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Planing: Garden Cart from Lawn Tractor components

642 Views 4 Replies 4 Participants Last post by  cricketo
What I'm working on is a little electric garden cart for my wife to putter around on our little one-acre property to her various raised garden beds. They're scattered around depending on the ideal light conditions her various plants need. She hates the idea of driving something like a gas lawnmower pulling a cart because of how loud they are and how much physical work they are to maneuver.

I plan on using mainly lawn tractor components. Ideally, I'd find a junked lawn mower with a bad engine and no mowing deck. I'm still working out what kind of configuration to go with. I can either keep the motor and batteries up front, driver in the middle with an add on bed or box on the back for carrying tools and what not, or I can put the driver up front, the motor and batteries in the middle between the frame rails and the bed in the rear. That will depend on my wife and the tractor I find as a base. I believe the total weight when complete and fully loaded with driver will be around 1000 - 1500 lbs. I have no problem, cutting and welding to stretch out this cart. I just need to get a welder first.

This thing doesn't need to be a speed demon or have too much range. Its only meant to stay on our property which is relatively flat with a slight slope. I'd like to use a geared transaxle to drive as opposed to a hydrostatic transaxle simply because that way, she uses the electric motor to actually control the speed. Plus, if she wants to go a little faster, just slip it into a higher gear or need more torque, drop it down in gear.

The controls will be simple. A keyed switch to turn it on, a foot pedal for speed, digital voltmeter. Other features of the cart would be headlights, maybe a light pole with overhead lighting, a 25 or 50 gal water tank with a 12V water pump for watering her garden beds, simple tow hitch and be able to seat two people. Obviously I'd need a stepdown transformer for the 12V applications.

Now, I actually built exactly one successful EV conversion for a college class a few years ago, but with that project, we were given most of the components and we had to mostly just assembly it. It wasn't all bolt on, but I think the most complicated thing I had to do was build a mounting board for all the controls and electronics the bolt onto.

Now for my questions:
What batteries would be best? I was thinking of going with two 12V deep cycle batteries in parallel, but then I realized that there may be something better or that 24V might not be the best.

What motor would be best in terms of volts and speed? Because I'm using a geared transmission with a reverse gear, I don't think I'll need a reversable motor.

What kind of charging system should I use? This thing would be parked in the garage when not in use.

Now for my budget. This is going to be on the cheaper side, but mostly because I like using re-cycled components. About the only new components I'll use are batteries and motor controllers. I'm open to using new or used electric motors. I hear old hydraulic forklift drive motors are still a good option?

I'm sure I'll have more questions or thoughts later, but that's the overall gist of my project.
Thanks for the help in advance.
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Discussion Starter · #3 ·
Ignoring the obviously low IQ comments on here, I decided to do some number crunching last night with regards to power requirements and I'd appreciate any feedback on my calculations.

First, after a little research and some estimation, I figured my final design would weight around 750 kg, when taking into account the weight of my starting vehicle (lawn tractor), addons such as comfortable seats, water tank, tool bed, etc. I admit, its maybe a bit over, but I'd rather assume more mass than less.

Because this is just something for the wife to putter around with on our property, max speed should only be about 2.25 kph (5mph).
Because of the low speed, wind resistance would be negligible, so that factor will be a 1. Same with the cross-sectional area since that will be about 1 m^2.

Because I'm not sure on the efficiency, I'm going to follow the 80/20 rule and assume my electrical losses due to inefficiencies to be about 20%.

Now, I plug all that into the power calculation formula and get a whopping 16.5kW.

With those number, I divide by 12, 24, 36 and 48 volt power banks and determined that my A-hours to be:

12V - 1716 Ah
24V - 858 Ah
36V - 572 Ah
48V - 429 Ah

Now that I know what kind of power ratings I can expect, I have a question to help me with motor selection. Because I'll be using a geared transmission, will the mechanical advantage of the gear ratios affect the motor output? What I mean is, if a gear ratio has a 10:1 ratio, will that translate to a 1.65kW power output requirement for the motor while also increasing the electrical consumption by the motor?
 

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Buy a Ryobi electric riding mower from Home Depot (or better a used one on Craigslist). You have too many gaps in your knowledge and a trivial project, few people will be interested in engaging you with constructive comments.
 
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