Riding lawn mower electric conversion

Discussion in 'General' started by Daox, Feb 27, 2009.

  1. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    You guys have huge lawns!! :eek:

    My parents' house down in SC is on a 1/2 acre lot and my house sits on approximately 1/3 acre. I use a push mower and typically finish mowing in just under 50min.
  2. Daox

    Daox Well-Known Member

    Wow, thats quite the lawn! Too bad about the bylaws. You could always mow the grass in two cycles. I'm considering that if batteries becomes too large of a pain to find.

    I also just dropped $120 on very nice sealed Timken bearings last night. I took apart the deck last weekend and found that about half the bearings were bad. This is by far the most I've spent on the mower thus far. Grand total puts me at about $160 into it. The original goal was to keep it under $300, or the price of a semi-decent used gas mower. Thankfully, the only thing left to get is batteries.
  3. Daox

    Daox Well-Known Member

    The mowing deck has been rebuilt for a while now. I've been slacking (actually busy with other stuff) and haven't worked on it recently, until now.

    Test number two went off last night. I lowered the voltage and only used two batteries instead of three. Oddly enough, my amperage was lower than with three batteries (which is opposite of what it should have been). My guess is that the lower motor rpm really helped out. At 36 volts (3 batteries) the motor spins 1.5 times as fast as 24 volts (2 batteries) since rpm is proportional to voltage. At 24 volts, my motor rpm is much more in line with what was originally intended for the mower. The result was driving around still took 20-25A, but the mowing deck only took another 25A! This is HALF the amperage of the last test. It seems that the deck rebuild and proper pulley sizing really works out well. I am very excited about this!

    The next step is to procure some actual deep cycle batteries. My power died off insanely fast because I just used two semi-dead staring batteries. One trip around the house and they were already puking on me. The local battery store has some group 24 deep cycles for $70 which seems pretty reasonable. I'll have to look into what else they have too. I might want to go with something larger. I'd ideally like a 1 hr run time. At 50A, I would get about 40 minutes of run time since you don't want to cycle batteries beyond 50% to retain good life. The alternative is to go back to 36 volts, but resize my pulleys. This theoretically would drop my amperage down to ~33A. This would (again theoretically) give me right around an hour of run time and maybe a bit more because of the peukert effect. So, 36 volts is looking like the way to go.
  4. ksstathead

    ksstathead Moderator

    Any thought of using 2 24V pairs in parallel? Extra battery to buy and carry, but the pulleys can stay the same?
  5. Daox

    Daox Well-Known Member

    That is definitely an option. However, it'll depend on the availability of batteries.
  6. Daox

    Daox Well-Known Member

  7. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    Sweet! I just have to wait for my Lawnboy to die... ;)
  8. Daox

    Daox Well-Known Member

    Looks like its time for the monthly update of this thread. :) For those who would like to catch up quickly, I have actually written a blog series that sums it up fairly quickly.

    EcoRider blog series

    More in depth info is available in the build thread on ecorenovator. Or, you can ask questions here.

    I also recently made an evalbum page for the mower too.

    Okay, on to the updates! :)

    At the end of last month I picked up the batteries that I'll be using on the mower. They are group 31, 150Ah flooded batteries. I picked them up from a local fork lift repair place. They did blow the budget for the project kinda as they are new. But, with the life I expect out of them, I'm not going to worry about it. I couldn't find any good sources for used batteries at the time.


    Last weekend I started the battery racks. I went with two in the back for traction, but one in the front to keep some reasonable distribution of weight (no wheelies!). The rear rack is all done, but the front one still needs to be made. I like how the rear rack will give me very easy access to the batteries for checking the water level. The one up front won't be quite as easy to check since it'll be under the hood.



    I'm really looking to get this thing running soon as I need to cut the grass before the snow starts to stick. My in laws recently took back the push mower we had borrowed from them so I have no way of mowing right now.

    However, I'm also a bit excited to see the snow fly so I can get a chance to try out this attachment I got a few weeks back. :)

  9. Daox

    Daox Well-Known Member

    The front the battery rack was made over the weekend. Here are some pics.

