Recently was gifted a Ryobi 40V 20” brush-less self propelled mower and I just don’t get it. What am I missing? Have been using various push mowers for many years starting with a basic 3HP gas model to my most recent 12 Amp 120V (1.9HP) corded mower that were not self propelled. The self propelled models that I have had started at 4.5 HP and I think one was 6HP.
How does this battery powered mower do it? Is my math wrong? The blade seems pretty similar to any other blade – a bit lighter than the gas ones. The deck is pretty similar. Width is the same as corded and only an inch or two smaller than the gas. It performs on par with any other mower that I have used and that just does not make any sense to me.
Can’t find any specs on the motor power. However the battery is claimed to be 6Ah at 40V. I measured 41.7V on a fully charged battery. It is made up of 20 18650s. This would be 240 Wh of energy or about 12 Wh per 18650. Entirely reasonable specs and they match my experience using it. Claimed run time of up to 42 minutes. Just to make the math easier lets take that down to 30 min. That would mean 12 A of current and about 480 Watts. Roughly 0.64 HP and we know that it doesn’t actually use that much when doing the job of mowing and moving itself. How is it possible that this machine can do the same job with so much less power? I know that electric is more efficient than gas but we are not talking about how much gas it takes to vs how much electricity. Power is power isn’t it? Somehow the same job gets done with dramatically less power available. What am I missing?
Heat. A lot of waste heat. If you could have harnessed that heat in 30 minutes of mowing you could probably heat 100 gallons of water.WAG. Just because you had 4.5 and 6 hp motors did not mean they needed to be that big. One to two inch less cut area is a lot. I’m guessing the ryobi is lighter. Edit. Plus it’d be more like 40 flashlights running on 20 batteries for 30 minutes to drain them completely.
A gallon of gasoline contains a huge and incredible amount of energy, but the machines that convert that chemical energy to mechanical rotary motion available for useful work are not efficient.
The piston travels up and down while stopping at each end of the stroke, so it has to be rapidly accelerated back and forth from the resting positions. Then this linear motion of the piston must be converted into rotational motion of the crankshaft. Burn a bit of fuel in an explosion and use the pressure to push the piston, makes a lot of heat and has a low conversion efficiency (maybe 20% ?).
Lithium batteries contain quite a bit of energy considering the package size, and the chemical energy reaction of a cell directly produces lots of electrons—the source of the electrical current used in electromagnetic machines which convert this energy directly to useful rotational motion with an efficiency of 80-90%. Continuous rotation without the linear start/stop motion of a piston.
The interaction of magnets and magnetic fields with electrical currents to produce torque is quite amazing. For example the traction machines of large heavy trains; diesel engines are used to run generators, but it is electric motors which drive the wheels.
i’ve been driving a Mitsubishi EV for 7 years and the amount of energy in a full battery pack is equivalent to about 1/2 gallon of gasoline, but i can easily drive 60 miles on that charge (my daily commute is 40).
Ryobi says you can get 42 minutes (0.7 hrs) of mowing from the 6Ah battery, so that’s about 9 Amps draw. At 40V that is 360W input power or about 1/2 Hp. i don’t know what speed they turn in the mower, but electric motors make their full torque right from the start up to the RPM of peak power.
s. Several years ago i was asked to evaluate a device where a fellow from Nevada got a Patent for a new and novel type of motor. In his invention he mounted the linear-action plunger of a solenoid valve to the crankshaft of a weed-eater to produce a rotating shaft! i saw of video of it in operation and dubbed it the “clacker motor” based upon all the noise and racket that it make while it vibrated itself on the bench top. And he paid over $15k in legal fees to get his patent.
Seems about right. I just got 38” Ryobi ride on mower and it is so much better than petrol. Drive motor is 1.2kw, mowing deck is 1.5kw, so roughly 20% of the equivalent internal combustion ride on.
If I put a 25kw electric motor in a go-kart that would be the like putting 125kw petrol motor in it.
