First off let’s define a few things:
- An ampere hour (Ah) is the amount of charge in a battery that will allow for one ampere of current to flow continuously for a full hour. Source
- A 2 Ah battery will allow for two amperes of current to flow continuously for one hour OR for one ampere of current to flow continuously over two hours time.
- An ampere is a measurement of the flow of a current.
- If a 2 Ah battery is discharged in 30 minutes then that means that the current was 4 amperes.
- Watt-hours is a measurement of how much energy is used or available for use over time. It’s like how big your tank of gas is in the car.
- Watts is a measurement of how much power is being used at a single moment in time.
A 100 Wh battery can put out 100 watts every hour, or 50 watts per hour for two hours, or 200 watts per hour for only 30 min, etc.
Batteries however have a maximum watt output, like a speed limit.
Also good to keep Ohm’s Law in mind:
Current (Amps) x Resistance = Voltage
or
Resistance = Voltage / Current (Amps)
Now let’s get to the topic for the day; battery mower strength and how misleading or “not particularly helpful” battery mower specs can be.
How can we tell which battery mowers are actually strong?
It’s hard to tell without using them in the field. Most battery mowers only blatantly advertise Max Voltage and Ah which aren’t that useful without knowing a few other inputs.
It would to be beneficial to know the current draw in Amps from the battery to operate, how much internal resistance there is to overcome and the maximum power draw it can handle under heavy load for both sustained periods of time and peak moments.
All of these are basically impossible to know without using the mower and recording data.
In 2020 I produced a large series of videos reviewing and comparing some of the highest end battery powered rotary mowers on the market. I included brands from Toro, Snapper, Yard Force, Greenworks Pro, Sun Joe, and EGO. All of these mowers are high voltage units marketed at 56-volts and higher.
In those videos I used all the mowers in my lawn through the Spring and summer and reported what I found without going into the electrical math.
Today I want to touch on the electrical math without going too far down the rabbit hole because battery electronics and electricity are not my strong suit.
What I want you to realize is that the numbers you see on the packaging can sometimes be misleading and sometimes point you in the right direction… unfortunately this doesn’t help and makes them all a little suspect.
One of the most important numbers you should note with a battery mower is actually the Wh (Watt-hours) of the battery you choose or get with the mower. This is like the amount of gas in the tank – it’s how much work the mower can put out over the course of an hour.
A 288 Wh mower like the 4ah Greenworks Pro model can idle for 1.86 hours using only 154 watts leaving roughly 50 more minuets in the tank to keep idling.
In layman’s terms centered around lawn mowers, the formula is Wh / Watts Required to Turn The mower blade under no load = Hours run.
Watts has to be stable though like in an idling state, like a toaster, you turn it on and run it and then turn it off. Everything has to be constant.
If we were to boost the resistance by cutting very uniform grass to the point where we were able to discharge a full battery in 30 minutes then our watt number jumps to 576. Using the same formula.
With mowers the internal components and the blade spinning at full RPM under zero load all contribute to form resistance… but cutting the grass will make it harder for the mower to turn the blade at the correct speed so the resistance is higher when under use or load. To maintain blade speed the mower has to overcome more resistance and thus draws more current.
We can calculate the current but only by using the mower. Here’s the basic formula:
Volts x Watts = Amps
For that same Greenworks mower we take the battery’s Ah and divide it by the run time to expel the battery 1.86 hours to get the current flow or amps. This would be a 2.15 amp current.
If we take the nominal voltage of the mower (72 not 80) and multiply it by the 2.15 amp current we get 154 watts, just like we got by starting with Wh. The numbers check out.
The point here is we can’t figure out how much energy the mower needs to operate under no load without figuring out how much time it takes to dispel the battery. This needs to be field tested because the numbers are never provided.
Under the heavy load hypothetical scenario of a 30 min stable discharge (which never exists in a lawn anyway) the amp or current flow jumps from 2.15 up to 8.
Now, for a almost apples-to-apples comparison lets consider the 7.5 ah EGO select cut mower. One of the most popular high-end battery mowers on the market these days.
I can idle that mower under zero load for 1.9 hours. The battery is advertised as a 7.5 ah battery and a 420 Wh battery.
Doing the same calculations the EGO mower uses 220 watts to idle over an hour and if we were able to discharge the battery under a stable heavy load scenario in only 30 min then it would use 840 watts.
Remember the GW Pro model was more efficient: 154 & 576
154 to 220 & 576 to 840.
In terms of amps or current the Greenworks mower requires a 2.15 amp current to idle for 1.86 hours while the EGO model requires 3.95 amp current to idle for 1.9 hours – a difference of about 2.5 minutes of idle time.
For a hypothetical 30 min heavy load complete battery discharge the GW mower requires a steady 8 amp current while the EGO mower would require a steady 15 amp current. Pretty big difference I think.
I’ve said consistently on this channel that I believe the GW Pro 80V mower is the strongest in terms of cutting the grass of all the options on the market – at least those that I’ve tried. And I think these numbers help prove it. I have never stalled the GW mower in the toughest of my mowing conditions but I can usually bog down or stall the EGO mower in the same conditions if I try.
