Electric Go-kart

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22 May 2013 - We've bought a go-kart

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A neighbour and I have gone shares in a home made go-kart, bought at a garage sale for $100. It's big and runs from a clapped-out old lawn mower motor. As it happens - I've been doing some work for EVLAb and they have an old electric motor in good condition that should work well. So I plan to convert it over to electric drive.


26 May 2013 - Cutting it down to size

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Daryl from EVLAb has a friend, Bernie (Metal Technics Ltd), with a good metal workshop at home. We took the go-kart there to build a mount for the electric motor. We quickly realised there were a lot of reasons why we couldn't simply replace the old engine with the electric motor;

  • it was only 1 wheel drive and 1 wheel brake - I wanted to have two wheel drive and braking so that it might work on grass OK
  • the back end was way bigger and heavier than we needed
  • our electric motor drives in one direction and freewheels in the other - which meant the motor and drive sprocket needed to be swapped to the right hand side
  • the back end of the frame was warped
  • the rear axle was a mish-mash of different part sizes; 25 mm and 1 inch

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So we spent three hours cutting most of the back end off.

Attach:IMG_20130526_132558_0_Small.jpg Δ We must have cut at least 300 mm off the rear end, and lost about 10 kgs.

Next we'll buy some new bits to rebuild the rear axle properly.


2 June 2013 - Wheels and axles

Another session at Bernie's place. This time we got the rear axle mounted and cut off the rubbish that had been welded to the rear hubs.

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15 June 2013 - The back end

After our last session Bernie reconstructed the rear hubs so that they would fit onto our new axle properly. Now they'll be mounted with grub screws which lock into indents drilled in the axle.

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Today Bernie and I spent several hours working out how to mount the motor. This involved removing another superfluous cross tube.

Attach:IMG_6104_Small.jpg Δ|Bernie milling out slots in the base plate that the motor will be mounted on.

In our last hour we finished the motor mount and then worked out a very quick mount for the disk brake.

Next we need to:

  • trim 1 mm off the thickness of the sprocket
  • mount the batteries
  • work out a robust accelerator system
  • paint the frame to prevent rust

Adam is most excited by the potential for flashing lights on the go-kart... so I'd better add those too.

There is a chance we'll have something to test ride next weekend.


24 June 2013 - My first test drive, list of tasks

On the weekend Bernie finished off a lot of nice little details that will eliminate future hassles; like drilling dimples on the axle where the grub screws go.

Tonight I quickly strapped the batteries on and connected up the wires to give it a test drive.

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Success! It actually moves. It's very slow - but that's probably a good thing for 5-year-olds.

It's got plenty of torque: I drove it up the stop-bank behind our house. The tires work fine on the grass. It's pretty heavy with the current SLA batteries.

List of tasks

  1. Put No-more-flats in the left rear tire - which isn't sealed properly on it's rim. Done
  2. Wrap preotective foam around edge of roll bar to cover the sharp edges of the grill. Done
  3. Kids test drive(s). Done
  4. Add a meter to monitor battery voltage. Done
  5. Put shielding around the chain/gear to prevent little fingers getting munched. Done
  6. Paint the bare steel with undercoat. Done
  7. Move the volt meter out of the way.. Done
  8. Add a cut out switch at the back - makes it much easier to pull the cart backwards and good as an emergency cut-off switch for adults monitoring new drivers.. Done
  9. Try moving the batteries to the back - see what that does to the steering.. Done
  10. Upgrade the battery to Li-Ion so it's lighter. Done
  11. Add extra bracing to battery mount.
  12. Improve the throttle mount (so it doesn't wobble).
  13. Fix up the front steering - cut off some more metal, shorten the wheel base, reconfigure the handlebar arrangement, reduce the width between the front wheels to match the back wheels
  14. Change the rear sprocket for a smaller one to make it go faster.
  15. Fix the brake so that it doesn't rub.
  16. Add a big on/off switch and power on light for the driver.
  17. Add flashing police lights for Adam.
  18. Upgrade to larger/faster motor.
  19. Rear axle differential?


25 June 2013 - Adam's first drive

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He likes it.


27 June 2013 - More kids testing

We had four more kids doing very short test rides this afternoon. At one point there were two kids standing on the back with another driving. I really need to put a cover over the chain to prevent little fingers from getting mangled.

Minor tears when some of the kids didn't get a long enough turn.

I added a little meter tonight to monitor the battery voltage.


29 June 2013 - Chain guard added

I added a chain guard. It should keep little fingers out of the way.


30 June 2013 - A longer kids session

The co-owner came round with her two kids and spent at least an hour playing with it. Then another friend came round with their 3-year-old grandchild.

