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I started my project by phoning all the lawnmower repair places in my part of town, asking if they had any 20-22" mower decks, minus the engine, in reasonably good condition. One kind gentleman had exactly what I was looking for, a 21" Troy-Bilt mower deck (engine was blown), rear bagger version, complete with blade and dead man handle/mechanism (which I wanted to use for a start/stop switch) - he charged me only $25 for what turned out to be an excellent deck.

That size mower is usually powered with a 4.5 to 5.0 HP gas engine, so I started looking for a DC motor of approximately 1.5 HP, the rule of thumb being that a 1 HP electric motor is equivalent to a 3 HP gas engine.  I found and purchased a 1.5 HP 24VDC motor for $69.95 - this turned out to be a new Tecumseh motor actually designed for lawnmower use (I called Tecumseh and tried to get some additional info on this motor or its application in a lawnmower, but none of the people I spoke with knew anything about it...)

I did some research on batteries and quickly decided that two 12V batteries were much cheaper and more easily obtainable than one 24V battery, and almost as quickly decided on the new AGM spill-proof batteries. What I needed to know, but could not find, was the power consumption of my new motor. The only thing I knew about it was the sales pitch: 4.5 Amps no-load and 1.54 HP. A little math resulted in the surprising figure of at least 44-48 amps at full load (1 HP = 746 Watts, or 1 HP = 746 x volts x amps) - and that's assuming 100% efficiency. The confusing part was that the motor leads were #10 gauge stranded copper, which is rated for only 30 amps. In the end, I decided to split the difference, and assume 40 amps. I then purchased two 22AH batteries, which by my calculations would give me approx 30 minutes of continuous mowing time before it was time to shut down and recharge the batteries.

Since I was still a bit unsure about the power consumption of my new motor, I also decided to hedge my bets with circuit breakers (actually a combination switch and circuit breaker) and ended up purchasing three different sizes: 30A, 40A, and 50A.

One lesson I learned is that components as described above cannot usually be found at your local hardware store or even an electrical supply house. There is apparently very little local demand for high current DC parts such as switches, circuit breakers, AGM deep cycle batteries, gauges, etc. Your best bet locally will be boating/marine supply outlets, and the internet for everything else.

Anyway, the day finally arrived when I had all the components laid out in front of me, and I could start assembling them. I mounted the motor on the mower deck first. Before the deck became weighted down with everything else, I installed the blade (easy, since the motor I had purchased had a keyed shaft with standard mower threads) and adjusted the cutting height so it was comparable to the height on the gas mower it replaced - then I removed the blade and set it aside. I experimented with various configurations and placements for the two batteries - I ended up mounting them side by side just aft of the motor - this seemed to give the best overall handling characteristics. I then built a battery platform and securely mounted it to the deck and motor housing - I added half-sides and a battery tie-down strap to keep the batteries from jostling around while mowing.

I built a "dashboard" for the switches and gauges and mounted it about 3/4 of the way up the mower handlebars. On it, I mounted a 15 VDC analog voltmeter with a DPDT switch so I could monitor the individual battery voltages to determine when to quit mowing; and a 0-50 DC amps analog ammeter and switch/circuit breaker so I could monitor the current flow to the motor, to determine if I was working it too hard.

When I had everything mounted I started wiring it all together. I used #8 gauge stranded copper wire since I was still operating on the assumption that my mower normally would be pulling approximately 40 amps. I connected the motor, batteries, two switch/circuit breakers (30 amp and 40 amp), and ammeter in series - the last connection was made very gingerly.

On the first test, the 30 amp circuit breaker repeatedly tripped as soon as both switches/breakers were turned on. After checking everything out and determining that there were no short circuits or something stupid like that, I replaced the 30 amp switch/breaker with a 50 amp switch/breaker, resulting in a circuit with 40 amp and a 50 amp breakers in series. Now the motor started running - after pulling about 35 amps on startup, it settled down to about 5 amps - just as advertised.

At this point, I physically removed one connecting wire entirely, flipped the mower onto its side, and bolted the mower blade on again.  Turning it right side up, I re-connected the wiring, and tried it again. It started right up - after pulling about 38 amps on startup, it settled down to about 25 amps. I wheeled the mower out to the front yard (which I had been purposely neglecting to mow for just such a test...) and started mowing. It worked great! The sound it makes is reminiscent of those big fans that churches used to have in the back of the church during a hot Sunday service in the summer... Very relaxing sound, and it brought back guilty memories of many sermons drowned out - yet it's not so loud you have to wear ear protection, like I've been doing with my gas mower.

Some observations:

(1) I replaced the 40 amp switch/breaker on the dashboard with a regular 50 amp SPST toggle switch - the main on/off switch - it has an orange ribbon on it to remind me to turn it off when done mowing. The motor circuit now has two 50 amp switches in it - the 50 amp main on/off safety switch, and a 50 amp switch/breaker that the dead-man handle is connected to.

(2) The motor will pull from 35 to 48 amps when starting up, depending on whether it's hot or not. There may be other factors too, but I haven't figured them out yet.

(3) When not actually cutting grass, the motor pulls 25-30 amps, and when cutting grass it will pull anywhere from 30-50 amps, depending on how high the grass is, or how fast I'm pushing the mower.

(4) I have found that I can mow my yard more or less continuously (stopping only to empty the grass catcher) for at least 25 minutes on one charge. The batteries were down to 12.2 volts at that point, so I probably could go 30-35 minutes total. This is more than enough time for me to mow my yard, so I am very pleased with the batteries I chose for it - beginner's luck.

(5) The weight of the completed battery-powered mower, including batteries and rear bag, is 92 pounds. The weight of this model of Troy-Bilt mower with a 4.5HP gas engine and rear bag, is 85 pounds. So mine is only 7# heavier - close enough.
 

Parts/Price/Source List:

(a) $25 - used mower deck (including blade, blade mounting hardware, grass catcher bag, and deadman handle/cable mechanism) - from local lawnmower repair business.

(b) $70 - 24VDC 1.5 HP surplus Tecumseh motor

(c) $66 - two UB12220 12V 22AH AGM batteries @ $33 each

(d) $16 - 50 amp AC/DC combination switch/circuit breaker

(e) $20 - DC ammeter, 0-50 DCA

(f) $5 - SPST toggle switch rated 50 Amp DC - from local boating place

(g) $5 - 20 feet #8 gauge copper wire - from local Lowes store

(h) $5 - bunch of #8 gauge ring terminals - from local boating place

(i) $0 - misc aluminum, plywood, screws, nuts, bolts, etc

TOTAL $212 (shipping charges not included)

This has been a fun project - it kept me amused and doing something that is at least marginally productive (mowing the lawn) - AND the best part is knowing that I built something with my own hands that eliminated one very noisy and smelly contributor to the sad state our planet is in.

Lee
mvemjsunp2007@yahoo.com

Xantrex C40

mower charging