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
Battery Powered Lawn Mower
2 - 12 volt Photovoltaic Solar Panels
4 - General purpose rectifier diodes
Nuts, Bolts & Washers
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.