A Solar Charger for Lee's Electric Lawn Mower

Latest update from Lee -- June 6, 2008 ...


Lee converted a Troy-Bilt gas gas mower into a battery powered electric lawn mower.  He has now added a solar charging system.  As Lee says, it is now a  TOTALLY GREEN MOWING MACHINE!. 
Details below.




From Lee:


Hi Gary,
OK, I've finally got a solar-charged lawnmower.  I purchased a 45W Solar Panel Kit from Harbor Freight a couple weeks ago (three 15W solar panels and a cheapie charge controller for $199), but had problems with the controller right off the bat.  I messed around with it for a couple of days before deciding that it was junk and started checking around for a better one.  On the advice of some friends and a silver-tongued tech rep at Xantrex, I purchased a Xantrex C40 solar controller on eBay for $112.  This is about twice what I had wanted to pay and a bit of overkill for my immediate application, but since I have a couple more solar electric projects in mind and wanted to keep my options open, I opted for the C40 because it seemed to have plenty of power (40 amps) and be the ultimate in flexibility.



Anyway, I mounted the three solar panels on my back porch roof, which faces south and has full sun all day.  I mounted the controller on a 4X4 support beam behind a couple of baffles to keep it dry if it ever decides to rain again here in Florida.  On the advice of the above-mentioned Xantrex tech rep, I wired the the three 15W solar panels in parallel, as well as the two 12V batteries on my mower (when mowing, the batteries are connected in series to power the 24VDC motor).  I also wired an SPST switch into the positive side of the input solar cable, and another SPST switch into the positive side of the output battery charge cable.

The C40 controller in charge mode can be configured a lot of different ways, e.g., for 12V or 24V or 48V output.  It has a variable charging rate based on the battery voltage and solar panel voltage, and you can tune (and fine tune, if desired) the voltages delivered to the battery during the bulk, absorption, and float stages.  Since the spec sheet that came with my AGM batteries specified the optimal voltages during the bulk and float charging phases, I simply adjusted the C40's potentiometers (pots) to those specifications.  This extra step will hopefully insure the maximum life for my batteries.
Charging the mower is fairly simple:  I throw a switch on the mower that isolates the two batteries, and then connect two short jumper cables from pos to pos, and neg to neg.  Then I connect the output cable from the solar controller to those pos and neg jumpers, and turn on the two SPST controller switches to start the solar juice flowing.  I've only charged the mower once and it took most of a day to charge the two batteries from a fully discharged state (approx 12.0 volts) to a fully-charged state (approx 13.3 volts).
But now I have a TOTALLY GREEN MOWING MACHINE!  Thanks to you and all your contributors for their inspiration and information.
Lee (mvemjsunp2007@yahoo.com)

P.S. The mower itself is still working fine - I've found that I can count on being able to mow my grass for 30 minutes before having to shut down for recharging - and that's just about what it takes to mow my entire yard, and it's also about as long as I can go without another kind of recharging, so it's working out perfectly.



Latest From Lee:  June 6, 2008

Lee's mower page has been getting a lot of interest -- over 600 visitors last week!


Here is an update from Lee:

A quick update on the mower:  it's still working, and I'm still using it once or twice a week, and still charging it with the solar array.  Some changes over the past year though:  

(1) I added three more solar panels (total of six now) and remounted them onto the roof of my workshop (see pic).  I have two large marine /deep cycle batteries (210AH total) being constantly charged by the array (except when the mower needs a charge) to which I have an 800W inverter connected, which now powers all the tools in my workshop except my 10" miter saw and air conditioner (this is Florida after all - temps are routinely in the 85-90 degree range) 

(2) One of the 22AH AGM batteries seems to be having a problem - it seems to have only about half the original capacity.  I've been giving it extra attention, charging it every other time with my smart battery charger (with a desulfination mode), which seems to help temporarily.  But long term, it looks as though I'll have to buy another set of batteries for the mower this year. 

(3) I bought a pair of used 12V 50AH GEL batteries on Craigslist for $40, and tried them on the mower in place of the 22AH AGM batteries.  Good news - I'll bet I could mow for a couple of hours.  Bad news - they are so heavy I can hardly push the mower any more.  So I bought a well-used electric scooter (off Craigslist) and cannibalized the 24V motor, controller, and twist throttle off of it and mounted it on my mower to power the rear wheels, but the motor was too small to do much good.  I experimented with various sized sprockets and finally got a combination that worked but the mower speed over ground was only about 1 mph, too slow even for me.  So I took everything off and am back to the original configuration. 


Lee's solar array that powers the lawn mower and shop tools.




Gary June 6, 2008