This winter, I converted my lawn tractor from gas to electric. Fortunately, saving money was not the point, or I would be disappointed. I still have a few small tasks to do but today I mowed up some leaves to test it. Learned a lot and had fun, and the best thing about starting with a piece of junk built by MTD is that I can deal with the idea that it isn’t going to last and I can transplant parts onto something higher quality later.

I’ll be mowing only half the lawn at a time until I have good data on how deeply I am discharging my batteries so that I don’t kill them–they were almost half the cost of the conversion so I want to take good care of them!


Michael K Johnson March 01, 2014 21:33

I mounted half the batteries directly in front of the seat instead of above the motor to keep the center of gravity low and not put all the weight on the little front tires. I know it looks funny, but today I tested that I can drive along my swale without danger of tipping. :-)

Jeff Garzik March 01, 2014 21:36

Are blade rpms and overall power similar to the pre-conversion levels?

Curtis Olson March 01, 2014 21:40

What kind of volts are you running?  How many mah does it take to mow your yard?  My yard is really hilly and odd shaped.  When I borrowed my neighbor’s rider one time I spend the whole time trying to turn 180’s and 360’s.  My other neighbor has a little electric push mower, but it takes him about 3 weeks worth of use/charge/repeat to do his lawn.  He’s dedicated!

Michael K Johnson March 01, 2014 21:42

+Jeff Garzik Blade RPMs are about the same. I used a motor intended as a replacement for typical lawn tractor engines. It’s a permanent magnet motor that at its rated 48V nominal has the same rated speed as the gas engine at full throttle. Power is a little lower. I’ll see how much it lags mowing thick grass this summer, but the torque curves are just different between gas and electric.

Michael K Johnson March 01, 2014 22:00

+Jeff Garzik The motor is rated at about 11.5 HP continuous (200A 48V 90% efficient) and about 23HP for one minute (400A); the gas engine was rated at 19HP. The batteries are 100AH at 20H AGM and should give me 100A for about 25 minutes before reaching 50% charge.

+Curtis Olson 48V nominal. I don’t know yet how many AH it will take to mow, since the grass wasn’t high enough to reach the blades; I just tested mowing up some leaves. I ran my foot through a running pushmower half a lifetime ago and I still don’t feel like using a pushmower if I can help it.

I don’t think I’d use one of the little electric push mowers anyway without replaceable battery packs. I’ve thought of buying a reel mower and converting it to electric, but I don’t have a handy motor for it. I do have a 3HP 90V DC motor but no 90V supply handy. ☺ I have a smaller AC motor (1/3 HP? 1/4 HP?) from a defunct air compressor but I don’t really think I want to tether.

Curtis Olson March 01, 2014 22:04

I’ve been slowly moving all my model airplanes over to electric.  It makes a lot of sense at that scale, but as size and power goes up, the costs of electric really start shooting up too.  My biggest model runs at about 18v with an 8AH battery … I’ve squeaked a 50 minute flight out of that with some help along the way from some friendly thermals.

Michael K Johnson March 01, 2014 22:13

The motor is in a plexiglas box to show it off. The orange thing above and to the right of the motor is the contactor that energizes the motor. At the top of that box I have mounted 4 48V muffin fans that suck air through a reticulated foam filter friction-fit into a custom-size metal picture frame in order to run clean air around and through the motor to keep it from chewing up its brushes by sucking in dirt.

The wood on the front is a 2x10 bumper to protect the batteries from clumsy driving.

The loop of wire between the steering wheel and the battery does double duty: I can put my clamp ammeter on it, and one end is hooked up to a powerpole emergency disconnect that is rated for connect/disconned while drawing 180A and can carry far more. I bought 4awg wire and then heard (wrongly, it turns out) that I should have gotten thicker wire, so I doubled the 4awg wire and used 1awg lugs (2 4awg wires have the same cross sectional area as 1 1awg wire). Turns out that my wiring harness has enough ampacity for many electric cars, let alone lawn tractors… Oh well, it won’t overheat this way!

Michael K Johnson March 01, 2014 22:41

Mowing in the rain is bad for the grass, so rain was not a design consideration.

I plan to cover the rear batteries with something (naugahyde?) for safety, but have no actual waterproofing intentions.

Michael K Johnson March 02, 2014 06:51

It  just has to put up with it, the same as the rider!

Michael K Johnson March 02, 2014 08:21

+井上エイド Ah! I had been trying that (web client), but only after opening a comment posting box, and it silently did nothing. Now I discover that it works only if the comment box isn’t open yet. Would be nice if it would insert a mention at the cursor point, but at least now I know what to do. Thanks.

Scott Leslie March 05, 2014 11:04

I hope you have a cover for those terminals in front of the seat.  Let me know if you want any laser cut acrylic parts for that.

Michael K Johnson March 05, 2014 13:21

+Scott Leslie that would be cool! I had been vaguely thinking of Naugahyde as the easiest solution but clearly laser cut acrylic would be more beautiful… :-)

Michael K Johnson March 22, 2014 17:32

+Jeff Garzik I tested today and at around 150-160A continuous my batteries start to sag. So in practice I don’t think I’ll exceed 10HP. I mowed for 20 minutes today pulling about 120A most of the time and that’s probably about as deep as I want to discharge the batteries. I’m monitoring them to see how well they recover so I have a good measurement of state of charge before I start recharging them.

Michael K Johnson May 28, 2014 12:50

As long as I mow often enough that the grass doesn’t get too long, I can mow my entire lawn without exceeding 50% discharge. I have once slightly (by about 0.04V) exceeded 50% discharge. If grass is heavy, I’ll mow the halves of the lawn on adjacent days.

I reduced the current draw by 10A or more by replacing the non-OEM drive belts with OEM drive belts, which helps make me more likely to be able to mow the whole lawn on a single charge.

Michael K Johnson May 28, 2014 21:18

Heat increases self-discharge but over the summer it doesn’t sit long enough for self-discharge to be an issue. For my batteries, at a constant 40°C, they will self-discharge to 50% after about 5 months. Not really significant over a week…

Higher temperature (at least within normal range) won’t reduce available power or capacity. Cold will reduce available power; thus the emergency “screwdriver trick” in the frozen north where a car with a cold, weak battery can be “jump started” by shorting the battery with a screwdriver momentarily, using the battery’s internal resistance to heat it up so that more power is available. Doing so merely carries the risk of maiming or killing you… (Click and Clack recommend that if you want to try this, you use jumper cables and do the short 20 feet away from the battery for a modicum of protection from explosively burning hydrogen and spraying sulferic acid. In the alternative, just don’t do that, then!)

It’s possible that I’ll be able to pull a few more amps out of a slightly warmer battery, but since the cost will be disproportionately shorter runtime (“peukert exponent”) that’s not necessarily a win. ☺

I’m unlikely to actually measure any difference in battery performance between uncomfortably warm and uncomfortably hot.

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