DIY Solar Lawn Mower

Build a solar-powered lawn mower that you don't have to worry about keeping charged or plugged in, simply leave it outside in the sun.

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courtesy by Skyhorse Publishing

I’ve had battery-powered lawn mowers before and they are a real pain to keep charged. You have to either plug them in or take the battery out and that sucks.

This is one solution to the problem. Install solar panels on the mower and just leave it parked in the sun to charge it.

Here’s how I did it!

You can purchase this book from the Mother Earth News store: Do It Yourself Projects to Get You Off the Grid.

Tools and Materials Needed:

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
red lawn mower sitting on dirt with a clipping bag attached
solar panel surrounded with red silicon leaning against the black and white box that it came out of
roll of white double stick tape sitting on a brown table
metal nuts, bolts, and washers piled together on a brown table

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.

a solar power battery sitting on a brown table with two strips of double sided tape on it with a person pulling the backing off of one strip
four solar batteries taped together with double sided tape

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.

illustration of wiring to hook up the wiring of the pv panels to charge the solar battery

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.

four solar batteries lined up together with double sided tape holding them in place and two interconnects hooking them up in pairs
voltage tester hooked up to solar batteries taped together with double sided tape and connected with interconnects

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.

an interconnect with yellow ends sitting on a brown table
interconnect with insulation removed in the center with wires exposed
modified interconnect with wire wrapped around the center where insulation has been pulled back
solar batteries connected with interconnects sitting inside of the open top of a lawnmower

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.

hand holding red wire that has been split
red wire being worked on with soldering iron
red wire with diode soldered on to it

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.

wire cutters being used to peel back insulation on black wire
soldering iron attaching diode to 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.

pv panel plug with cigarette lighter car adapter with person holding wire cutters to the cord
pv power wire with end removed

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?

cigarette lighter plug with one side panel removed showing wiring and motherboard inside

Step 7: Continuing with the Wiring

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

top view of four solar batteries wired together with black and red wiring and yellow capped interconnects

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.

person holding a black wire with white heat shrink tubing slipped on to it
person holding a cigarette lighter flame up to heat shrink tubing on a black wire
person holding lighter up to heat shrink tubing that has almost completely shrunk on black wire

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.

two connectors wired through diodes to the solar batteries
person holding volt checker reading zero against connector

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.

voltage checker wired to solar batteries reading 13.28

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.

person holding a small wrench to a nut on a screw to create a stud mounting
cover of pv panel with two screws with nuts threaded on to them near the top

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.

person holding silver sharpie to black plastic with pv cover with screw placed next to it for reference point
electric drill placed against black plastic cover

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.

ruler placed next to screw stud placed in hole drilled in black plastic cover
white plastic spacers placed on stud mounts made of screws on red plastic cover
side view of solar panel placed on black plastic cover with plastic spacer over screw stud

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

screw top with plastic spacer covering threads pushed through black plastic cover
person holding wrench to screw head inside black plastic cover
red plastic protected solar panel mounted to lawn mower cover

Step 11: Run the Wiring

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

drill bit pushed through plastic of lawnmower cover
wire cutters being held against plastic cover of lawnmower above drilled hole
person sliding black wires through cut in black plastic cover towards drilled hole
two wires being knotted together coming out of a drilled hole in the plastic cover of a lawnmower
two black wires knotted together on black plastic cover
stapler pressed against black plastic cover of lawnmower below hole through which two wires are threaded
two staples in the plastic of the lawnmower cover closing up a cut made by wire cutters to drilled hole
two wired plugs resting on the inside of a lawn mower 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.

person holding voltage reader showing 17.95 volt output

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.

person holding wires connecting pv panels and battery banks
voltage checker hooked up to solar battery array in lawn mower reading 26.2 volt output

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:


green and white book cover with potted plants, solar array, and a small building made of recycled windows on it

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.