Michael,
Hard to believe you made this post 2 years ago - I've been holding on to
a few of these for a plan of my own.
I've now cut the grass half a dozen times with my electric tractor
conversion. I've wanted to convert something bigger for ages, but I've
had to settle for starting small first.
I have had some fun just driving around in a nearly silent go-kart, that
just happens to also serve a useful purpose when I turn on the mower
deck. :)
Pictures -
http://www.tmcom.com/~tsm1/gallery2/main.php?g2_itemId=2060050
Action video - I should get some audio to overlay on it, but haven't.
https://youtu.be/wUHULS-56vo
Brief synopsis:
Used Craftsman 42" / 15.5hp tractor, with a hole in the block.
It sat for a few years before I finally pulled the trigger on the
conversion.
EVDrives supplied an ME1004 and 48V contactor.
I pulled the gas engine/fuel tank/muffler and then fabricated an
aluminum engine adapter plate, along with a spacer for the pulleys -
while the pulley fit the shaft just fine, I had some height to adjust.
I wound up using some reduced head "header" bolts in a few spots on the
motor mount - mostly to avoid cutting the tractor frame to make it all
fit (this way I kept the rolled lip around the old engine opening).
At one point, I had a line on some used 80AH 12V UPS batteries, but I
didn't grab them at the time.. and after much contemplation, I bought a
Chevy Volt pack from Hybrid Auto Center. 12S1P, "48V" pack, charge &
balance BMS, and a 9amp charger.
The motor was ~$600, the battery and accessories another ~$800 IIRC.
Yes, I could've just bought a new gas tractor (or just kept using the
gas tractor I already/still have) for less.
(4 12V lead batteries at ~$150 each came in closer to the Lithium cost,
at greater weight and packaging problems, and likely reduced life)
This is a 47ah battery, or 2kwh. Charges to 50V, and I stop what I'm
doing when I discharge to 42V.
The Volt battery sits where the muffler used to be. I have room to put
a second battery between the electric motor and the dash if I had to,
and maybe a third under the seat / above the transaxle.
This mower uses a 6spd transaxle.
So, 48V relay, 400amp fuse.
My wires are a hodgepodge - the negative cable is a short run of 2/0
cable I had lying around. The positive has a 2g battery lead to the
fuse, because I had a used automotive battery cable that was 2g. Then
the fuse to relay, and relay to motor is doubled-up 4g cables in both
cases - because I had other used automotive battery cables. (Bad ends
removed, new ends crimped in the right spots, etc - just used wire saved
for this purpose).
None of the wires seem to get hot - the motor is more hot than the wires.
The short story is it works great. The 2kwh Lithium pack between 50V
and 42V yields around 20 minutes of mowing time with the deck going. It
is enough to mow either my front or my back yard. So I can mow one,
charge over night, and mow the other. OTOH, if I stay on top of it and
focus on the task at hand, I can do either side in a little over 10
minutes - so maybe with a little more practice, I could get to do the
entire yard in one charge.
A second battery would solve that, of course - but the current situation
is very workable.
I had hoped for 15 minutes to an hour of mowing, depending on actual amp
draw. I seem to be closer to the 125amp side of things (125amps @ 45v
for 1/3 of an hour is 1.873kwh). I have not tried just "driving" around
to see if I can make it to the corner store and back on one charge. 6th
gear isn't as fast as I'd like for that.
I haven't noticed the electric motor "bog" down like my ICE mower does
from time to time - it just pours on the torque.
You get a little bit of a whir and whine driving it around - but
compared to the ICE, it is silent. It does still make a racket when you
start spinning the blades.
On the over-doing-it side, I also bought a ~$6 48V to 12V water-proof
down-converter to run the headlights, rather than a "house" battery. I
also added some power-poles, so I could conceivably use the 2kwh pack to
run the radios during Field Day just because.
So far so good - mowing the lawn hasn't been this fun since I was a kid
and didn't know that mowing the lawn was a chore. :D
So - thank you, Michael, for posting your plan and your follow up.
I hope mine helps the next one..
-Tom
On 7/28/2014 8:59 PM, Michael K Johnson via EV wrote:
I got lots of useful help here on my lawn tractor conversion project —
http://www.evalbum.com/4841 — so I thought I'd post some lessons
learned, both positive and negative. None of this is intended as
advice for anyone else, for obvious reasons. I'm not doing everything
as safely as I might! This is just the "brain dump" I wish someone
else had written for me to read when I was getting started.
Biting the bullet and buying real batteries (Deka Intimidator 9A31)
was a good idea. Mowing my half-acre lot that's not much more than
half lawn sometimes brings me close to 50% DoD, occasionally perhaps
very slightly below. It took a bit of convincing myself to spend $1K
on batteries—almost twice what I spent on the Motenergy ME1004 motor.
