Newbie Learning the Kart Ropes. Lots of Questions...HELP!?

[higgypoo]

Hey all. New here to the forum. I'm excited to start reading and learning as much as I can from many more experience people than myself. This is my first project...heck I don't even own a welder yet. So, there is a lot of learning and discovery going on. I'd like to explain the build my son and I have been designing on paper so far and see if I could get some help with some newbie questions, so if you have time to answer, talk to me like a 3 year old so I can follow Thanks in advance!!!

GOALS:​

1. Quiet
2. Cart weight with batt (150lbs) and carry me (200lbs) for total carry weight of (350 lbs)
3. 35mph
4. 30 min drive time

Performance Parts list:​

Motor: 72V 3000 watt brushless DC 6500rpm max This motor and ESC
Speed controller: packaged with motor
Sprocket / gears: 11 tooth : 54 tooth
Battery (option 1): 72v 40ah 0-3000W Lithium Ion
or
Battery (option 2): 12v 8ah (x6) 0-3000W rechargeable SLA Neptune
Tire size: 12.75” diameter total

Questions:​

1) Is my gear ratio correct for a slightly Gear ratio correct for slightly higher speed? I want more speed than torque..
2) Is the battery / motor combination going to work? Am I missing something there?
3) Does the speed controller do what a solenoid would otherwise do or do I need both?
4) Battery connection wire gauge? I was thinking 6 gauge..is that overkill?
5) will 1" conduit will be robust enough for a frame? I think with enough bracing it could, but want other opinions.
6) does anyone know of a good step by step guide to building one of these? otherwise, I'm gonna wing it haha.


Thanks again! Can't wait to hear from you all.

 

[madprofessor]

I'm going to stay out of doing the advice column thing on this one, as others here specialize in electrics while I don't. I'm not only a fossil, but strictly a fossil fuels kart guy. Functional Artist and Epsilon Zero are just two I can think of that know it all about electrics. They'll chime in, listen to what they say, they won't steer you wrong.

 

[EpsilonZero]

Questions:​

1) Is my gear ratio correct for a slightly Gear ratio correct for slightly higher speed? I want more speed than torque..
2) Is the battery / motor combination going to work? Am I missing something there?
3) Does the speed controller do what a solenoid would otherwise do or do I need both?
4) Battery connection wire gauge? I was thinking 6 gauge..is that overkill?
5) will 1" conduit will be robust enough for a frame? I think with enough bracing it could, but want other opinions.
6) does anyone know of a good step by step guide to building one of these? otherwise, I'm gonna wing it haha.

1. You have way too much gear. Theoretically, your top motor speed would get you over 50mph with that configuration.
2. You don't have any battery specs listed besides Ah and Voltage. The controller and motor combo seems to work up to 70A and it would be a good idea to match that with your battery. Your option for 6x 8Ah batteries are still only 8Ah in series so you won't get much run time at all.
3. Follow the directions for your controller. If it calls for a relay, then use one. Only my big kart needs a relay.
4. 6 should be fine.
5. Conduit as in for routing cables? No, it won't be enough.
6. I'd suggest search some youtube videos on karts, electric motors, and controllers. Read the documentation and look up anything you don't understand. Make sure you understand what you are buying.

 

[madprofessor]

Answering question #5, you can use 1" EMT conduit if you sleeve it over 3/4" EMT and plug weld them together every few inches, very strong that way, I've done it myself. Went 2X sleeving with 3/4" and 1/2" making a functioning monoshock springer front end for a minibike. For the long stretches of a kart frame though, you wouldn't do as well, unless you paralleled 1" and maybe 3/4" about 3" apart, with opposing diagonal struts between them to make a truss frame (NOT a ladder frame!) A truss has diagonal struts opposing each other, a ladder frame has straight struts like a ladder. The truss is amazingly strong to be so lightweight, but just because you could so easily do it, I'd sleeve in 3/4" into the 1", and 1/2" into the 3/4" anyway. Just because it's so simple to do.
NOTE: You CAN bend 3/4" EMT with 1/2" EMT inside of it with a common manual 3/4" conduit bender. I did it, and I only weighed about 150#. Did have to bounce up/down on it, but it did great.

 

[EpsilonZero]

I wouldn't advise welding zinc-coated conduit unless you have to and know what you are doing. Just get some proper steel or get an assembled frame.

 

[higgypoo]

Thanks so far for the feedback guys. I'm really trying to soak up as much as I can. I've decided to build the frame from 1" square tube steel 16g. I've also settled on getting a lithium Ion battery, Now I just need to make sure I get the right size for a 72v, 3000watt motor with matching controller...suggestions? So in short, i'm missing the understanding of batteries and how they work compared to the motor. Can someone make a generalized recommendation of specs for a battery I should be looking for for a go kart using a 3000watt 72v motor? Thanks!!

I'm going to stay out of doing the advice column thing on this one, as others here specialize in electrics while I don't. I'm not only a fossil, but strictly a fossil fuels kart guy. Functional Artist and Epsilon Zero are just two I can think of that know it all about electrics. They'll chime in, listen to what they say, they won't steer you wrong.

