One Man's Adventures into Renewal Energy
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DIY Solar Powered Cordless Lawn Mower - The Build

9th July 2014

DIY Cordless Mower Brushless
Here is the donor mower that I plan on using.
It's a "Lawn Pro 41". 41cm cutting width. Adjustable height.
I bought it off Ebay, with handles, but without engine, for £10.

When buying your mower, you need to stick to some guidelines -
1 - Try and get a petrol mower, because these will be the easiest to modify.
2 - Plastic or aluminium, to avoid corrosion.
3 - Consider battery placement. ie, is there a flat'ish section on the mower.
4 - Do you want a bag. I've decided to 'mulch' my lawn, so a bag isn't one of my requirements.
5 - Adjustable height - It'll most likely be quite difficult dictating the height of blade, so instead you need another method of dictating grass cut length.

DIY Cordless Mower Motor Mount
I need a way to mount the motor the Mower deck, as well as filling the gaping hole where the combustion engine originally was.
The simplest is the use a disc.

I cut the above disc out from 10mm plywood (actually 2x5mm pieces glued together).
It's not the most circle of circles - blame my jigsaw skills for that!
Gave it a coat of varnish to protect it.

I would have preferred to use an aluminium disc for this, for reasons of strength and weather proofing. But I was quoted £23 for such a disc...which I thought was a lot.
I'll see how my wooden disc wears - if in time it starts to look tired, I'll have to upgrade.

DIY Cordless Mower Motor Mount
Here is the disc mounted to mower, with the motor attached.
It all feels very solid.
However, I am a little concerned how wood would hold up if the blade were to hit a heavy clump of grass, therefore exerting a lot of rotational torque to the motor, which may shear the bolts from the wood.
I am just gonna have to see how it goes.

DIY Cordless Mower Motor Mount 2
So after I mounted everything up and attached a blade, I discovered the blade was far too high up in the chassis.
To fix, I re-mounted the disc to underneath the mower - as seen in the pic.
This has lowered everything by a good inch or two and now the blade is a lot lower to the ground.

DIY Cordless Mower Motor Mount Collet
Probably the biggest issue you'll have with your build is how to mount the blade to the motor shaft.
The last thing you want is the blade detaching at high speed and causing damage to the mower, or more importantly, you!

I had two requirements -
1 - It had to be none-flexible and strong.
2 - It had to be an off the shelf, no welding, metal work, etc.

So reverting back to my Radio Controlled Airplane days, I thought about how I would attach propellers to motor shafts. The thing I would use was called a collet.
These collets are very strong and come in a variety of sizes.
They basically squeeze onto the shaft - the more you tighten them, the more they squeeze.
And they also have the effect of self tightening - provided you spin them clockwise (make sure you get a clockwise mower blade).

The one in the pic above is a "Prop adapter to suit 10mm motor shaft (collet)" I bought from HobbyKing for £3.

DIY Cordless Mower Motor Mount Collet
So here is the blade attached to the mower using the collet.
You can see I've used a lot of washers to increase the depth of the blade. It might look a bit precarious, but I can assure you, this is super solid.
You'll also notice the blade is a bit small for the mower deck - just ignore this, I've got a new blade on the way which is the right size.

DIY cordless mower
So here is a pic of everything loosely put together, ready for 1st test.
The batteries are loose.

DIY Cordless Mower Battery Mount
So thinking about powering the mower, I originally looked at sealed lead acid batteries (the kind that are used in mobility scooters), but to get a decent Amp-Hour (ah) rating, I'd have to be spending over £100.
The other issue with these kinds of batteries is they don't provide a huge amount of CCA (cold cranking amps). I want a decent amount of CCA so that the mower doesn't bog down through lack of current.

So instead, to keep things simple and cheap, I've decided to use a standard car battery to power the mower.
I bought the smallest car battery I could find, for the princely sum of £29.
This battery is rated at 43ah, which is more than enough for my needs.
This battery will supply over 200CCA, which is more than double my needs. This will ensure the mower never bogs down through lack of amperage.

