Published Friday, May 20, 2011.  Updated October 1, 2012

Lawn Mowers Part I

I prefer electric lawn mowers to gas ones because they are super quiet, require almost no maintenance, they don't pollute and I don't have to buy gas.  The only problem is corded electric models are a hassle and cordless mowers, due to limited battery capacity are only suited for small lawns.  For my ½ acre lot, neither option was acceptable.  

John's Rule #106:  If you can't find what you need, invent it!  

My lawn mower uses a 115 Amp-Hour, 12V deep cycle battery to power a 750 watt AC inverter that in turn powers a 157 VDC motor that has been connected to a bridge rectifier so it can run off of normal household AC power. Granted, it’s kind of stupid to convert DC to AC and back again to DC but that’s exactly when I have done.  The down side to my Frankenstein mower is that it's not self propelled and the deep cycle battery on board weighs a lot. 

I'm still looking for the parts so I can connect a 12 volt motor to the back wheels.  In the mean time, mowing the lawn is good exercise. 

When my mower is drawing the maximum amount of power, the 12 volt battery  has to provide 62.5Amps continuously.  That gives me 1 hour 50 minutes before the battery goes totally dead.  This isn't a problem because my lawn only takes about an hour to mow and the mower doesn't always draw maximum power anyway.  

746 watts is only 1 horsepower but an electric motor is way more efficient than a gas engine. My lawn mower has the equivalent grass cutting abilities of a 3.5HP gas mower.

This season, I have been baffled as to why it has been taking me 2 deep cycle batteries to mow my large lawn instead of only one. Last season, I would boast that I could mow the entire lawn and only use ½ the capacity of my battery.

Having my lawn recently aerated left thousands of little grass poops all over the lawn. You should have seen my 2-year-old daughter, (thinking they were dog poops), try to walk across the lawn with ever-so-much care and caution. She had such a cute, disgusted and worried look on her face.

Mowing the lawn after aeration, the occasional lawn poop would hit the blade with a “twang” before being obliterated. It was fun to do but it wrecked havoc on the blade.

I sharpened my spoon sharp lawnmower blade to that of a spinning-death steak-knife. The revived blade helped a moderate amount but my mower would still bog down in long, thick grass. What is going on here?

Then it occurred to me that the first time I mowed the lawn this season, I lowered the blade down to a 2" cutting height and I neglected to raise it back up to the original height of 2 ¾”. After raising the blade back up, I was able to mow the entire lawn using only one battery.

After measuring my lawn mower's performance (with a Kill-A-Watt meter) and applying some crude calculations, 

I arrived at these surprising statistics:   

  • A sharp lawnmower blade will use 33% less energy than a dull blade. 
  • Raising up the deck only ¾", extends the range of the lawn mower by 40%. 
This is true for gas or electric lawn mowers.  
Unless you have a putting green, there is no practical reason to cut the lawn short.  

I found that cutting the grass a little bit longer will in return yield several benefits:
  1. A healthier lawn.
  2. The lawn remains greener with less water.
  3. Takes less time to cut.
  4. The lawn mower does a better job of cutting/mulching the grass.
  5. And amazingly, it uses less energy.  

After collecting data from a few of my neighbors about their lawn mowers, I found that a ½ acre lot can be mowed using 1 tank of gas (0.4 gallons) at a fuel cost of $1.50/week, spanning a 21 week grass mowing season.  Using these figures, my $84 battery needs to last at least 2.4 years to pay for itself.  
Well, it has already done that since I am into my 3rd year of using this battery. 
After mowing, the battery is only 45% discharged.  Conveniently, this also extends the overall life of the battery.  

The x-axis is the discharge percentage of the battery.  The y-axis is the number of charge cycles. 

Amazingly, only discharging 45% will allow a battery to last 1200 charge cycles, (1200/21 = 57 years).  Due to the cheapness of my battery, I'm conservatively estimating mine will only last for 250 charge cycles or 12 years.  We'll see. 

My ½ acre lot with ~¼ acre of lawn requires ~0.7KWH of energy from the battery and 1KWH of electricity to recharge the battery. That’s about 10 cents per week to mow my lawn.   Since solar panels provide all my electricity, my lawn mower is effectively solar powered.

Nerdy Fun Facts:
  • There are 125,000 BTU in a gallon of gasoline and 3412 BTU in a KWH.  An equivalent gallon of electricity would be 36.6KWH. 
  • Gasoline has an amazingly high energy density.  Storing the same amount of electrical energy (as a gallon of gas) into cheap deep cycle lead acid batteries would require 2036 lbs of batteries.  Since you can never retrieve 100% of the energy stored in a battery, you would actually need more than this. 
  • A typical push mower has a 0.4 gallon gas tank.  It takes about 1 gallon of gas to mow 1 acre of lawn.  My electric lawn mower uses 15 times less energy to mow a lawn than a gas mower. 
  • The deep cycle battery on my lawn mower weighs 50 lbs but only releases the (usable) equivalent energy contained in 2¼ ounces of gas. 
  • The ideal grass cutting height (in my opinion) is: 2 ¾“.  This saves the most energy but still keeps the grass relatively short and manicured. 
  • You can measure your own lawn mower's cutting height by placing the lawn mower on flat cement.  Measure from the ground to the bottom of the lawn mower deck.  The cutting height will be 0" -¼" higher than this. 
  • An electric lawn mower is 16dBA quieter than a gas mower.  You can mow your lawn at night with an electric mower without disturbing your neighbors.  

The weak link in the run-time for my lawn mower is the inverter's low voltage cutoff circuit.  Once the voltage at the inverter drops below 10.7 volts, it's game over.  After drawing 60 amps from a 12 volt battery for 50 minutes continuously, the battery voltage (while drawing a heavy load) is about 10.9 volts.  The voltage drop across the 18" cables running from the battery to the inverter is about 0.2 volts with a resistance of 0.004 ohms.  After beefing up the cables with much larger gauge wire, the voltage drop is only 0.03volts and only 0.0007 ohms.  

The small red and black cables were what was used before.  I added the larger red and blue cables.

I can always tell when my lawn mower battery is around 55% because (while pulling 60 amps), that is when my inverter starts squawking about low voltage.  Before beefing up the cables, I was never able to discharge the batteries below 50% before the inverter's low voltage circuit pulled the plug on my mowing "fun".  After I reduced the voltage drop across my cables, I can complete the entire lawn without the inverter voicing a single complaint.  This effectively bought me an extra 25 minutes of run-time.

I am toying with the idea of swapping out my 78 lb lead acid battery for a 28 lb, 100AHR LiFe batteryBut at over $500, can I justify the cost?  Dang!  Science can be expensive.