H. G. Dietz
http://aggregate.org/hankd/
Department of Electrical and Computer Engineering
Center for Visualization & Virtual Environments
University of Kentucky, Lexington, KY 40506-0046
Original April 28, 2020, Latest Update June 15, 2023
The opinions expressed in this document are those of Hank Dietz speaking for himself personally, not as a Professor at the University of Kentucky.
I've always liked the idea of electric vehicles. My first real experience was in the mid 1970s, when my Dad and I modified my soap-box derby car, which was styled like a 1910 Mercer Raceabout, to be powered by the starter motor from a 1965 Ford Mustang. The belt drive didn't gear-down the motor speed enough, and I remember getting up to truly scary speeds driving it on the then-empty airfield at Mitchel Field. Later, my Dad's company built a not-for-road-use utility vehicle with a similar styling. It was available as gas, gas with electric start, and electric versions, and the electric one was displayed at IEEE Electro 77 as an example of an exceptionally efficient electric vehicle -- the trick was, it had a clutch and 5-speed transmission for its 1HP electric motor... Here is a little bit about that car we built in 1977; that page was put together for the senior project team that I supervised in the 2022-2023 conversion of the last gas with electric start version to modern Lithium-powered electric. Here's a photo of it:
It might be the cutest little tractor ever made. ;-)
Anyway, in 2001 I moved to a house on a 10-acre lot in Lexington, KY. I never had more than 1/4 acre of yard before, and this 10-acre rough field quickly proved to be a challenge. We started by having a commercial mowing company "knock the grass down" to 6" every couple of weeks, but they were neither very good nor very cheap. So, I bought a 48" "Scotts by John Deere" riding mower to take the grass down to 4" or so... and soon after, I was actually mowing everything with that. It was challenging to say the least, although I have to admit that having the optional bagger did give me a way of collecting tons of grass mulch in minutes.
Clearly, I needed a bigger mower. After having a few come for trial runs, I decided on a 72" Scag Turf Tiger. It's a monster, but it did the job... so I eventually sold the 48" tractor. However, the Scag isn't great at towing things and maintenance service is slow (presumably lower priority than folks using Scags for commercial mowing businesses), so I bought another John Deere -- this time, a 54". That wasn't a bad combo, but as COVID-19 hit the USA, both my tractors needed servicing. I had been thinking about building or buying an electric tractor for twenty years, so I looked again to see if there was a viable electric replacement. Honestly, Mean Green makes what could be a great replacement of the Scag: the lithium-powered EVO-74" ZTR... but they're not really around here and the tractor isn't cheap. So, I looked more toward an electric that could replace the 54" rather than the 72". That's how I ended-up buying my 42" Ryobi, which is powered by four 100AH deep-cycle batteries. So, let's see if we can answer the question: are electric riding mowers really viable yet?
One big table can summarize a lot:
| Property |  |
 |
 |
|---|---|---|---|
| Model | John Deere LA175 | Ryobi RY48ZTR100 | Scag 72" Turf Tiger |
| Price (new) | $2,800 | $4,200 | $12,000 |
| Run Cost/Acre | $2-3 | $1-2 | $3-5 |
| Deck Width | 54" | 42" | 72" |
| Travel Speed | 5 MPH | 7 MPH | 12 MPH |
| Cut Speed (in use) | 2 MPH | 5 MPH | 5 MPH |
| Max Deck Height | 4" | 4.5" | 6" |
| Tall Grass & Weeds | Cuts, but can Clog & Stall | Mulching no, Discharge yes | Cake |
| Twigs | Unhappy | Very unhappy | You call that a tree? |
| Turn Behavior | Large radius | Zero radius | Zero radius kills turf |
| Ride | Tall, but stable | Bumpy, can slide | Like a tank |
| Noise | Engine+deck == earplugs | Deck only, can talk over | Engine+deck > earplugs |
| Cuttings go where? | On lawn, often clumps | On lawn, as they should | On you & everywhere |
| Towing stuff? | You feel it | No problem | Works, but not intended |
| Safety? | Interlocks, & stable | Interlocks, but can slide | Scary, but belt & rollbar |
As I write this, all three tractors are fully working, and in the past week I have done hours of mowing with each. They're all ok. So here's the summary:
So, is the Ryobi electric a viable answer?
The answer is mostly yes. The "bursty" mowing style my Scag nudges me towards really isn't the best approach. Things look better doing the incremental cutting that the Ryobi inspires, and mowing is more pleasant in shorter doses with less noise and mess. Not having to deal with gas and belts is also a big win. I also have to give the Ryobi high marks as a farm utility vehicle, because it really works well for light towing.
The "no" part of the "mostly yes" answer has to do with the ability to convert a rough field into what looks like a trimmed lawn. The Ryobi is a lawn mower, and it doesn't deal as well with cutting non-lawn stuff -- weeds are ok (and the 4.5" max cut height helps), but it doesn't like twigs. If I run over a small branch with the Scag, there's nothing left but mulch. The John Deere mowing a twig makes some unhappy sounds, and twig pieces will survive the mowing, but there's really no harm done. The Ryobi doesn't do much to twigs, and in wet or really heavy mowing will need to have the discharge path cleared regularly. So, for acres like mine, the Ryobi alone isn't really sufficient; arguably, it plus a bush cutter would suffice, but using the Scag for rough cuts and Ryobi for frequent cuts seems to make the most sense.
