An Analysis of Lawn Mower Battery Life

To power my Ryobi 40V cordless electric mower I purchased two additional batteries.  These batteries are made by a company called GERIT BATT and are 6 AH models.  Currently, on Amazon these batteries are available for ~$80which is what I think that I paid for each earlier in the year.

The mower came with a Ryobi branded 4 AH battery and charger.  Thus far it appears that the charger has no problem juicing the off brand batteries, which is a complaint that I read about from some reviewers.  I will note that if I pull the battery off the charger when it claims to be full it runs for a shorter period of time compared to when I leave the battery on for some time after a full charge indication.  Maybe there is some trickle charging going on or a quirk in the firmware of the charger.  I do not know.

Below is a table showing all of my mowing this season and the times each battery lasted while mowing.  I have been recording these results over the course of the lawn mowing season because I have found very little concrete analysis of battery life in cordless electric mowers.  Batteries A and B are the GERIT BATT models while Battery R is the included Ryobi battery:

	Battery A	Battery B	Battery R
May 1	32	To complete	NA
May 8	34	To complete	NA
May 17	To complete	32	NA
May 22	40	To complete	NA
May 27	To complete	25:28	NA
June 1	18:03	21:37	17:38
June 5	26:42	24:15	To complete
June 8	To complete	38:51	NA
June 12	No time	No time	NA
June 21	To complete	37:25	NA
June 26	41:57	To complete	NA
July 2	39:44	To complete	NA
July 8	To complete	48:16	NA
July 13	45:02	To complete	NA
July 20	41:51	To complete	NA
July 28	37:05	To complate	NA
August 14	To complete	36:23	NA
August 28	To complete	39:04	NA

After the first mowing I stopped using the self-propel feature of the mower believing it to use more electricity from the batteries than it was worth.  The mower is quite light and my yard is not steep enough to really justify the extra juice.  Plus, if I consider it a workout it’s better to make it more difficult.  Right?

The biggest change in battery duration seems to be the dampness of the grass rather than the length.  Early in the season we had a lot of spread out rain that kept everything fairly moist.  Even mowing late in the day it would still be damp.  Compared with later in the season when I let the grass grow long as the rain quit and the temperatures rose, mowing damp grass was just an energy suck.  Check out June 1.  It was like mowing through wet concrete.

You can see that the last month was a hot and dry period of time.  I have really only mowed the lawn to even out some spots that grow long while large swaths of my yard stay at about 4-5” of grass height.

Regardless, it seems like mid-30 minute duration is consistent with some deviation to the high or low side dependent upon local conditions.  Given that my yard is about two-thirds of an acre in size I would imagine that most suburban lots—about one quarter of an acre in size—could get buy with one 6 AH battery.  The possibility of large emission reduction from replacing small engines with electric motors is gigantic.

Note: I bought the two GERIT BATT batteries with my own money from Amazon.com.  I have received nothing in compensation for writing about or linking to the batteries.

Deeper Decarbonization by Intent and Accident

One of my 2020 “goals” was a deeper level of decarbonization.  In prior years I have installed solar panels (twice) and purchased an electric vehicle.  However, for 2020 I wanted to examine other parts of my lifestyle and see where I could decarbonize even more.

The first area was lawn care and its attendant equipment.  As anyone who cares about the air we breathe knows, lawn equipment powered by small gasoline engines is one of the dirtiest sources of air pollution that we use on a regular basis.  God help you if you are still rolling around with a two-stroke mower or string trimmer.

For the lawn care “season” so far I have mowed my lawn 11 times with my new battery electric mower and string trimmer.

Each mowing session represents approximately 1 hour of small engine runtime eliminated.  Depending upon the study and assumptions, an hour of small engine runtime is equivalent—in terms of emissions—to approximately 100 miles of automobile travel.  Considering I a running 100% “on the sun” right now you could say that my change in mowing equipment has resulted in the equivalent of reducing driving by 1,100 miles.

While reducing CO2 emissions is a big deal, it is even more important to reduce other types of emissions like small particulate and gasses other than CO2.  This is where reducing small engine runtime is so beneficial.  Without catalytic converters or other advanced emissions equipment, small engines are essentially belching out pollutants like it was 1972.   There are no hard estimates, but there are guesses that for every hour of mowing something like34 pounds of “other” pollutants are shot into the air. This includes things like nitrous oxide, carbon monoxide, and particulate matter.  The image of an American lawn as some kind of green idyll takes on a whole other bent when you consider the maintenance costs.

The second area where I was going to decarbonize was commuting.  Well, commuting is out because of coronavirus.  By accident both my wife and I have ended up cutting our commuting dramatically since mid-March.  Through June 30^th we have avoided commuting for 63 days—not counting furlough, vacation, or holidays—which works out to a combine savings of ~2,500 miles driven and ~3,300 pounds of carbon dioxide emissions.

My original goal had been to replace 500 miles of driving—EV or otherwise—with the equivalent of human powered transport.  However, as you can see my household is blowing those goals out of the water in a totally different way.

There are some other areas of my household that I am looking to decarbonize in the coming months, but restrictions and closures due to coronavirus may impeded the progress toward those goals.  Regardless, I think that between the revised lawn care routine and cessation of commuting there has been some good progress made toward any goal of emitting less carbon dioxide and other emissions.

How Long Will the Battery Last in My Cordless Electric Lawn Mower

This weekend was the first time that I actually used my new cordless electric mower to cut my lawn.  Prior to this I had tested the mower to ensure that it worked—in case I needed to return it since I bought it on mega discount at the end of last year’s outdoor season—and to make sure that the third party batteries I purchased online worked as well.  Nothing would have been more disappointing then heading out to mow my lawn and hearing nothing.