Thanks for the support!

1. You have way too much gear.
2. You don't have any battery specs listed besides Ah and Voltage. The controller and motor combo seems to work up to 70A and it would be a good idea to match that with your battery.
3. Follow the directions for your controller. If it calls for a relay, then use one. Only my big kart needs a relay.

Thanks for the responses. So, i've decided on getting a Lithium Ion battery pack. The motor is 72v 3000watt and matching controller...so I need to figure out what kind of battery I should pair with it. So far, I was just trying to match the numbers with a 72v 40ah battery. Am I right in thinking that is a good match and would provide me with roughly 30-40 min of run time? Or am I understanding that incorrectly? Would something like a 72v 200ah battery be a better fit for longer ride times?
Last question I asked, I think I was mixing my terms. I am asking about 'prechargers' Are those needed? with a lithium ion battery?

Answering question #5, you can use 1" EMT conduit if you sleeve it over 3/4"

Pretty clever. Yeah, i think i'm just going to use plain steel. I'd been looking at the electrical conduit before because I was a dumb dumb and priced out steel pipe at home depot and it was SO expensive. Finally found a steel yard that has really reasonable prices, so i'll just be going to square tube now...easier to cut angles on too.

I wouldn't advise welding zinc-coated conduit unless you have to and know what you are doing. Just get some proper steel or get an assembled frame.

Thanks for the heads up. yeah, had a friend just tell me that too. don't want to die, so i'll stick with normal steel haha

 

[EpsilonZero]

I think you're talking about a pre-charge resistor installed across a relay. It prevents a huge inrush current from blowing up your controller by allowing the controller capacitors to charge slowly before closing the relay. You'll likely be wanting a flyback diode as well. I can take a picture of one of my relays if you need illustration.

You need to know what kind of continuous discharge current and max discharge current the battery pack can handle and make sure you aren't going to exceed that with the motor and controller combo. You will also need to balance charge the pack and, while a BMS isn't absolutely required, it is recommended to keep your battery from detonating. That's another thing about lithium; if it isn't lithium iron phosphate (LiFePO4), it is much more likely to experience rapid unplanned hot disassembly. Those types of lithium are usually just called "lithium ion" or "lithium polymer". They are more energy dense batteries, but I wouldn't charge one in my house without safety precautions.

 

[higgypoo]

I think you're talking about a pre-charge resistor installed across a relay. It prevents a huge inrush current from blowing up your controller by allowing the controller capacitors to charge slowly before closing the relay. You'll likely be wanting a flyback diode as well. I can take a picture of one of my relays if you need illustration.

You need to know what kind of continuous discharge current and max discharge current the battery pack can handle and make sure you aren't going to exceed that with the motor and controller combo. You will also need to balance charge the pack and, while a BMS isn't absolutely required, it is recommended to keep your battery from detonating. That's another thing about lithium; if it isn't lithium iron phosphate (LiFePO4), it is much more likely to experience rapid unplanned hot disassembly. Those types of lithium are usually just called "lithium ion" or "lithium polymer". They are more energy dense batteries, but I wouldn't charge one in my house without safety precautions.

Ah, ok. yeah if you wouldn't mind taking a picture of where you've wired up your precharger and flyback diode, that would be amazing. I just can't seem to find much in way of illustrations showing the wiring of these DIY motors...i'm not an engineer type, so this is all new territory for me haha.

As for the battery pack issue...thanks yeah that helps clear up the 'requirements' issue I was having. So tell me if I'm missing any specs below, but it seems like this could be a good fit/match between the motor/esc and a battery. What do you think?

Motor: Motor listing /specs
50 amp @4900 RPM (Rated 4900 ROM @3000 Watt)
72V DC
Controller:
rated for 36-72V
rated current 65-100A
Battery pack: battery pack listing (72v 40ah)
BMS preinstalled
72V 40Ah 0-3300W(20S1P)
Nominal Voltage: 72V
Rated Capacity: 40Ah
Max Constant Discharge Current:60A (BMS)
Apply for: 200W-3300W

 

[EpsilonZero]

Motors can pull more current than their rating; potentially that whole 100A that the controller can supply. Theoretically the BMS will prevent damage, but the lower end controllers, motors, and batteries made of an unknown array of cells are sort of a gamble more than usual. Many manufacturers hide lower quality cells in packs or even produce counterfeit cells. If you were feeling squirrely, you could buy your own cells and make your own pack to be sure. I don't think that pack is 20s1p though... that's just a series of 20 cells.

That being said, you won't see max amperage being drawn outside of hard acceleration or hill climbing. I guess the answer is that it will "probably be fine". Just make sure you don't store that battery pack on the kart or where you don't mind stuff burning and make sure you balance the cells (I top balance).