Finally, it's worth saying a bit about safety. The Ryobi is certainly the inherently safest of the three to operate, but it really doesn't feel all that safe. The safety belt on the Scag feels like overkill, and I can't imagine what it would take to need the Scag's rollbar (other than power-flipping it by catching the rollbar on some overhead structure -- don't ask). However, the Ryobi bounces around a lot, and a safety belt would be a reasonable addition (the John Deere should have one too). The Ryobi's sliding potential also means I have to be very careful on slopes near my two ponds -- that, plus the fact the deck doesn't stick out the sides as much as on the other tractors, means I keep a little extra distance from most obstructions and can't cut under obstacles like fences. The John Deere's center of gravity is much higher, but the weight is evenly distributed between the engine up front and you in the back, while the Ryobi has the batteries under you with no weight up front -- it feels like you could pop a wheelie under the right circumstances. The Ryobi is also fast, and I can imagine it would be a little too responsive for someone not used to zero turns (e.g., it was for my wife); Ryobi does have a lower-speed mode for inexperienced operators.
So, electric mowers are pretty viable... as long as your property doesn't have lots of rough areas and slopes. Acres of grass, even tall grass with weeds, are not a problem.
All three of my tractors have tires go flat regularly. Why? Ossage orange trees. Many of those twigs I was talking about have tire-killing spikes on them. I know there are now airless tractor tires, and I thought those should be the standard tires... until I looked at Tweel prices. They're running an order of magnitude higher price. That's insane. Let's hope that the industry soon realizes that airless tires should be the low-cost, high-volume, norm for slow-moving off-road vehicles.
Well, I've now had the Ryobi roughly one and a half years (two peak-growth seasons), so it seems time to post an update. It's been used a lot, probably close to 200 charge/cut cycles, partly because it is so pleasant to use but also because I've been home a lot due to the pandemic and it encourages incrementally cutting because the battery cannot do 10 acres in one cycle. When new, it did 1-2 acress per charge, and that's still about right. Disturbingly, if the Ryobi has been sitting for a week or longer (on charge), the battery indicator often will say 70% rather than the 100% it shows when it's only been on charge overnight, but it seems to get about the same run time either way.
The Ryobi continues to cut well as long as the grass/weeds being cut are dry -- wet grass clogs the side chute quicky, often requiring manual removal of the clog. I'm now on my 3rd set of blades; they don't dull, but they do get bent when they hit nasty things like 2" thick twigs. There's also a little chunk taken out of the blade mount on the left blade -- it doesn't seem unsafe, but I think this should be replaced, so I might need to get it serviced. It's not fun, but I had no problems changing the blades myself and have never had the tractor professionally serviced.
Overall, if you only have 1-2 acres of lawn, this Ryobi would handle it effortlessly. For my 10 acres, I find myself using it on the lawn-like ~4 acres nearest the house and closer to the road, but resorting to the Scag to knock-down the field-of-weeds remainder of the property.
The original batteries have gotten down to cutting about 1/3-1/2 acre, just 20-30 minutes, before refusing to spin the blades. Full charge is usually reading 70%, but it actually hits 100% if I don't have it sitting on charge for too long. In any case, it's time to do something about the batteries.
Step one was getting a new charger, this HTRC one, which is capable of charging the LiFePO4 batteries if I switch to them. However, it also has an SLA battery repair cycle, so I tried that first on the original AGM SLA batteries. To do that, I needed to fix a little issue with the charger's RXV plug not fitting the Ryobi socket. This could be done either by breaking some plastic tabs in the Ryobi socket or by swapping the connectors between the Ryobi charger and the HTRC. The swap was easy, but here's a photo of the connectors and socket:
The original charger had black, white, and green wires while the HTRC had just blue and brown with an unconnected pin in the third position. Well, blue is the new black, brown is the new white, and I left out the unconnected pin because I didn't want to risk it rattling into a short inside the connector. The missing green (earth) connection seems to be harmless, although I've seen it suggested that a 3300ohm 1W resistor from that pin to ground might be a good idea (to cause the green charging light on the socket to light when charging), but that seems a bit arbitrary. The really interesting thing is that the guy who suggested it has the exact opposite wire color code of what my HTRC charger has! He has the resistor to the 3rd pin connected to what is the brown wire in my connector (but is blue in his)... we agree that pin should be ground.
After that connector swap, I raw a repair cycle that took about 24 hours to complete. Then I ran a regular charging cycle, which did not complete by itself in 12 hours (with the batteries somewhat warm it had dropped from 13A to 8A charging), but the Ryobi showed 100% charge after. Mowing with it I cut perhaps 2/3 acre in about 40 minutes without fully draining the battery, and I put it back on charge. The next charge cycle also failed to stop itself. Then I cut a very rough 1/8 acre and put it on charge -- this time, it quickly charged and stopped itself as "FUL."
I'll update this as appropriate. It looks to me like the batteries are now getting better formed, but are not quite where they should be yet. Thus, I've just run another repair cycle, which again took 24 hours. It's not as close to good as new as I'd like, but an upgrade to LiFePO4 batteries seems premature. I'll probably wait until next year, and see if there are any bigger, better, alternatives then. I do think LiFePO4 batteries make more sense than the other Lithium chemistries being used in most electric mowers; certainly, LiFePO4 offers the best battery life with the lowest cost and greatest safety.
The only thing set in stone is our name.