My yard is just under half an acre, but not all of that is grass.  A large portion is the house, driveway, sidewalk, etc.  There are also quite a few trees and landscaping with more on the way this summer.  I do not have an exact figure, but it is a large lot.

Most of the reviews of the cordless electric mowers that I have seen were conducted on small yards in places like Florida and California.  These are the kind of yards that take twenty minutes on Sunday morning to mow.  In the case of my yard it is more like 45 minutes to an hour depending upon mowing direction and how many foam darts I have to pick up.

To power the lawn mower I purchased two third party 6 amp hour batteries.  These batteries were decently reviewed and I hoped that six amp hours would provide me enough extra capacity to mow my lawn without stoppage.  Originally, the Ryobi lawn mower came with a single 40V 5 amp hour battery that I am going to use with a battery electric string trimmer.

Well, the first battery—from full charge until the mower stopped cold in its tracks—lasted just over thirty two minutes.  This is a far cry from the forty five minutes I had read that a five or six amp hour a battery would last.  However, I was using my mower’s self propelled feature so there is probably a decent hit to run time caused by the extra oomph it provided.  Given how light the mower is the self propelled feature is more of a “nice to have” than a necessary evil.  The next time I am going to forgo using that feature on the totality of my lawn to see the impact on battery life.

The second battery lasted until I completed my lawn approximately 20 minutes later with no fade in power.  That is approximately 50 minutes of run time with one battery completely empty and another with electrons in the tank, so to speak.  My plan is to keep track of the batteries’ performance across the season to see how operator error, weather, and cutting conditions impact performance.

Lawn mowers are pollution bad actors.  The numbers are hard to pin down because there is so much variation—small engine, riding mower, CARB compliant, etc—but there is no doubt that an hour of mowing is worse than an hour of driving a car.  It’s not just the carbon dioxide that is emitted by small engines that should concern you the most.  These engines are veritable factories for compounds like carbon monoxide and nitrogen oxide.  Nitrogen oxide, along with its friend sulfur dioxide, is the bad chemical compound that combines with atmospheric compounds to make acid rain.

For the season, I have eliminated ~50 minutes of gas-powered lawn mower operation with a solar powered battery electric lawn mower.  Stay tuned for the ongoing accounting of how much I “decarbonized” my lawn care routine.

First Step in a Path Toward Deeper Decarbonization

Once you have purchased an electric vehicle—in my case a used 2015 Nissan Leaf—and installed solar panels—in my case a total of 24 panels for a nameplate capacity of ~7.5 kWh—you are left with a question: how do I further decarbonize my household?

If you live in a single family home in the United States there are a surprising number of places where fossil fuels are being used on a daily basis.  Most home owners do not really consider these sources of carbon emissions.

Consider the lawn.  Anyone with an inkling of environmental conscience understands that the turf grass monoculture that dominates our landscape is essentially a hellscape of inappropriate plants, harmful chemicals, and energy intensive maintenance.

In my household we have abandoned the chemicals and I am ripping out sections of turf grass as often as I can in order for it to be replaced with perennials suited for my region.  However, I am left with some amount of turf grass and social expectation that this grass be mowed on a semi-regular basis.

Trust me, I have pushed the bounds of both social expectations and legal ramifications over the years by allowing parts of my lawn to go weeks without seeing the spinning blade of a lawn mower.

Nonetheless, I am bound to some degree to maintain a well-manicured lawn.  As a good suburban homeowner I spent the last nine years mowing my lawn with a traditional gas powered push mower.  I dutifully filled it up with a small amount of ethanol free gasoline every few weeks and spent about an hour clipping my grass down to the maximum height setting.

Thankfully, a series of mechanical mishaps aligned with my desire to rid myself of this pollution spewing beast.  How much pollution does a mower release, you ask?  It depends upon the source and methodology, but according the EPA lawn mowing accounts for up to 5% of the United State’s total air pollution.  Not to mention the millions of gallons of gasoline that are spilled filling mowers.   Add in the oil required for four stroke engines and you have a lot of fossil fuels being consumed to keep our lawns high and tight.

Now, I could have rolled old school with a reel mower as someone will surely point out.  I would also ask that person if they have ever mowed more than a few hundred square feet with one of these contraptions.  Seriously, another eco-minded neighbor bought one and every household with an interest tried it once.  Reel mowers are the Zima of lawn care.  You try it once and never think about it again.

Strolling the aisles of my local Home Depot—an activity one is likely to engage in when waiting for your child to complete soccer practice—I noticed a clearance sticker on a Ryobi cordless electric mower.  Now was the time to jump on the electric lawn mowing bandwagon.

For less than the cost online of a regular push mower—battery electric or ice—I took home a battery electric self-propelled mower.  The 40V mower came with a single 5-amp hour battery.  If I believe the online reviews this battery should provide about 45 minutes to 1 hour of cutting depending upon usage.  We shall see.

Additionally, I purchased an extra battery online.  The cool thing about the 40V Ryobi tool line is that with such a large installed base there is a healthy aftermarket in third party batteries.  I was able to get a compatible battery rated at 6-amp hours for less than $80.  With two batteries I should have more than enough capacity to complete mowing my lawn.  Again, we shall see.

For the first time in forever I am looking forward to the beginning of lawn care season if only to see how the electric revolution applies.  The march toward a deeper level of decarbonization carries on.