 

[higgypoo]

Motors can pull more current than their rating; potentially that whole 100A that the controller can supply. Theoretically the BMS will prevent damage, but the lower end controllers, motors, and batteries made of an unknown array of cells are sort of a gamble more than usual. Many manufacturers hide lower quality cells in packs or even produce counterfeit cells. If you were feeling squirrely, you could buy your own cells and make your own pack to be sure. I don't think that pack is 20s1p though... that's just a series of 20 cells.
Just make sure you don't store that battery pack on the kart or where you don't mind stuff burning and make sure you balance the cells (I top balance).

I mean, other than the cost, I thought lithium Ions were supposed to be a safer and better option than say a series of SLA batts? Is there really a safe option here?
As for the balancing, the manufacturer made it seem like the bms balanced the cells for you,...is that possible, or are they yanking my chain?
Btw, thanks a TON @epsionzero for taking all the time to help me out man, I really appreciate the wealth of knowledge.

***edit: just did some more research....i've changed my mind. going the LiFePo4 route. I can get a higher A discharge safely and they're about the same cost.

 

[EpsilonZero]

I mean, other than the cost, I thought lithium Ions were supposed to be a safer and better option than say a series of SLA batts? Is there really a safe option here?
As for the balancing, the manufacturer made it seem like the bms balanced the cells for you,...is that possible, or are they yanking my chain?
Btw, thanks a TON @epsionzero for taking all the time to help me out man, I really appreciate the wealth of knowledge.

***edit: just did some more research....i've changed my mind. going the LiFePo4 route. I can get a higher A discharge safely and they're about the same cost.

I wouldn't assume anything with the BMS. It should detail what it does. It could protect from overcharge voltage, charge overcurrent, undervoltage, discharge overcurrent. It might balance with resistors or balance actively with capacitors. It might use Bluetooth, CAN, or proprietary interfaces for communication or just exist with no feedback. It might support LiFePO4, LiPos, SLAs, or other chemistries or a combination of them. Specifications vary wildly as well and some allow for lots of configuration. There are lots of possibilities; just take a look at the DALY BMSes for a good BMS overload.

SLAs are safer than LiPo (Lithium Ion). I'd say that LiFoPO4 are safer than SLAs in my experience. I've had LiFePOs fail and SLAs fail and the SLAs are the ones that got hot while the LiFePO4s just leaked a little fluid and sometimes made a little hiss noise.

 

[higgypoo]

Agreed. Yeah this is all excellent info. And if you do wind up getting the change to take pictures of that precharger and that flow back thing you mentioned, thats would be amazing. Thanks!

 

[madprofessor]

I only just now noticed your nom de guerre...............Congrats on the best name I've seen here yet. Makes me giggly too, higgypoo.

 

[higgypoo]

I only just now noticed your nom de guerre...............Congrats on the best name I've seen here yet. Makes me giggly too, higgypoo.

Haha thanks

 

[EpsilonZero]

Agreed. Yeah this is all excellent info. And if you do wind up getting the change to take pictures of that precharger and that flow back thing you mentioned, thats would be amazing. Thanks!

 

[Functional Artist]

Howdy

Sorry I'm late, I was riding (I mean testing) a kart.

The wiring "definition" for that Boma motor doesn't list a contactor
...& the contactor, is where the pre-charge resistor is usually mounted.

I have used Boma motors on a few different karts (Aerial Atom & Excalibur)
...& I just mount my pre-charge "circuit" right on the circuit breaker (ie mechanical contactor)

Here is a video, I made describing & explaining this DIY pre-charge circuit

 

[higgypoo]

Howdy

Sorry I'm late, I was riding (I mean testing) a kart.

The wiring "definition" for that Boma motor doesn't list a contactor
...& the contactor, is where the pre-charge resistor is usually mounted.

I have used Boma motors on a few different karts (Aerial Atom & Excalibur)
...& I just mount my pre-charge "circuit" right on the circuit breaker (ie mechanical contactor)

Here is a video, I made describing & explaining this DIY pre-charge circuit

haha, I watched several videos on YT yesterday trying to figure out how to wire one up...and I saved your video without knowing it was you haha. Its by far the most clearly explained one I could find. Thanks for putting that together. Where did you wind up getting your precharge circuit and breaker? Is there any special formula to determine the right specs for those 2 items? Thanks again so much! HUGE help!

 

[Functional Artist]

Yup!
The thread on my Excalibur kart has lots & lots & lots of info, that you may be interested in

The 60V info (circuit breaker specs./link & precharge info) starts in post #175
https://www.diygokarts.com/community/threads/2020-excalibur-electric-racing-kart.43462/

Doin' Donuts

 

[higgypoo]

Yup!
The thread on my Excalibur kart has lots & lots & lots of info, that you may be interested in

The 60V info (circuit breaker specs./link & precharge info) starts in post #175
https://www.diygokarts.com/community/threads/2020-excalibur-electric-racing-kart.43462/

Doin' Donuts

hell yeah! Thanks a ton, gonna watch/read all this today!

 

[higgypoo]

Yup!

Hey, I've been reading through a lot of your super helpful build notes, particularly on electronics. I'm trying to get parts together to build a precharge circuit, but I'm finding it really hard to find a breaker for a 72V system. Got any suggestion? WWFAD (what would functional artist do)??