  • The steering is the major performance issue to address. It's currently pretty hard to steer.
  • Right now the top speed as a slow walking pace - which is perfect. I want more speed, but the kids don't need it yet.
  • I need to move the volt meter to somewhere out of the way - on the top of the steering wheel is to easily knocked.
  • The 3-year-old enjoyed driving it but steering was too hard and he couldn't understand that you needed to let go of the accelerator stop or slow down. So I suspect 3 is just too young. 5-years-old is fine.
  • It would be good to have a cut-off switch at the back so an adult can kill the motor easily from the back.
  • They love to stand on the back almost as much as driving it.
  • The batteries were still OK.
  • One of the kids was hesitant to try it. But after seeing the others enjoy it gave it a go. She was nervous on her first go, then over the course of the hour became progressively more confident. By the end she was keen to jump in and drive.
  • One of the kids had his foot run over - twice. There was tears the first time. He didn't need reminding after the second time. The tires are soft enough for it to not cause an injury.
  • The old bungy cords holding the batteries down seemed to work fine.
  • Adults love driving it too.

A very successful afternoon.


7 July 2013 - Neighbourhood kids

The Moera community lunch was held today at our local hall. Afterwards I brought the go-kart round to the hall/playground for all the neighbourhood kids to play with. I was really thankful for the low top-speed!

The kart was a roaring success. The kids loved it - even the older 10 years olds.


13 July 2013 - Moved batteries to the back

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I've moved the batteries to the back, behind the seat. This clears the front end ready for rebuilding the steering.


20 July 2013 - Ninja go-karting

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Adam driving, Max running.

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Max driving, Adam shouting.

Note the addition of an emergency stop button at the back. I thought this would be useful for adults, but it looks like it'll mostly be used by passengers on the back to annoy the driver.


31 July 2013 - Rear lights and stuff

I've added rear red LED strips so it's easy to see when the system is on. And the emergency stop button works now too.

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"Dad! Full steam ahead!"

Attach:IMG_20130731_162054_0-Small.jpg Δ|Falk, Finlay and Officer Adam screaming along at 2kph.


8 August 2013 - Lithium Ion battery

Attach:IMG_6305_Small.JPG Δ|"New" Lithium Ion battery installed.

The new (2nd hand) battery is much smaller and lighter. I've not yet tested it to see how long it will drive for. It came with a nice slide on/off mount so it's easy to pull out for recharging.

Attach:IMG_6300_Small.JPG Δ|New Lithium Ion battery in place.


22 August 2013 - Lithium Ion battery charging notes

My charger is rated at 29.33 volts, which equates to 4.19 volts per cell (29.33 / 7 = 4.19) which is close to the 4.2 volt standard for Lithium Ion batteries.

However reading through How to prolong lithium batteries indicates that if you only charge to 4.1 volts, or even less, you significantly increase battery life.

Discharge cycles and capacity as a function of charge
Charge level (V/cell)Discharge cyclesCapacity at full charge
4.30 (bad!)150 – 250110%
4.20300 – 500100%
4.10600 – 1,00090%
4.001,200 – 2,00070%
3.922,400 – 4,00050%

Every 0.1V drop below 4.20V/cell doubles the cycle; the retained capacity drops accordingly. Raising the voltage above 4.20V/cell stresses the battery and compromises safety.
[Source: How to prolong lithium batteries]

So if I only charge the battery to 4 volts (70% capacity) I get four times the battery life. I need to run some test to see if 70% charge will run the go-kart enough to satisfy the kids.

The other way to increase battery life is to discharge it less deeply - in other words; not run it dead flat, or even close.

Cycle life as a function of depth of discharge
Depth of dischargeDischarge cycles
100% DoD (dead flat)300 – 500
50% DoD1,200 – 1,500
25% DoD2,000 – 2,500
10% DoD (light discharge)3,750 – 4,700

A partial discharge reduces stress and prolongs battery life. Elevated temperature and high currents also affect cycle life.
[Source: How to prolong lithium batteries]

Chargers for cellular phones, laptops, tablets and digital cameras bring the Li-ion battery to 4.20V/cell. This allows maximum capacity, because the consumer wants nothing less than optimal runtime. Industry, on the other hand, is more concerned about longevity and may choose lower voltage thresholds. Satellites and electric vehicles are examples where longevity is more important than capacity.
Environmental conditions, and not cycling alone, are a key ingredient to longevity, and the worst situation is keeping a fully charged battery at elevated temperatures. This is the case when running a laptop off the power grid. Under these conditions, a battery will typically last for about two years, whether cycled or not. The pack does not die suddenly but will give lower runtimes with aging.


1 Sept 2013 - Lithium Ion battery monitor

I added a Low Voltage Battery Meter. It tells me the voltage of each cell in the battery pack and the total of the whole pack.


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Page last modified on September 05, 2013, at 09:43 am
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