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Built by Lee in Florida
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
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
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
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
Later . . .
OK, I've finally got a solar-charged lawnmower. I purchased a 45W
Solar Panel Kit from Harbor Freight a couple weeks ago (three 15W solar
panels and a cheapie charge controller for $199), but had problems with
the controller right off the bat. I messed around with it for a couple
of days before deciding that it was junk and started checking around for
a better one. On the advice of some friends and a silver-tongued tech
rep at Xantrex, I purchased a Xantrex C40 solar controller on eBay for
$112. This is about twice what I had wanted to pay and a bit of
overkill for my immediate application, but since I have a couple more
solar electric projects in mind and wanted to keep my options open, I
opted for the C40 because it seemed to have plenty of power (40 amps)
and be the ultimate in flexibility.
Anyway, I mounted the three solar panels on my
back porch roof, which faces south and has full sun all day. I mounted
the controller on a 4X4 support beam behind a couple of baffles to keep
it dry if it ever decides to rain again here in Florida. On the advice
of the above-mentioned Xantrex tech rep, I wired the the three 15W solar
panels in parallel, as well as the two 12V batteries on my mower (when
mowing, the batteries are connected in series to power the 24VDC motor).
I also wired an SPST switch into the positive side of the input solar
cable, and another SPST switch into the positive side of the output
battery charge cable.
The C40 controller in charge mode can be
configured a lot of different ways, eg, for 12V or 24V or 48V output.
It has a variable charging rate based on the battery voltage and solar
panel voltage, and you can tune (and fine tune, if desired) the voltages
delivered to the battery during the bulk, absorption, and float stages.
Since the spec sheet that came with my AGM batteries specified the
optimal voltages during the bulk and float charging phases, I simply
adjusted the C40's potentiometers (pots) to those specifications. This
extra step will hopefully insure the maximum life for my batteries.
Charging the mower is fairly simple: I throw a
switch on the mower that isolates the two batteries, and then connect
two short jumper cables from pos to pos, and neg to neg. Then I connect
the output cable from the solar controller to those pos and neg jumpers,
and turn on the two SPST controller switches to start the solar juice
flowing. I've only charged the mower once and it took most of a day to
charge the two batteries from a fully discharged state (approx 12.0
volts) to a fully-charged state (approx 13.3 volts).
But now I have a TOTALLY GREEN MOWING MACHINE!
Thank you Guy for all your inspiration and information.
(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
(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.
(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.