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Discussion Starter · #1 ·
Hey, I'm an engineering student from australia and i have challenged myself to build a three-wheeled electric gokart for my final project. being a student, money is definitely a factor, I have found a place to purchase 6x12V 12Ah for around 110 AUD which is around 75USD, it is a good deal but i want to make sure that lead acid is good.
I have done as much research as i can justify as I am in year 12 aswell, but I'm stuck on the power source.
I have purchased This motor, its rated for 3000w, 72V and it says 50A. I assume that means that under load it pulls 50 amps of current, meaning that if I have 50 amps of battery, in an ideal situation with no loss i would be able to run it for an hour.

My main Problem is that I think lead acid batteries wont be very good because the maximum speed will decrease as the battery is used.

I don't have much reference, my family has a kids electric scooter, it has a small (450w give or take) electric motor, and when powered with two 12v 9ah lead acid batteries, it goes well (I'm too big for it now, so it goes much slower) but the max speed decreases pretty fast and I hear that that's the voltage decreasing as the battery depletes.
I also hear that this isn't a problem for lithium batteries as they maintain voltage until they are nearly dead.
I'm trying to figure out my estimated top speed, speed decay rate, and any other ideas or info. I'm new to batteries and any help would be GREATLY appreciated. if you have any questions, ask and I will reply as soon as possible, although my time zone is weird. thanks
 

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without spending too much time writing a long reply, here are my quick comments
  1. You are mixing too many systems together. The electrical (battery) system affects your top speed, but the drivetrain (motor power, gearbox) also does. Try to think of each system separately and understand each of them before understanding how they each influence each other
  2. Kid scooter is not a good comparison with a gokart because they are 2 different vehicles. weight is different, traction is different amongst other things
  3. Ultimately, you didnt mention what kind of top speed you are looking for (as well as other key parameters like weight, acceleration, range, overall cost), so it's hard to really help even if we wanted to.
 

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Howdy,
As with most DIY projects, ya gotta do lots & lots of research (first)

Look at what others have done & how
...make notes on the stuff you like (or makes sense)
...then, copy/mimic when designing & building your kart

Here is a video of a 3 wheeled kart, I built, kind of based on a Polaris Slingshot
48V 1,000W brushed motor (~1.2HP)
Top speed seems to be ~16 MPH
 

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Discussion Starter · #4 ·
without spending too much time writing a long reply, here are my quick comments
  1. You are mixing too many systems together. The electrical (battery) system affects your top speed, but the drivetrain (motor power, gearbox) also does. Try to think of each system separately and understand each of them before understanding how they each influence each other
  2. Kid scooter is not a good comparison with a gokart because they are 2 different vehicles. weight is different, traction is different amongst other things
  3. Ultimately, you didnt mention what kind of top speed you are looking for (as well as other key parameters like weight, acceleration, range, overall cost), so it's hard to really help even if we wanted to.
thanks, that's really helpful.
I understand how each system works, my main question is how max speed will decrease within an hour of use, regardless of what that top speed is.
I have watched as many relevant build videos as I could, I feel like I have everything else set up well, it's just the battery desicion.
my estimated specs are as follows;
weight, 100kg with rider, 50kg alone.
top speed, 30km/h
acceleration, 3m/s/s
overall cost is tricky, since I'm doing it with my engineering tafe, they are buying the raw materials, I just have to worry about the motor kit (it has everything but the batteries) and then the batteries. I have the motor kit, I bought it for 400 aud (180 + 200 shipping + 30 handling)

my top speed is largely dependant on the motor, as long as the batteries can keep up the volts and amps
 

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Discussion Starter · #5 ·
Howdy,
As with most DIY projects, ya gotta do lots & lots of research (first)

Look at what others have done & how
...make notes on the stuff you like (or makes sense)
...then, copy/mimic when designing & building your kart

Here is a video of a 3 wheeled kart, I built, kind of based on a Polaris Slingshot
48V 1,000W brushed motor (~1.2HP)
Top speed seems to be ~16 MPH
dude thats sick!!
it's funny, aside from the front suspension, that's is almost exactly what I'm designing.
do you have links for the HUD elements?
 

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The joke used to be ' There are liars, damn liars, and battery salesmen'.

It is not exactly fair but the way batteries are rated leads to misunderstandings. Batteries are built differently to achieve different goals. I am guessing the batteries you are looking at are meant to be used in UPSs. The defining critea is 'cheap' since most of us rarely need the battery backup that our UPS gives us.

Lead Acid batteries are commonly rated at the 1/20C rate. This means that if you discharge them for 20 hours, the total Amp Hours will add up to 12 Amps. This means that they can deliver 0.6 of an amps for 20 hours.

If you discharge them at a higher rate, say the one hour (1C) rate, you will not be able to discharge them for an hour at 12 amps. I would guess you would be lucky to get 6 amps for an hour.

If you tried to discharge them at the 50 amp current you mentioned, the voltage would sag probally in half, so you might see 36 volts * 50 amps briefly and would probably damage one of the cells.

This is because these cells are designed for low currents. If you really want to stick to lead acid, you need to look for ones rated at the 1C rate like Optimas. These tend to be expensive.

I would strongly suggest looking at lithuims. We use ones sold for R/C planes and cars. They run arond the same cost as the expensive lead acids but the cycle life is much greater making them a bargain.
 

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<I assume that means that under load it pulls 50 amps of current, meaning that if I have 50 amps of battery, in an ideal situation with no loss i would be able to run it for an hour.>

Rereading your first post, I just want to make sure you understand that if you wire 6 of the 12 volt 12 amp hour batteris in series, you will have a 72 volt 12 amp hour pack (rated at the 1/20 C rate), not 72 volt 72 amp hour pack.
 

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Discussion Starter · #9 ·
The joke used to be ' There are liars, damn liars, and battery salesmen'.

It is not exactly fair but the way batteries are rated leads to misunderstandings. Batteries are built differently to achieve different goals. I am guessing the batteries you are looking at are meant to be used in UPSs. The defining critea is 'cheap' since most of us rarely need the battery backup that our UPS gives us.

Lead Acid batteries are commonly rated at the 1/20C rate. This means that if you discharge them for 20 hours, the total Amp Hours will add up to 12 Amps. This means that they can deliver 0.6 of an amps for 20 hours.

If you discharge them at a higher rate, say the one hour (1C) rate, you will not be able to discharge them for an hour at 12 amps. I would guess you would be lucky to get 6 amps for an hour.

If you tried to discharge them at the 50 amp current you mentioned, the voltage would sag probally in half, so you might see 36 volts * 50 amps briefly and would probably damage one of the cells.

This is because these cells are designed for low currents. If you really want to stick to lead acid, you need to look for ones rated at the 1C rate like Optimas. These tend to be expensive.

I would strongly suggest looking at lithuims. We use ones sold for R/C planes and cars. They run arond the same cost as the expensive lead acids but the cycle life is much greater making them a bargain.
thanks, that's super helpful.
I think I would be right in saying that using 6*12v 28A batteries would give me half an hour of charge?
 

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One thing you need to learn, which is not easy, is to read (& accept, if credible) what you don't want to hear vs reading selectively with bias to suit your agenda. It was clearly stated to you:

"If you discharge them at a higher rate, say the one hour (1C) rate, you will not be able to discharge them for an hour at 12 amps. I would guess you would be lucky to get 6 amps for an hour."

So, no, you're not going to get 50A for a half hour. Nothing even close.
 
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