Part of what got in the way of accepting that reality early on was a
silly idea that I could fit everything under the existing hood and it
would look the same, just be quieter. Things got a lot easier when I
decided to ditch the "hood" and embrace as part of the conversion the
fact that this was going to look way different when I was done. If I
were starting over today, I'd seriously consider getting together with
friends on breaking a scavenged Volt battery and doing a 2p15s or
3p15s arrangement (I don't recall the capacity of the volt cells).
Scavenged Volt batteries seem to be going for around $2K so if enough
people wanted to get together on it, this would have been an
opportunity to get more capacity for less money. I hope that by the
time my current batteries are going downhill, lithium cells are a dime
a dozen.
Installing muffin fans pulling cooling air through a reticulated foam
filter (like they use for aquariums) to provide positive pressure
cleaned air in a plenum around the motor was a good idea. The motor
stays clean. I don't know if it matters for cooling, since I'm running
the motor well under rated load. But cleaned air can't hurt brush
life. I count that as worth the 1.2A continuous draw. Using plexiglass
to make that plenum was a great idea. Using 1/8" plexiglas for the
sides and 1/4" only for the top where the muffin fans were mounted was
a false economy; if I were doing it again I would make the whole
plenum out of the more expensive, but more robust and easier to work
1/4" plexiglass. Given the frame I had to build to support the 1/8"
plexiglass, I think that it would have taken me less than half the
time if I'd done it all in 1/4" plexiglass. However, using a sectional
picture frame (the kind where you purchase two packages to make one
frame, one package for each dimension) as the holder for the
reticulated foam worked quite well. I'd do the same thing again even
if I weren't so incredibly lucky as to find the frame packages on sale
for 95% off... I used a combination of glue and brass screws to hold
the plexiglass together; if I were doing it again I'd just use the
brass screws. They look pretty cool in the plexiglass box if I do say
so myself.
Talking myself into doubling my 4awg welding cable for the equivalent
cross sectional area of 1awg was almost certainly overkill. Joe
Lorenzi has 8awg in his JD with the same motor, and he told me that it
gets a little warm and thicker than 8awg would be useful, but I really
don't think I need 1awg equivalent. Even immediately after mowing
through thick grass with no breaks for nearly half an hour (down to
50%DoD) my cables are cold. If I were doing it over, I would just use
the 4awg cable. Would be easier and use lighter, easier-to-manage
lugs, and routing would be less of a challenge than it was.
I bought both 400A and 200A fuses, not sure whether it would blow the
200A fuse. I needn't have worried. I do see over 100A continuous, but
the 200A fuse hasn't blown. The batteries just can't push that much
current through that motor...
Hooking up my power switch so that the charging cable is connected in
one direction, and the contactor in the other, means that I can't
accidentally try to charge it and run it at the same time, and makes
it less likely that I'll drive away and leave the charger connected.
Buying SB50 anderson connectors for charging at 48V 6A max was silly;
the charger came with C15 (just like in the back of your computer)
which works fine. So I have some unused SB50 anderson connectors...
I was originally going to connect both sides of the motor through an
SB350 anderson connector for a disconnect. Instead, I put a single
pole PP disconnect (using exactly the same internals as the SB350)
inline next to the fuse as my emergency disconnect / safety
maintenance disconnect. Requires only half the effort to pull it open
in an emergency, and the loop of wire that doubles as the emergency
disconnect handle is also convenient for my clamp ammeter! So I also
have some unused SB350 anderson connectors...
I wanted to avoid a 12V house battery and run everything off of 48V
due to my initial expectation to install one of my batteries under the
seat, before I settled on the 9A31 batteries that couldn't possibly
fit. After deciding on batteries, I didn't go back and reconsider my
design. Because of that, it was hard to source the muffin fans (found
surplus fan FRU for rack-mounted hardware), and the contactor solenoid
draws more power than it otherwise would. And now I need 12V house
power anyway if only because the amphour meter I purchased requires
isolated 12V power, so I intend to put the original starting battery
back in the empty hole under the seat and use it for house power, and
regret that it's not running my fans and contactor since they are 48V
units. It also got in the way of taking Lee Hart's excellent advice
for using two contactors to reduce the power draw. And it also kept me
from hooking up the safety interlocks — which it turns out would be
more of a pain than I expected for other reasons, so even when I add
the house battery I'm not sure I'll get the safety interlocks
connected. I do keep that in mind when using it, though. I did spend
my early childhood (as soon as I could physically reach the pedals)
driving around a couple of simplicity tractors with no interlocks...
