DIY battery electric push mower (cordless)
 

For a while now I've considered making my own battery powered push mower. The electric riding mower I have is great and I love it. But, it can't get everywhere and I end up using the weed wacker to mow areas larger than I'd like. I'm not ready to pull the trigger and start making one, but did want to start a thread to collect ideas and improve the design. I think it is entirely feasible to design a DIY battery powered push mower for less than the cost of the cheapest brand new gas push mower. I'd also like to use as many off the shelf parts so that it is easy for others to replicate.

I looked up the price on lowes.com and saw their cheapest mower is about $150. It doesn't have very good reviews either... In any case, the goal will be to keep the price under that.

So, lets start looking at possible parts.

mower
This is the easy one. Find one for free! There are PLENTY of dead mowers out there that people are throwing away all the time. Put an ad on craigslist.org or freecycle.org. Someone is sure to be happy to have you take their 'junk' away.


motor
I've been looking at electric scooter motors. They're very low cost, about $0.10 per watt. I think a 300W motor would do just fine for this application, so you'd have to pay a whopping $30 for one. Most of these motors are 24V. If you go to a higher wattage motor you're probably looking at 36V motors.


batteries
For batteries, you're stuck paying whatever they're going for unless you're like MetroMPG and know people. The sealed lead acid batteries seem to ideal for this setup as you can get them in many different capacities. What you go with is completely dependant on how much run time you need. For me, I honestly only need 5-10 minutes, so two 7Ah batteries would probably be more than enough.


charger
Again, scooter chargers are incredibly cheap. You can get a 24V charger for $15 shipped off of ebay. It is of course a cheap Chinese made (what electronics components arent though) charger, but it will work well for the application.


blade to motor adapter
This is the only thing that I'm having a bit of trouble finding. I'm still looking for ideas. Obviously, I'd like to keep this as something you can easily make, or buy for cheap somewhere. Its also a pretty critical part since its holding a sharp steel blade that will be spinning at about 2500 rpm!


power switch
I think a ~30A switch from radio shack or whatever local electronics store will do just fine. It should cost about $5 would be my guess.

For the adapter between the motor/ blade, I'd be looking at somehow re-using the connection that comes with your dead mower. So you don't have to reinvent the wheel.

As for batteries - there are all kinds of free, small batteries out there: alarm companies, mobility scooter sales/service places, motorcycle shops, etc.

Just depends how much time you want to spend searching/waiting for a functional, re-usable one vs. just buying a new one. I like the thrill of the hunt, plus it's more satisfying to make functional stuff with as little money and as much re-use as possible.

But I still think you should totally go the ghetto route and find a beefy cooling fan motor from a junked truck, or even better: a starter motor! :) You'll be famous when you post the YouTube video of your hacked starter motor mower with a litle 12v car battery on top and jumper cable wiring.

I searched Kijiji and found some for $50. or less, cordless mowers for sale, some good some with bad batteries.
From what I've seen if you don't keep the batteries charged or hooked to the charger as the manufacturer says they don't last.
Here's a short video of a 24volt, 17amp/hr, 20"cut, Yardworks cordless mower cutting some grass. Estimated run time 45minutes.
Yardworks 24volt mower - YouTube

That's long grass to cut. I don't know if I'd subject my revived M3300 to that.

dh1 - where in Ont. are you? (Roughly, if you don't want to reveal specifics.)

Markham near Pickering border.

I was walking a little slow in the video as the mower was loading up pretty good.

Aha - I'm east of Kingston, though I grew up in east Scarberia.

To me the whole point of a power mower is as a time saving device: I could use the push/manual reel mower once a week (or more often, in the spring) as long as I don't let the grass get too long.

With a power mower, I can wait more days between cuts, and it takes fewer passes to do the job than my manual reel mower.

But the M3300 is not a mower I'd subject to heavy loads. Yours looks better

And Daox's 300w spec on his theoretical DIY mower is probably a wise move.

I grew up in western Scarberia.

I haven't had time to do any test, amp meter readings on my cordless, the heavy wire size is 12ga. Double the voltage 1/2 the amps for the same power.
Plan on trying a extra battery, 36volt set-up, on a 24volt yardworks push mower. Be nice to have more power.

A battery you might want to look at is the lawn tractor battery, 34 amp/hr, you can get the sealed type at CTC or WallMart, about $100. last time I checked.

Currently working on this as time permits>http://ecorenovator.org/forum/lawn-g...ower-deck.html

Also I have a Suzuki Sidekick with a good body and a ADC FBI4001A
hope to someday make a battery powered EV out of the 2.

If you can't find a 30 amp switch then you might try a starter push button and a lower amp switch as the wear in the switch comes from making contact and again from breaking contact, or just a wire bale that holds the push button down.
I really like the idea of using scooter parts, if the motor is powerful enough of course, so you might try to find a whole scooter that has issues and rip in to that.

heres a point of reference.
i have a 48v 20" battery powered push mower by ryobi.
i live on .63 acres of land.
one battery charge lasts for half of my yard. the grass is not too thick, roughly 1 hour of use. side discharge. no self propel use.
the battery takes 12 hours to charge.

ryobitools.com/catalog/outdoor_products/mowers/RY14110

I've been watching craigslist and either people around here don't have them, or they want a mint for them. Plus, DIY is a fun project and if I can come up with an easy way for others to do it too, that would be great.

Thanks for the reference. Do you have any idea how many amp-hours your battery pack is?