At some point the batteries on these mowers can’t keep increasing their Amps to deal with heavy load conditions – it’s just not possible. None of the mower companies that I know of state what the maximum amperage their batters can sustain nor do they state what they can hit under short bursts of heavy load.
Through use however I know that if I want to bring the height of my grass lower or need to mow wet grass or need to tackle a thicket of overgrown weeds and grass the GW mower is capable of getting it done better than the EGO Mower.
It’s not going to be as good as EGO in other areas but for cutting strength on the grass itself it is stronger – probably because it works more efficiently.
Unfortunately you can’t calculate many of these things without actually using the mower and even still I have no idea how much sustained load or current flow these mowers can withstand; it’s not advertised anywhere that I know of but I do know that each battery can only handle so much load before power is cut off or inadequate to get the job done.
Anyway, for anyone out there that want to add to this discussion please put it all down in the comments below. I’m not an expert in this so it’s possible that I may have made some errors if so please discuss them civilly in the comments below.
I’ll also add a link to the text version of this video over on my website. Please feel free to check it out if you like. In the future I’ll edit the text version if needed to keep it accurate – hopefully those edits won’t be necessary.
Thanks a lot and I hope some of you found this video to be a valuable addition to the battery mower topic here on the channel.
Now lets look at some of the data I recorded at my home on a handfull of my own mowers. Some of these numbers I had to record myself such as hours run and others I had to calculate.
Greenworks Pro 80V Mower Using the 80V Max 288 Wh Battery
Idling
Battery Wh / X Watts of Average Power Flow = Hours Run
288 / X = 1.86
x = 154
Mower Uses 154 Watts of Average Power To Idle @ 2800 Blade RPM Speed
4aH battery discharged in 1.86 hours
4 / 1.86 = 2.15 amp current
Volts x Amps = Watts Consumed
72 x 2.15 = 154.8 Watts Consumed
Heavy Load
Battery Wh / X Watts of Average Power Flow = Hours Run
288 / X = 0.5
x = 576
Mower Uses 576 Watts of Average Power To Mow at Heavy Load @ 2800 Blade RPM Speed
4aH battery discharged in 0.5 hours
4 / .5 = 8 amp current
Volts x Amps = Watts Consumed
72 x 8 = 576 Watts Consumed
Toro 60V Mower Using the 60V Max 324 Wh Battery
Idling
Battery Wh / X Watts of Average Power Flow = Hours Run
324 / X = 1.27
x = 255.1
Mower Uses 255 Watts of Average Power To Idle @ 2700-3000 Blade RPM Speed
6aH battery discharged in 1.27 hours
6 / 1.27 = 4.72 amp current
Volts x Amps = Watts Consumed
54 x 4.72 = 254.9 Watts Consumed
Heavy Load
Battery Wh / X Watts of Average Power Flow = Hours Run
324 / X = 0.5
x = 648
Mower Uses 648 Watts of Average Power To Mow Heavy Load @ 2700-3000 Blade RPM Speed
6aH battery discharged in 0.5 hours
6 / 0.5 = 12 amp current
Volts x Amps = Watts Consumed
54 x 12 = 648 Watts Consumed
Snapper 82V Mower Using the 82V Max 144 Wh Battery
Idling
Battery Wh / X Watts of Average Power Flow = Hours Run
144 / X = .75
x = 192.4
Mower Uses 192 Watts of Average Power To Idle @ 2800 Blade RPM Speed
2aH battery discharged in 0.75 hours
2 / 0.75 = 2.67 amp current
Volts x Amps = Watts Consumed
72 x 2.67 = 192.24 Watts Consumed
Heavy Load
Battery Wh / X Watts of Average Power Flow = Hours Run
144 / X = 0.35
x = 411.4
Mower Uses 411.4 Watts of Average Power To Mow Heavy Load @ 2800 Blade RPM Speed
2aH battery discharged in 0.35 hours
2 / 0.35 = 5.71428 amp current
Volts x Amps = Watts Consumed
72 x 5.71428 = 411.4 Watts Consumed
EGO 56V Mower Using the 56V Max 420 Wh Battery
Idling
Battery Wh / X Watts of Average Power Flow = Hours Run
420 / X = 1.9
x = 221.05
Mower Uses 221.05 Watts of Average Power To Idle @ 2700 Blade RPM Speed
7.5aH battery discharged in 1.9 hours
7.5 / 1.9 = 3.947 amp current
Volts x Amps = Watts Consumed
56 x 3.947 = 221 Watts Consumed
Heavy Load
Battery Wh / X Watts of Average Power Flow = Hours Run
420 / X = 0.5
x = 840
Mower Uses 840 Watts of Average Power To Idle @ 2700 Blade RPM Speed
7.5aH battery discharged in 0.5 hours
7.5 / 0.5 = 15 amp current
Volts x Amps = Watts Consumed
56 x 15 = 840 Watts Consumed