many KwH is the battery? You’d need 4 or 5 of these battery packs to run one race day.

I’ll let Matt give you the nerdy math, but so far so good. Battery testing is underway. The goal would be 2 packs and/or one charge per race day.

Most places near us do practice (sometimes x2) heat then feature. With 15 minutes of real race power you should be able to get through no problem. You add in all the down time during a race day and you got plenty of time to charge up as well.

I’m excited Matt has chosen to share this build with this forum. Tons of cool stuff he has done.

I’m confident we’re going to hit the market with the simplest, lightest, most affordable Ekart package.

3 Likes

How I cam up with 15 mins at race speeds

Battery specs: 4.2 Ah per cell, 8 parallel cell groups = 33.6Ah battery pack
Real life specs of cell @ discharge rate of 20A per cell (20A per cell, 8 parallel cell groups = 160A draw)
3.8Ah *8 = 30.4Ah of usable battery power

Side note: to answer your question the battery pack is 2.4Kwh

30.2Ah capacity 150A draw continuous for the motor gives us .201 hrs or 12 minutes of full power draw.

Educated guess on % time at full throttle around local track is 70% giving us 17.25 Mins of power.

The extra benefit of this setup is it has coast down Regen, so during all braking zones you are gaining some power back, nothing crazy but enough to help tilt the scales in the correct direction.

with ~17 mins of race time I expect 15 mins would be doable with this setup.

For our local club which Derek eluded to our local track is 5 mins practice, 8 lap heat race 20 lap feature. Usually about 1 hr in between each on track session, I would say with the charger I have for this (16Ah) I would need 1 hour after the heat race to get the battery to 100% before the feature, or makinng sure it gets plugged in after each track session.

I think the biggest need for multiple packs would come from practice days, 15 mins of track time on a practice day typically goes by pretty quickly, it would be nice to swap out to a fresh pack to start it charging without interrupting your day. Which this setup is capable of doing, no need to undo any hardware to remove teh battery, just unplug, unlatch and pull out.

5 Likes

Are you actively cooling the battery? Heat might become a problem.

We currently do not have any active cooling for the pack, we believe we will stay within the margin for the cells without it.

Also from the testing docs a 20A draw increases the battery temp 40C over the full capacity. The upper temp range for the battery is 60C for long life and can reach up to 100C without failure. With those in mind I would expect the pack to get close to the 60C after a feature race when it’s 80F+ outside.

The pack in this kart has a BMS in it which monitors the pack temperature and will shut the kart off it gets too hot 80C.

Can these run in hot climates?

It should be able to yes, we need some practical testing though. I would expect seeing the pack go from 45-80C in extremely hot climates

1 Like

Matt, what you are doing is ambitious and us arm-chair engineers really shouldn’t be scrutinizing your efforts, at least not too much but most questions are purely out of curiosity. I do have an interest electric drives and batteries, as I was in the electric vehicle charging business for 20 plus years (retired). Started with fast charging electric forklifts, all the way to charging electric cars and buses.

I’m not a battery expert but I do know that single cell performance is much better than when they are packed together because the inner cells have no where for the heat for the heat to go to.

Really cool work and the concept of fitting the battery where the fuel tank was is great. Can I ask how much you’re costs are for a KwH?

I just wanna race the darn things which is why I’m curious. I’ll take e-racing over no-racing any day.

1 Like

That it seems reasonable so long as you don’t have too many missed starts. It should be fun!

PS, I added your battery configuration to the spreadsheet.

Larry I welcome scrutiny and questions, there very well may be items I overlooked.

I do agree in a pack heat isnt dissipated like a single cell, in the report I am referring to they leave the cell in an insulation box to do the test, I am hoping that having the pack “open” to some airflow will help mitigate the heat buildup in the pack. Again I didnt do a thermal model, I know middle pack will get the warmest during the discharging.

I will have more information to share as I continue running it as I’ve got 4 temp probs placed throughout the pack to help track temps.

Just the Cells were $760 for the 2.4 Kwh
All in for the pack with cell tray/copper/nickel/wire/connectors/BMS ~$1000

so about $420 per Kwh

2 Likes

Agreed, though on pace laps I would not expect near the amp draw since we typically go half speed at most.

I’m trying to understand battery terms. I’m looking at BSR 25KW in the range of Rotax Senior. I was thinking your
2.5 decimal was in error but it’s not.

I prefer ION to gas but as you note, time is short requiring multiple batteries and chargers. It was expensive to begin with but multiples to stretch time makes it cost prohibitive.

What am I missing between your 2.5KW v 25KW? BSR is in Latvia. They do a lot of battery racing in Europe, nothing in USA.

When dealing with batteries and electric motors, many get the terms kWh and kW mixed up. kWh is an abbreviation for Kilowatt-Hour and is a measurement of energy typically used to measure the energy capacity of a battery or how long it can put out a certain amount of power. So a 2.4 kWh battery would be able to put out 2.4kWh for 1 hour or 14.4 kW for 10 minutes. kW is an abbreviation for Kilowatt and is typically a measurement of power of a motor, it’s the same as Horsepower. 25KW = 33 HP.

You need to check out Vextrek, LLC in Evans Georgia. They already are making a conversion package from gas to electric for karts. The tests that they have done are matching TAG times at Lamar County Speedway in Barnesville Georgia and they have also turned in good time at Atlanta Motorsports Park in Dawsonville Georgia.

1 Like

Larry nailed this explanation better than I could have, thank you.

I have seen their kart as well yes very cool build.

@Matt_Geist thanks for sharing your project.

Speed has never really been a challenge for electric karts (or cars) in recent history honestly. TAG\Shifter speeds and more were surpassed some time ago. Run time (per charge) is solved too.

The biggest challenge has been controlling heat dissipation (Both batteries and motors) and (of course) weight. There was an electric class run somewhere in cali a couple of years ago, times were right there between an 80 and 125cc shifter… But keeping them cool was the challenge.

More electric topics here: #electric_karts

1 Like

Cost is the other thing that has been prohibitive until recently. If Matt can keep the package simple, light, and affordable, I think there is a huge untapped market there.

2 Likes