NP. glad to help. i dont know off hand. will update when i get home...

sorry for the double post.

i looked at the manual,
Battery = 48 Volt, 10 A/hr sealed maintenance free

I guess it depends on where you are as to whats available, I just thought that cordless mowers would be a good source of parts.
I picked up 4 altogether so far, 1 was good sold to my friend the other 3. I was able to get 1 good 12v 17 amp/hr batt, 3 good motors, 1 24volt relay with a 100amp fuse, micro switches, and 2 hi amp switch, plus they all came with blades. These motors were made to turn a blade so you don't have to worry about that.
I don't think you can burn out the motor on these as the batteries can't deliver enough amps for a long enough period of time, maybe if the blade was jammed but then the fuse or cb would blow.

Getting back to the first post
For relay you might want to look at some 12volt RV relays. Maybe some auto relays, ex fuel pump relay I think there 30 or 40 amp. You could also use a standard 120volt household circuit breaker, 40, single.
For batteries what ever you can find, there are as I said before 12 volt sealed lawn tractor batteries, but $100. each that gets expensive for 2.
Scooter parts there are a lot of suppliers around for that stuff.
Hope this helps.

Keep use posted I'll be watching this tread, I'm sure to learn something.

20Ah batteries are only like $35. Unless you're looking for real long run times it shouldn't be that much for batteries.

Well, I've started collecting parts for the project.

Over the weekend I picked up a 300W 24V scooter motor for $30 shipped off ebay. I'm really hoping this is large enough to spin the blade and cut grass. I think it might be a little on the small side, but the price was right and I couldn't pass it up. If its too small I'm sure I can find another use for a 300W permanent magnet motor.

Last night I picked up the mower for the project too. Its my uncle's 20 year old Honda push mower. He recently replaced it with a reel mower since the engine was making odd sounds. Its a bit on the heavy side I think, but it should work. One downside is it doesn't have a side discharge. Its really only setup for bagging which I have no interest in using a bagger. So, I'll have to make something up for side discharge.

Pictures to come. :)

I'm still looking for suggestions too. I'd still like to keep the build price under $150 but exceeding that a little is okay too.

I'd just put a plate on the discharge and let it mulch. As long as you don't let the grass get too long, shouldn't be a problem.

Do you know the eBay motor's physical size? FYI, the motor in my B&D M3300 mower is ~3.5 inches diameter by about 5 inches.

I'm not sure of the exact size of the motor. It looks to be more like 4-5" in diameter and 3-4" long. I'll take measurements when it gets here.

I thought about the mulching option, but I would think it takes more power to mulch? I'd like to keep the power usage as low as possible.

True. All depends how long you let the grass grow & how wet it is.

That's the biggest problem with estimating run time on a mower: the grass is a huge variable.

As some of you know I have my own battery powered mower in the works. I have the motors mounted and have done some testing.
I am using motors form a Yardworks cordless mower, 20" cut, 24volt with 2- 12volt 17amp/hr batteries, that's what it was made with when new.

Tests> I wanted to know how many amps one of these motors draws with the blade on spinning free not cutting any grass.
Found that the motor at 24volts draws about 20amps, my amp meter is not the best. Picture shows the reading I got, the needle at start up goes higher then falls back to what you see.

Thought this might be useful as a point of reference.

Thanks Doug, that is helpful.

I grabbed a picture of the mower tonight before I went in for the night. I'm not even sure what the specs on the mower are so I'll have to check that out and report back.

http://ecorenovator.org/forum/attach...1&d=1314153707

I know on some brands of rear bagger mowers you can get a shoot that goes in place of the bag and directs the clippings out the left side (opposite of a normal side discharge). I've seen 2 different sears models that had them.

We'll see. The budget is tight enough on this project, I don't want to be buying parts if I don't have to. If worst came to worst and it doesn't work out with this mower I'm sure I can find another dead mower out there somewhere pretty easily.

Anyway, the motor arrived today. It is 4" long and 4" in diameter. The shaft is 3/8" in diameter with I'm guessing 3/8-16 thread on the end. It also does have a flat spot for a setscrew. I was hoping the shaft was a bit beefier, but I guess this'll have to do.

http://ecorenovator.org/forum/attach...1&d=1314243100

I've really been trying to come up with a good coupling solution but am not getting any good ideas. You guys have any? Again I'm trying to keep this a real simple build that anyone can do. That means off the shelf parts and/or simple modifications to parts like hand drilling, maybe tapping, but no custom parts.

What was originally connected to that motor?

Chain sprocket? Hub or no hub? Maybe you could get one and adapt it to mount the blade (drill & bolts).

There is a clutch mechanism on this mower. You can run the engine without engaging the blades. I honestly haven't looked to see if it can be adapted, but I'm guessing not since the engine shaft is way bigger than 3/8".

This is how the blade is mounted on a Yardworks cordless mower.

1> is the assembly
2> the motor shaft
3> all the parts,
Spacer>Blade with fan>Square plastic washer>Steel washer>Nut
4> the parts on the motor shaft, minus the blade, fan and plastic washer

The blade, fan, plastic and steel washer all lock together.
The steel washer locks on the motor shaft.

IMO if you can get some kind of a collar locked on the motor shaft in the right spot so it doesn't move, slid up and down, then a thick fender washer, then the blade, then another thick fender washer, and finally the nut it should work.
Whatever you do you want to make sure the blade is not going to come flying off.

That motor looks like a fair size and I see it has a threaded shaft on it.
I think it's going to work.