Solar Lawn Mower!

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I'm an environmentally conscious experimenter who loves to bring people together, build things, a...

Intro: Solar Lawn Mower!

Let's go green!
I've had battery powered lawn mowers before and the biggest problem I've had with them is keeping them 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!

Step 1: Tools and Materials Needed

Tools
  • Soldering iron
  • Wire stripper
  • Volt meter
  • Screw driver
  • Wrenches
Materials
  • Battery Powered Lawn Mower
  • 2 - 12 volt Photovoltaic Solar Panels
  • 4 - General purpose rectifier diodes
  • Double-stick tape
  • Nuts, Bolts & Washers
  • Solder

Step 2: Evaluate the Lawn Mower's Current Condition

I had a DR Neuton Mower, but this Toro came up on Freecycle (www.freecycle.org) . 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 3: 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 so the solar panel can charge the battery, but the battery cannot 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 4: Hook Up the Batteries.

Returning to the battery pack.
Let's treat these four batteries as 2 sets of 2.
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 5: 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 6: 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 7: 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 8: 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 +. 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.
If you are REALLY doing this project. I mean if you have the parts and everything and are actually in the process of assembly and having trouble, send me a private message and I'll be happy to call you (if you're in the USA).

Step 9: Check Your Wiring!

At this point you should have two connectors wired through diodes to the batteries. Check these with a volt meter and 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 10: 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 11: Mount the PV Panels

Now that the hard part is out of the way, lets 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 under side that had to be removed. Tin snips and an xacto 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 12: Run the Wiring

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

Step 13: Check the Solar Panel Output

OCV (Open Circuit Voltage) of these PV panels is on the order of 16-20 volts. If there is an appreciable amount of light out, this is the reading you should get.

Step 14: The Final Hookup!

Connect the PV panels the the battery banks.
Next, check your voltages. You should have two banks of 12-15 volts and the overall voltage should be 24 volts+.

Step 15: There It Is!

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

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    83 Discussions

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    NRG4UandMe

    3 years ago on Step 3

    amorphous panels (like those 1.5 Watt HF models) are low llife and effeciency - I tried thouse in the car once thinking it would help my small car battery in the hybrid, Not even the amount the computer uses while 'asleep' - these panels are next to useless, - If you get 5 or 10watt mono or poly then there is something more useful...

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    michaelb2

    5 years ago on Step 15

    The panels provide no appreciable power when the mower is in use, why attach them to the mower? Now the mower must be left out in the elements so that the panels have access to the sun.

    1 reply
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    Marshmichaelb2

    Reply 5 years ago on Step 15

    So what? Leave it out. It's not like the sun is going to hurt it. Any kind of plug will eventually wear out. Been three years and still works fine.

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    Captain John

    8 years ago on Step 15

    Would this mower work on the solar cell alone, or is a batery needed? I guess you can just park it in the sun and operate of the battery. Will the charging stop if the battery is fully charged?

    5 replies
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    vmod32Captain John

    Reply 8 years ago on Introduction

    lol... no way it would run off just the solar panels.. I think you have a major misunderstanding about how much power solar panels produce. Those panels could hardly power an MP3 player, never mind a lawn mower

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    Sassah122vmod32

    Reply 5 years ago on Step 15

    Actually no, I have exactly the same panel and I can power my 12 volt dvd player on a sunny day. But you are right, there is no way you could power a mower!

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    MarshSassah122

    Reply 5 years ago on Step 15

    It takes a week to recharge the mower for it's next use.

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    Marshvmod32

    Reply 8 years ago on Introduction

    These panels keep the batteries charged and ready all of the time. They're not very powerful. They probably won't overcharge it.

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    vmod32Marsh

    Reply 8 years ago on Introduction

    Exactly, they just slowly charge the batteries. Very slowly since there are a lot of batteries. If you have a small lawn that does not need cutting very often, they may provide enough juice to never have to plug in the mower. Or at least extend the time between needing to plug it in. But there is no way the panels could power the mower without the batteries. They probably provide 1/1,000 of the power the mower would need to run. But store up all that power over several days in the batteries and your running the mower.

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    patenaude

    8 years ago on Step 15

    I'd be worried that if you left it out for long periods (weeks->months), that the panels would eventually cook the batteries.

    2 replies
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    9w2xyz

    8 years ago on Step 3

    nice. Did the same thing with my truck. Came back to see the panels stolen. At least they left me the diodes...... pfffftttttt....

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    wirecutter

    8 years ago on Step 3

    Nice device but you only need ONE diode on each cell ( it is a series connection you can redraw it with the two diodes connected next to each other) as with two in Circuit you lose about 1/2 a Volt and that in the UK's sun is important in giving you a charge in to the battery. ( Image = 2 diode ) Even better is to join the positive of one panel to the negative of the other battery get rid of the junction to the middle of the batteries and then you only need one diode in the circuit and you are charging at 24 volts ( Image = 1 diode ) ++++++++++++

    2 diode.jpg1 diode.jpg
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    kill-a-wattwirecutter

    Reply 8 years ago on Introduction

    ayup. both will work fine. First will charge even if the sun blocks one of the panels (though only one pair of batteries will charge), and the second will avoid a drop of six tenths of a volt by removing one diode.