Tuesday, June 14, 2016

A new meter for my solar charged electric lawnmower conversion

Okay, so I am a geek!  11 years ago, I converted an old gasoline lawnmower to electric by replacing the gas engine with an electric motor and a deep cycle 12 V battery that I charge from solar panels.  I wrote a detailed blog about the construction of this mower and many other people have built their own.  While commercial cordless lawnmowers exist, they are relatively expensive, plus it is much more fun to build your own.

The original design included a voltmeter, amp meter, charging jack and circuit breaker power switch mounted near the handles.  The meters are standard automobile components, but they were unreliable and the Amp meter would get stuck often and I never really bothered to look at it very much.

An electronics parts supplier I use a lot (MPJA) has been getting in a line of digital meters that can display multiple properties and I decided to replace the old meters with a meter that can display Volts, Watts,  Amps and Watt hours.  At a cost of around $18, I consider this sexy meter a real bargain.  Click here to get a copy of the manual. 

 The image above shows the meter installed with the motor running before I took the mower for its first major test drive.  So with the blade spinning, it is drawing about 140 W and as I mowed into the taller grass, that peaked at around 250 W.

It took an hour or so to reconfigure the mower wiring to make it compatible with the meter and its current sensing shunt. 

After mowing a section of lawn for 10 minutes or so, the screen above shows that I used 34 Watt hours and the battery voltage slumped below 12 V on my brand-new deep cycle AGM battery.  This is the first time I have been able to quantify how much energy the mower actually uses!  I was watching the Amp numbers ranging from 12.5 to 20 while mowing (or about 150 to 250 Watts).

When I plugged it into the 40 W solar panel mounted to the roof of my garden shed, the voltage came back up to 12.56.  The battery is usually topped off in five or six hours of good sunlight.  Note that the display has a cool blue "techno glow" back light - not something that is very useful for device used mostly outdoors in bright light, but it sure is pretty.  I installed a power switch to the left of the meter to prevent it from draining the battery between uses.  The meter only uses about 2.8mA though, so I could just leave it on.  

6 week later
I recorded the Watt Hours (Wh) for each time I used the mower and found that I am ranging from 28 to 53Wh each time depending on how much area and how tall the grass is.  This is not a lot of power - and of course it's all free from the sun.  I also burn a lot of calories pushing this Beast around!