    I started out again making it a bolt on item. This will allow me to modify or alter things as needed. Its also easier to make it since it won't be welded right to the chassis.




    I was also able to do some actual mowing over the weekend as well. I wasn't able to do the entire lawn all at once, but there are a number of factors working against me right now. There is still a bunch of tweaking to do!
  10. drimportracing

    drimportracing Pizza driver: 61,000+ deliveries

    It's really coming along nicely. Did you resize the pulleys for the 36volts? :D - Dale
  11. Daox

    Daox Well-Known Member

    Nope, not yet. That is one of the more important tweaks to do. :)
  12. nidly

    nidly Well-Known Member

    Daox, your mower conversion looks great. Would you be able to assist me in sizing a motor and then perhaps finding some prices on one?

    If so, my application will be an old mtd 18 hp with a 46" deck. Are you using a 2 hp motor to replace the gas job? My mower rarely takes 5-8 hp to do it's job with the gas engine.

    I was thinking to match the engines rpm (it may be around 3600) so I could leave the tractor/pulleys, ect. alone. I have no idea how much hp I need though and the voltage can be probably from 12,24,36,48 volts.

    I'm still in the tire kicking stage, but your project inspires me to do some checking.

  13. Daox

    Daox Well-Known Member

    Sure nidly, I'd be happy to help you out. I'm no expert, but I know a thing or two.

    I've heard that a general rule of thumb is take your gas horsepower and divide by 3 to get roughly what you want your electric motor's horsepower to be. I am actually a fair bit underpowered according to this formula. My motor is rated for 1.6 hp and the gas engine was originally an 8 hp engine. My motor did get warm while I was mowing, but not hot. From talking with other owners, a warm motor is fine, just as long as its not too hot to touch. So, there is definitely some play in that formula. Of course, it will depend on your motor too. Bigger motors with bigger brushes are more capable of producing more power than they are rated for. If the motor does get too warm, you can always add a fan to it for cooling. Its not the most efficient option, but a lot easier than finding a new motor!

    Matching the motor rpm to the gas rpm is necessary. However, you can fix that with different pulleys if necessary. My motor is rated for 3200 rpm @ 24V. Since I'm running 36V it is running 1.5x as fast. So, I'm running around 4800 rpm right now. Lemme tell ya, that deck will REALLY shoot that grass out. :) However, running everything that fast is sucking the amps, so I'll be lowering the rpms back down where they should be.

    As for voltage, I would see how many batteries you think you can fit on the mower. If you can go to 48V thats a good thing since it'll lower your amperage which minimizes the peukert effect (discharge a lead acid battery 2x as fast and it lasts less than 1/2 as long) and you'll get longer run times. Just keep in mind how this will effect motor rpm.
  14. nidly

    nidly Well-Known Member

    Yep, gotcha. I can fit as many batts as I need since it will not have a driver (radio control saves weight)

    I may look for a 48 volt motor with about 4-6 hp then. I'd like to use all stock pullys and speeds since they are about right for good mowing. I can just slow the tractor down if need be.

    Now to find a used/surplus of dc motors and some kind of controller, hehe. This will be as much of a "radio controlled" project as it is an "electric conversion" I'm hoping that losing a 200pound rider will make a big difference.

    One thing I'm worried about is keeping the batteries in balance since there may be 4 in series. I guess it is no different than 6 series cells (in one battery) though.
  15. Daox

    Daox Well-Known Member

    Yeah, since a 12V is just a bunch of series cells, balancing really isn't a big issue with lead acid. Every once in a while you can just top them off individually. Or, just use individual chargers.

    I can tell you right now, saving 200 lbs isn't going to save you hardly anything. On my mower, just driving around the yard pulls ~25A, but engaging the blades and mowing takes an additional ~75A. So, the majority of the power is going to the mowing deck.
  16. nidly

    nidly Well-Known Member

    Yep, thanks for the info.

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