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CONVERTING A GAS LAWN MOWER TO
SOLAR CHARGED ELECTRIC POWER

HOME PARTS LIST REMOVING
GAS ENGINE
INSTALLING
ELECTRIC MOTOR
INSTALLING
COMPONENTS
WIRING
THE MOWER
MOUNTING
THE BLADE
SOLAR
CHARGER
TEST
DRIVE
OTHER
CONVERSIONS

Solar Charging

My charge controller inside Since solar charge controllers are relatively expensive - at least for the good ones, I searched eBay for a good deal.  What I found was a bare circuit board for a 150 Watt Solar charger.  Since I'm am electrical engineer, putting it in a nice box, mounting a meter, and terminals was simple!  I sprayed the front panel with the same green hammer tone paint that I used on the mower.

If you're not an engineer then look for a new or used Trace, Xantrex, or Outback unit on eBay.  Harbor Freight also have some inexpensive units.  You should get a charge controller that is rated for more Watts than your anticipated panel Wattage so you have some "head room" and won't stress your controller.  I recommend a bare minimum of 60 Watts or a controller rated fro 5 Amps

My controller has 3 lights:
GREEN = charged
AMBER = charging
RED = blown fuse
2 connections for the solar panels, and 2 for the battery.  What could be simpler.

I added a 10 Amp meter so I can observe charging rate.

My charge controller front panel
Note my color coded screw terminals.  Polarity is very important!
I colored them with magic markers.

I originally planned to use an extension cord for the charging connector, but the exposed male plug on the charger side made me nervous, since the pins could short to bare metal.  So I looked around for another safer option and found an old computer power connector and a heavy duty power cord for it.  This way there are no exposed live connections.  I mounted the connector next to the power meters on the handle.
So here is the charger sitting on a shelf just inside my solar shack/garden shed.  I am using the 40 watts of solar panels that would normally sit unused during the summer that power the circulation pumps for my solar heated radiant floor in my workshop.  I have 2 - 10 Watt, and 1 - 20 Watt panel which is enough to charge the mower fully in one long sunny day.

Battery Charging
(81K pdf file)
Why use a charge controller?

If you simply connect your solar panels directly to the battery, you will over charge it and ruin it.  12 Volt solar panels can produce over 22 Volts in direct sun and this is too much.  Also when a battery reaches full charge the charging current needs to be removed or reduced to a trickle to protect the battery.  That is what a Charge Controller does.  It monitors the battery voltage and limits the solar voltage and current as needed to safely charge the battery.  Once the battery reaches full charge, then a trickle charge is used to maintain the state of charge.

Solar Panels

I had thought that 40 Watts would not be enough, but have found in practice that it is enough to re-charge the battery in 1 or 2 days of full sun after 20-30 minutes of use.  So the mower is always ready to go by the time I need it.

Choosing a Solar Panel

You will need enough power from the panels to charge your mower before the grass gets too long.  There are a lot of variables to consider if you are using different parts than I did:

  • Your average sun hours per day (solar insolation)

  • Size of your battery

  • Current draw of your motor

  • Frequency and duration of use of the mower

I happen to have 40 Watts of panels on my workshop roof that I use to run my solar heating system pumps in the winter months, so I can switch them over to charging duty in the summer.  In the overlap months where I need both heating and charging I just use a battery charger (see below).  Since we have opted to get our electricity from wind power it is clean power by default.  If you have the option of selecting your electric supply then consider switching to green sourced electricity.  We also have a 4kW solar array that produces enough power to spin the meter backwards on sunny days - so even using a plug-in charger is "solar powered" in our home.

If you get an average of 4-6 clear sun hours per day and use the mower every few days, I would suggest starting with 40 Watts or so and adding from there as needed.  There are many complex and confusing formulas for calculating the required panel size for any solar need, but I favor the empirical method, blended with some rule of thumb guess work to get started!

Alternate Charging Method

The other way to charge the mower that I use on cloudy days is a good old fashioned automobile battery charger.  Mine is an automatic unit that is relatively inexpensive.  You can find them at Sears or auto parts stores.  My 6 Amp unit will recharge the mower in 3-6 hours depending on how depleted the battery is.


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HOME PARTS LIST REMOVING
GAS ENGINE
INSTALLING
ELECTRIC MOTOR
INSTALLING
COMPONENTS
WIRING
THE MOWER
MOUNTING
THE BLADE
SOLAR
CHARGER
TEST
DRIVE
OTHER
CONVERSIONS