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. _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
