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.

Comments

[kinggargantuan]

I have that same fisher space pen. :)

[Captain John]

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?

[vmod32]

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

[Sassah122]

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!

[Marsh]

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

[Marsh]

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

[vmod32]

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.

[sandybrown097]

It is a nice idea to use solar panel on lawn mower
If you want to make it you need a 2-12 Volt Solar Panel
Here you need a cheap Solar Panel
You can use Surplus Solar Cells You can get pay back in 1-2 years
There is an Engineer from Chicago his name is John Sommer
He explain it All in his Blog http://topdiysolarpanels.com/

[patenaude]

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

[Marsh]

time will tell.

[sbeazley]

Let us all know when/ if that happens. Would be good info.

[9w2xyz]

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

[wirecutter]

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 ) ++++++++++++

[kill-a-watt]

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.

[lbrewer42]

Goats work well for lawnmowers :^)

[Marsh]

No they don't. People fall in love with them and start feeding them Barleycorn and molasses. From that point on, they stop eating grass.

[static]

That's a people problem, not a goat problem. :)

[blained]

Only because barleycorn and molasses-fed goats TASTE better.

[kill-a-watt]

this wouldn't last a week in my backyard unless locked in the shed.

[Marsh]

Ever consider getting a dog?

[kill-a-watt]

yes, but not for the sole purpose of guarding my lawnmower vs. locking it up.

[Marsh]

Ask any cop. The general consensus is that people who have dogs don't have their stuff stolen from their back yards. Even if it's just a yappy little rat-dog. You could always mount the PV panels on the roof of your shed and set it up to plug in when you park the mower inside. The kit I used must have come with 20' of wire and I re-used the plugs in the project.

[xenor]

Someone stole a bag of dog food from my backyard (even when guarded by yappy chihuahua)

[thebriguy]

...or add a car alarm (haha:-)

[luvit]

you mow dirt, like me. you are full of awesome. i also mow my trash.. you?

[EmmettO]

I was going to say, "Of course the solar panels are enough to mow your lawn! I don't see any grass!" Just kidding though. Lawns are evil. I try to kill mine as much as possible by putting in gravel play yards, gardens and a big pit that is supposed to be a workshop one day.

[Marsh]

They had a program in Phoenix a few years back where they paid you to tear out your lawn and replace it with gravel. Now grass grows through the gravel and you can't get on it with a lawnmower because you'll break your windows with the flying rock. What a cruel joke! LOL!

[EmmettO]

We put down that black weed barrier fabric before putting down gravel and it's held up well. Weeds will root on the rocks themselves, but they're few and easy to pull out. Also if you're not chemical squemish, a $14 jug of round up goes pretty far. I tried avoiding it but when my driveway started to sprout plants that will kill my wife (allergic) I got a jug and went to town on all the places that I'd been trying to suppress weeds mechanically.

[Marsh]

I actually have a lawn out front. Now that I have a mower, maybe I'll put one out back too.

[luvit]

whoa! people can have 2 lawns? i wonder if rich people have side lawns..

[RaNDoMLeiGH]

Stupid people have side lawns. (What were we thinking?!) We had a sidewalk and a driveway that also needed mowing, because they were very old and full of cracks.

[TinkerWorkshop]

Would it be possible to get the schematics of the full circuit? You have a small schema of the "solar battery goes here", but you lost me with the cigarette lighter circuit.

[Marsh]

The full circuit is there. This unit doesn't use the cigar lighter plug. It was only shown to indicate the voltage regulator contained within...which we did not use.

[TinkerWorkshop]

Ok then I am unconfused. Thanks! Would it be possible to run a cost approximate? mostly I am asking about the land-mower.

[Marsh]

It's pretty comprehensive and all of the info you're looking for is there. Some of it is only on the photo notes so be sure to look at all the pics and mouse-over to read the associated notes.

[talonts]

A few comments: 1 - The wiring could be cleaner/shorter if you rotate every other battery in the pack, so the +- jumper is very short. 2 - most solar panels have diodes built into them to avoid reverse flow already - if your panel screws together, open it up and check it before wiring. 3 - I'd recommend using Velcro to hold the panels on when not in use, and removing them while cutting - panels aren't really designed to be bounced around on a mower, even the ones made for charging cars. The shocks from occasionally hitting rocks or twigs will shorten the life of the panels. Better to remove them while in use. It also allows putting the panels in a better location for sunlight, as you noted in comments later. 4 - if you make the panels removable, cut the plugs from the panels, and use a 4-pin Molex plug to attach both. Molex plugs (ones with release tabs) will come apart easier than the plugs the panels in the 'ible have.

[Marsh]

Connecting and disconnecting are precisely what I want to get away from.Keep checking back. I'll post updates when it dies or develops any problems.

[thebriguy]

You might consider a charging pad you just push the mower into and it charges - Either plugging it in when you push it in or make the contacts somewhere underneath so when it's positioned right on the pad it charges. Just some ideas. Thanks for the 'ible.

[rhackenb]

I have this Toro mower. I've gone through two sets of batteries over 5 years. When I need to replace the batteries, I buy them from Toro. Instead, I get two 12 volt batteries from Batteries Plus or some other dealer. The cost is usually around $100 for the pair. I glue the batteries and wire them in series so that I get 24 volts. You say that the batteries you get cost $18. I wonder what the physical dimensions are and the amp hour rating. The two batteries that I use fit perfectly into the mower without any extra space. The batteries you use must be thinner. I appreciate the detail you have gone to in designing this. It taught me some lessons on electrical design. However, I'm not sure if the purpose is to charge the mower without having to plug into a wall outlet. If that's the case, a much simpler solution might simply be to plug the solar panels into an inverter and then plug the mower into the inverter. We actually use this solution to charge other battery powered devices such as drills at a shed we have that is not connected to the grid. See http://www.instructables.com/id/How-to-mount-a-solar-panel/.

The real benefit to me would be if the solar panels actually extended the current charge of the batteries while I am mowing.  Do you think these two panels would be sufficient to extend the charge in any meaningful way.  For example, could I mow an additional 30 minutes?   If not, the downside of the design is that the extra weight of the panels will make the mower even heavier to push.

Very good instructable.

[Marsh]

The replacements I used are made by Powersonic and they are exactly the same size and rating as the ones that came out of it (presumably the originals.) The idea was that I would never have to plug it in. I just park it on the back 40 and it's ready any time I need it. It runs for well over half an hour even if the grass is wet. I mean, I'm done in 25 minutes and the charge indicator is still reading full. Your batteries should last longer than that. They're probably being over charged by the on-board charger.