This project uses lead-acid batteries harvested from other equipment; basic safety systems; and a permanent-magnet DC motor specifically designed for light EVs. Inexpensive “donor” lawn tractors are easy to find in the classifieds or at local junkyards. The technology for this project is proven and readily accessible, since it comes from the golf-cart, mobility scooter, and forklift industries. It can be a “just-for-fun” project for riding around the backyard, or a powerful addition to your garden tool stable. And, down the road, the skills and tools you’ve acquired in this project can be useful in converting other small vehicles.
The fundamental parts of any EV are the batteries, motor, and controls. Secondary parts include an on/off switch, speed control, safety systems, and the mechanical attachments and brackets. Finally, you will need a charging system for the batteries.
This project initially used four 12-volt sealed lead-acid (SLA) mobility-scooter/wheelchair batteries that were left over from another project. At 22 Ah and 48 V, they make a pretty small battery pack, but they’re handy and will give us real-world testing ability so we can figure out the battery capacity we really need. Used 12 V flooded lead-acid batteries are also an option—they’re inexpensive and readily available.
The “beta” motor, which we’ll use as a test motor, is a Briggs & Stratton 48 V, 10 kW ETEK. It’s small, lightweight, and fairly powerful. Another motor, the Motenergy ME1004, is designed specifically for an electric lawn-tractor conversion. It has a heavy-duty brush design and a 1-inch output shaft to accommodate the stacked pulley that drives both the tractor and the mower deck. It’s also rated at 48 V, but can provide more power and torque. If we can demonstrate that the tractor is worth the investment with our beta motor, the ME1004 will be a worthy replacement.
Since most mowers of this type are run at full-throttle most of the time, and the tractor already has a transmission, clutch, and brakes, you can skip spending a few hundred dollars on a DC motor speed controller or a throttle. We’re simply going to turn the motor fully on and off, and manage the rpm with battery pack voltage. Forty-eight volts should provide about 3,000 rpm on either motor, which is pretty close to what a gas engine will produce at full throttle.
Some safety mechanisms for the on/off switching are necessary. Almost every EV has a contactor or large electromagnetic relay that uses a small amount of current to control the full power of the battery pack. A fuse is required that can accommodate the motor’s 100 to 200 A draw. These parts are available from golf-cart/forklift parts companies or EV conversion suppliers.
Simple gauges, found through golf-cart part suppliers, will indicate battery state-of-charge (SOC). Since the tractor spends most of its life sitting idle, a large, fast, and expensive charging system isn’t needed. Although you can charge the batteries individually with an automotive 12 V charger, a low-current charger that matches the voltage of your pack is a smart purchase. Thirty-six and 48 V systems are very common, and a simple 2 A charger costs less than $50. It may take as many as six hours to fully charge the batteries (depending on the battery capacity), but that can be done overnight. You might also use a small PV array and charge controller to recharge the batteries on the tractor’s days off.