I also wanted to mount the battery between the two axles, but as far to the rear axle as possible. This is so that the mower is balanced and is easy to tilt.
The mower deck I have has a large gap just in the right spot, but it wasn't quite wide enough.
So using my dremel, I cut a hole in the chassis.
I also cut a slit for a battery strap.
I now have a perfect battery sized hole!

DIY Cordless Mower Battery Mount
Here is the battery - all fitted and strapped in.
Looks neat and feels secure.

DIY Cordless Mower Dead Mans Switch
Previously - you may have remembered me talking about a dead mans switch and how it's needed for safety.

Anyway - This pic shows the back side of the Servo Tester.
You can see I've soldered the two leads from my dead mans switch to solder joints of the Variable Resistor of the Servo Tester.
This effectively bypasses the Variable Resistor and instead gives you a full-on, full-off switch.
I secured the wires a tie-wrap.

DIY Cordless Mower Electronics
I needed to mount all the various bits of electronic gizmozetry.
Using a piece of mdf, I've mounted the various bits I need.

To go through the various elements -

ESC (Electronic Speed Controller) - This is the thing that controls the brushless motor. Mine is an 80amp controller, called a Turnigy Plush.
The one consideration with the ESC is air flow - ESCs can get very hot, so you need to ensure it has adequate air flow.

Shunt - This is needed by my amp meter.

DC Circuit Breaker - This is used to protect all the circuitry from excessive current draw and/or short circuit protection. It also acts as an on/off switch for the mower.
Mine is a 200amp breaker, which is way over the top - I've ordered an 80amp breaker to replace this.
You'll also notice I've got it hooked up the negative side (black) of the battery as opposed to the more conventional positive (red) side. This is because the amp meter I'm using takes its feed from the negative side.
I guess, logically, I could have wired this to the positive side, but it makes little difference.

Servo Tester - This controls the ESC, which in turns controls the motor. Because I did some soldering on this for the dead mans switch(above), I ended up exposing the circuit board - so to protect it from the elements, I've wrapped it in insulation tape.

Wiring - You'll notice some heavy grade wiring coming in from the battery. This is 10mm gauge wiring, which should supply more than enough current for the mower.
When spec'ing up wire, you have to ensure the wire you use is at least as thick as the wire used on the input side of the ESC.

DIY Cordless Mower Volt Amp Meter
I love stats!
So to satisfy my desire to get some stats from my mower, I've fitted a Volts/Amp meter.
This is a DC 100amp meter, purchased from EBay for £5.
Mounted to mower handle for easy viewing!

DIY Cordless Mower Electronics
Here are all the electronics, all mounted up.
It looks fairly neat, I think.
I may have to cover up the electronics to protect from the elements, but as I only mow when the sun is out, I may get away with not bothering. Plus, all the exposed wiring makes it looks a little bit like the DeLorean from Back to the Future!

DIY Cordless Mower Mulching Plug
You may remember me mentioning that I wanted to "mulch" my lawn clippings as opposed to bagging.
This is because I don't want to fill my household bin with clippings, and it is also beneficial for the lawn to leave the clippings on it - ie, let the nutrients go back into the soil.

My mower deck has a large hole which is used to vent the clippings out the back and into a bag. So to prevent the clippings from doing this, you have to plug this hole.
If you're lucky you can buy a purpose made plug for your mower, but I couldn't get hold of one for my mower, so I had to find a DIY way of doing it.
The expanding foam stuff you can buy from DIY stores seemed like a good way of doing this.
So that's what I did!

Geeeez! Talk about messy! Because of my lack of preparation and just diving in with both feet, I ended up getting this stuff all over the place - all over the mower, my clothes, my hair, everywhere.
Top Tip - Prepare yourself. Use masking tape or something to block up exit points for the expanding stuff. Lay something down on your garage floor.

Anyways - the end result looks a bit crappy, but it has worked - I've completely blocked up the exit chute of my mower, effectively creating a mulching mower.

DIY Cordless Mower Mulching Plug
The underneath of the mower showing the mulching plug.

DIY Cordless Mower Solar Charging
Here is the mower in my "Solar Shed", hooked up the Solar Charging system.
You'll notice I've fitted the plate to the back of the mower - this is purely aesthetic to cover up the ugly looking expanding foam rubbish.

