DIY Solar Lawn Mower

Tools:

• Soldering iron
• Wire stripper
• Volt meter
• Screwdriver
• Wrenches

Materials:

• Battery-powered lawnmower
• (2) 12 volt photovoltaic solar panels
• 4 general purpose rectifier diodes
• Double-stick tape
• Nuts
• Bolts
• Washers
• Solder

Step 1: Evaluate the Lawnmower’s Current Condition

I had a DR Neuton Mower, but this Toro came up on Freecycle. It was way more mondo than the DR, so I decided it would be the donor machine.

The first thing I did was check the batteries. They were toast, so I had to build a new battery pack.

I got four replacement batteries at my local electronic supply for $18.00 each. To keep them as a cohesive pack, I applied double-stick tape between each battery, just like the original setup had.

Step 2: How to Wire It Up

A photovoltaic (PV) solar cell has a power output recognized in watts. When the sun is shining, the potential of the PV cell is greater than that of the batteries, so energy will flow from the PV cells to the batteries.

But what happens when the sun goes down? Then the batteries have a greater potential. That means that if you don’t take steps to prevent it, energy will flow from the batteries to the PV cells. This energy will be wasted as heat emanated from the PV cells, ultimately burning them out and draining the batteries.

We can prevent this by installing diodes in the circuit. A diode is like a one-way check valve for electricity. It makes it possible for the solar panel to charge the battery, but impossible for the battery to heat the solar panel.

The circuit below shows the typical wiring for this type of application. This system uses four 6 volt batteries and is charged by two 12 volt solar panels. The overall system voltage is 24 volts. When you line up batteries, their voltage adds as you place more in the series. The panels are 12 volts so we need to isolate them from each other. The diodes also accomplish this task.

Step 3: Hook up the Batteries

Returning to the battery pack. Let’s treat these four batteries as two sets of two. Hook them together as shown and test the voltage to make sure they show 12 volts per pair. OCV (open-circuit voltage) may be on the order of 14 volts. This is normal. In fact, if it’s below 10 volts you may have a bad battery.

Finally, there will be an interconnect between the two sets. As shown in the schematic, we need to tap this interconnect to hook up our PV cells. Do this using a wire stripper. Do not cut the wire, just breach and separate the insulation.

Step 4: Install the Power Taps

Just as we did on the interconnect, breach the positive power lead and install a diode. Make sure the band on the diode is closest to the red wire.

Step 5: Repeat the Process

Do the same thing again on the negative side.

This time make sure the band of the diode is facing away from the black wire.

Step 6: Scavenge Some Parts

With this PV panel came a cigar lighter plug. Yes, I said cigar lighter. Read your owner’s manual. That heat source is a CIGAR lighter.

We’re not going to use it, but we need to take a look at it.

First, cut the PV connector off. Leave a foot or so of wire on it and strip the ends.

Set that aside and let’s look at what we have left.

Open up the cigar lighter plug. There’s a circuit board in there. What do you think it does?

Step 7: Continuing with the Wiring

We’re now ready to connect the power taps to the PV power plugs.

Slide heat shrink tubing over the wire before soldering the wire to the diode. Attach the wires to the diodes and solder them in place. Next, slide the shrink tubing over the solder joint and the diode and shrink it down to insulate the joint.

Make sure to get the polarity right! The stripped wire from the PV panel is positive. Make sure this wire is connected to a diode that points toward a positive terminal of the battery. I’ve tried to make it clear on how to make this determination.

Step 8: Check Your Wiring!

At this point you should have two connectors wired through diodes to the batteries. Check these with a volt meter; there should be no voltage present. The diodes are a one-way check valve for electricity from the PV panels to the batteries, not the reverse.

Step 9: Continue Checking Your Wiring

At this point you’re all wired up and you can make some voltage checks to make sure you can safely proceed.

Step 10: Mount the PV Panels

Now that the hard part is out of the way, let’s get to the easy stuff.

These panels have keyhole shaped mounting holes. Place a screw in the hole and tighten a nut down over it. This gives you a stud mounting.

Align the stud onto the cover and drill mounting holes for the PV panels. Next, cut spacers to conform to the contour of the motor cover. Don’t forget, it’s all plastic and the stuff flexes really well. It’s pretty forgiving.

In this installation there were some reinforcements on the underside that had to be removed. Tin snips and an X-ACTO knife took care of the offending plastic pretty quickly.

Use the other half of the contour-cut spacer to shim the bottom of the mounting.

Step 11: Run the Wiring

Now that the PV panels are mounted, run the wires into the motor cover.

Step 12: Check the Solar Panel Output

OCV (open-circuit voltage) of these PV panels is on the order of 16 to 20 volts. If it is especially light out, this is the reading you should get.

Step 13: The Final Hookup!

Connect the PV panels to the battery banks.

Next, check your voltages. You should have two banks of 12 to 15 volts and the overall voltage should be at least 24 volts.

Step 14: There It Is!

It works and really works well. I’ve been mowing my lawn every day for three days and the mower is fully charged every time I turn it on. All I need now is a lawn.

More from Do It Yourself Projects to Get You Off the Grid:

• DIY Electricity Wind Turbine
• DIY Algae Bioreactor from Recycled Water Bottles

From Do It Yourself Projects to Get You Off the Grid by Instructables.com (Skyhorse Publishing, 2018) Copyright Skyhorse Publishing. All rights reserved. Used by permission of Skyhorse Publishing.