The MTD transmatic belt-and-pully variable transmission isn't as bad
as I thought (it's not hydrostatic and by most reports it's more
efficient than a hydrostatic), but it's really imperative to get the
MTD belts and not belts advertised as "OEM replacement". The non-MTD
belts were both unsafe (the tractor wouldn't come to a complete stop)
and were so sticky that they cost me something like 20A. Also, using
them damaged the transmission a bit and made it harder to shift,
though that's gotten better since I replaced them with the MTD belts.
I was surprised to find that the bearings in the mower deck were free
and turned easily; I had expected to have to refurbish them but did
not have to do so in the end. I did replace the mower blades.
Having two of the batteries cantilevered over the front tires requires
excessive tire pressure and makes steering a lot of work. There is way
more weight forward than in the ICE configuration. I expect to replace
the steering pinion every few years; the first one lasted about 7
years but this is much heavier service. Having the other two batteries
between the console and the seat such that I have to straddle them
while I drive works, but it doesn't put enough weight at the back, so
I sometimes have to shift my weight to the back of the tractor to keep
the rear tires from spinning. The advice to instead mount them
cantilevered off the back of the tractor was good; I didn't do it
because it would get in the way of bagging. I normally mulch, but I
bag when I mow leaves in the fall. So I don't think I personally would
change that, but for anyone else considering this, if you don't need
to bag, I'd strongly suggest considering mounting rear batteries
behind the seat for better traction and balance. Better still would be
lighter batteries, like the broken Volt battery idea, or other lithium
ion, that can source the current and won't be hurt by much deeper
discharge. Would have taken a different charger, of course.
Mounting a wood bumper in front of the front batteries to protect them
was a good idea. Especially when I had the non-MTD belts installed
that didn't come to a complete stop. That protected those expensive
AGM batteries!
I definitely expect to have to sharpen the blades often; they will
draw more current if they are dull. If I remove the batteries, I can
easily and safely tip the tractor on its side to sharpen the blades.
I'd like to be very clear that this isn't advice or a recommendation.
But in doing that I know for sure that the motor can't start while I'm
trying to sharpen the blades! ☺
I did use a DPDT on-off-on switch to connect positive pole to either
the contactor or the charger, and I routed through both poles in
series so that each half sees 24V for less arcing in the switch for
longer life. I have not yet built a snubber, though on an oscilloscope
I measured a 1ms -100V pulse from the contactor solenoid. Basically,
that means that I got a 1ms -100V pulse with a 10MOhm 15pF RC snubber.
☺ I finally got an assortment of rectifier diodes and high voltage
polypropylene capacitors so I can cook up a RCD snubber to put on the
contactor solenoid thanks to Lee's excellent page on the
characteristics of various types of snubbers. No need to snub the
permanent magnet DC motor; I couldn't measure any back-EMF from it and
I have a vague sense that's typical due to the permittivity of
permanent magnets.
I'm considering making a simple PWM motor controller for the fans
since I think they are probably way overkill and I could reduce the
1.2A draw. But first I need the snubber to keep the power cleaner. One
I have that, I have a HV buck converter to supply the control circuit,
and it just takes a 555 timer, resistor, potentiometer, a few
capacitors, a few diodes, and a MOSFET to reduce the power draw from
the fans. But there's really no need. That would be just for fun.
I have not built the battery balancers. The batteries are all within
.02V of each other so far. The one balancer I built was a pain to
build. I can't figure out how Lee manages to solder the diodes to the
lugs without overheating the diodes. I had already ordered the parts
from mouser before I saw that Lee was selling them, at which point I
thought I'd just build them, and then I discovered what a pain that is
and gave up after building one that I don't really trust because the
zeners got so hot while I was soldering them into the lugs, despite my
best efforts. So if my batteries start getting out of balance, I think
I'll just cut my losses and buy a set from Lee who knows what he's
doing. I'll just add the zeners etc. to my kit of random electronics
parts and someday find a use for them.
Oh, and it seems like everyone I talk to about my project exclaims
about how cool it is that I can mow my lawn silently. Maybe they are
remembering how a Leaf or Prius or (if they are lucky) a Tesla can
sneak up on them. My motor running alone on the bench was
uncomfortably loud and I wore ear plugs for comfort when I was
breaking in the brushes while it was on the bench in the workshop.
Before the conversion, the ICE was louder than the mower deck but the
mower deck made the tractor much louder when it was engaged. Now, the
mower deck is much louder than the motor. I still wear ear plugs while
I mow the lawn.
But still, with all the mistakes and downsides and things I'd do
differently next time, I still get the EV grin while using it, which
is impressive since I really really hate to mow the lawn. Making me
happy while I mow the lawn is a definite accomplishment.
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