Testing, costs and conclusions -
The mower works very well. I managed to mow both my front and back lawns in less than 15 minutes. Approx 80m2. Grass was probably mowed about 1 inch in height. With my old plug in mower, what with all the hassle of flicking the power lead around, would take me double that time. Cordless mowing is a dream!

The battery went from 12.9v to 12.5, so still plenty of juice left in it.
The mower would bog down on occasion, but never stalled.
The mower feels lighter than my old plug in mower. I've seen other DIY cordless mower builds with huge DC motors on - I suspect they are very heavy. This brushless motor in comparison is tiny and probably just as powerful.

I'm still waiting on the larger blade. The larger diameter blade will have an effect on motor loading, but I'm sure the mower will still work fine.
I'll report back.

Using a cordless mower is also incredibly satisfying. Gone are the days of flicking the mains lead around and ensuring you aren't running it over.
The cordless mower is also a lot quieter than our mains mower.
Even the missus had a go, bearing in mind she isn't as geeky as me, to quote "this is good, it feels a lot lighter and quieter than our old one, I quite like it".

Costs -
Mower Deck - £10 from Ebay.
Battery and battery clamps - £38 from local motor factors.
Motor - £31 from GiantShark
ESC & Programming Card - I already had these, but they are approx £25.
Servo Tester - £1 from Ebay.
12v/100a Volt/Amp meter - £6 from Ebay.
Battery Strap - £2 from Ebay.
16" Mower Blade - £11 from Ebay.
DC Circuit Breaker - £6 from Ebay.
Sundries - £10
So a total of around £140
To buy a decent cordless mower, you'd have to spend double this. The other drawback to purchasing a cordless mower is the battery - the batteries don't last forever and replacing a dead battery, bearing in mind you're now tied in to using a battery supplied by the manufacturer, could cost you in the region of £100, every few years.
By building your own mower and using a cheapo car battery, you have greatly reduced your future expenditure.

Conclusions -
This has been a very enjoyable build. If you have a reasonable DIY skill set, you could easily build yourself a cordless mower.
The mower itself is performing brilliantly.
Things may change when I have to cut wet grass - but if I struggle, I'll just upgrade the motor. I'll report back if this ever happens.

The fact I've combined the mower with my Solar System, puts an even bigger smile on my face. I worked out that it costs me around 10 pence to mow my lawn with my old mains mower. It doesn't sound a lot, but over a year, that is around £3.
The other aspect to this is that the battery on the mower now also contributes to my solar shed battery bank, giving it more amp-hour capacity, which takes the strain from the pre-existing battery bank. So even when the mower is idle in the shed, it is doing good!

DIY Cordless Mower Blade
My 16" blade has arrived.
Nothing special to show you here, other than gap between the blade tip and mower deck. You'll notice the gap is very tight, less than 10mm. This is good because the grass, when cut, gets whizzed around in the chassis, and the only way for it to escape is by going through the blade and getting chopped into even smaller pieces...all part of the mulching effect.
The blade feels nice and solid and I only paid £11 inc P&P.

DIY Cordless Mower Brushless Programming Card
Now that I've got my new blade fitted, I've encountered a problem with the motor.
On start-up, it is "cogging". Cogging is where a brushless motor jitters and splutters and doesn't easily get up to speed. The route cause is because the ESC (electronic speed controller) is struggling with getting pole position from the motor. There are many reasons why this could happen and going into the detail of this is beyond the scope of this website.

Instead - I'm just gonna focus on fixing it.
When you buy an ESC, you also want to ensure you can buy a programming card for it, and buy both at the same time.
A programming card will allow you to tweak many aspects of your brushless motor setup.
If needs be, you can program your ESC without a programming card, but believe me, it's a massive ball ache without.

The above pic shows my programming card and the configuration I am using for my ESC/Motor setup.
I'll go through each section -

I bet you never thought you could have ever have a cordless lawn mower that was so programmable?!

Next - Solar Lawn Mower Motor


Jeroen - 18/07/2014 21:34:18
good day, the site looks good. I look with admiration all that you make. Would you like to mail? Schema of the solar tracker the schedule of the relay and possibly the Arduino code? Thanks in advance.


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