As lawn care and OPE (outdoor power equipment) continue their shift into more cordless products, the landscape isn’t easy to navigate. Bigger gas engines mean more power. Higher battery voltages seem to be what Pros and consumers look at as an equivalent measurement. The problem isn’t quite that simple, though. To help settle the argument, let’s take a closer look at volts, amps, and resistance.
Volts and Amps
Neither volts nor amps describe power by itself. That title goes to watts, and the calculation is pretty simple:
Volts (V) x Amps (A) = Watts (W)
Let’s say we need 2,200 watts to run a lawnmower. There are multiple ways to get there.
- 36V x 61.1A = 2,200W
- 56V x 39.3A = 2,200W
- 108V x 20.4A = 2,200W
As long as you can draw enough current (amps) from the battery, you can get the same amount of power out of many voltages.
So theoretically, the higher voltage doesn’t mean more power in and of itself.
A Quick Note About Volts
Voltage numbers like 40V, 80V, and 120V often represent peak (max) volts. This is the voltage you may measure right off the charger. As soon as you start using them, they settle into their nominal voltages: 36V, 72V, and 108V. Once you understand this, you can see that 18V = 20V Max, 36V = 40V Max, and so on. Pro Tool Reviews has a more detailed article titled 20V Max Vs 18V: Setting the Record Straight .
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Join the Resistance
As the energy reaches the motor, our V x A = W equation describes how much power it gets. Resistance gums up the equation, however, and keeps it from being a simple matter.
Let’s take a fuel line as an example. A thinner tube restricts how easily fuel gets to the engine. A similar issue exists for electrical energy.
Thinner gauge wire and lower quality materials restrict electron flow. Thick wire and higher quality materials allow electrons to flow smoothly. You may have experienced a voltage drop when using too thin an extension cord on 15-amp power tools. Or, you may have noticed your lights flicker when the AC in your home turned on.
It’s here that a guy named Ohm comes into play. He’s responsible for the resistance equation and a unit called “Ohms.”
Ohm discovered that resistance had a greater effect on current (amperes or amps) than voltage. If you try to send the same amount of energy through two different voltages, the higher voltage deals with less resistance.
Quick Case Study
Volts2 / Resistance = Watts
(V2/ R = W)
Current2 x Resistance = Watts
(I2 x R = W)
Note: Ohm’s Law uses “I” for current instead of “A”.
Let’s go back to our 56V example and see how things change when we apply Ohm’s Law.
56V x 39.3A = 2200W
In this example, the resistance equals 1.42 Ohms. (56V/39.3A = 1.42 Ohms)
Increasing the Voltage
Here’s what happens when we increase the voltage by 20% (67.2V):
(67.2V x 67.2V) / 1.42 Ohms = 3180.2 W
A 20% increase in voltage yields a 44.6% increase in power with the same resistance.
Increasing the Current
Now we’ll go back and increase the current by 20% instead.
47.2 amps x 47.2 amps x 1.42 Ohms = 3163.5 W
That 20% increase in current yielded a 43.8% increase in power. While similar in this example, it takes a bigger increase in current to get a higher power output than increasing the voltage. That doesn’t tell the whole story, however.
Back to the Discussion
One of the consequences of resistance is that it has an energy penalty. A higher voltage system is more efficient than a lower voltage since it experiences less energy loss from resistance given the same amount of power draw.
That’s all well and good, but what if you can lower resistance to allow more efficient energy transfer in lower voltage systems?
The 18V battery is a great example. Using 18650 lithium-ion cells and standard pack technology, those packs deliver 800 watts of power. That means manufacturers have the confidence to send up to 44.4 amps of current through it and expect it to last 3 years or more.
When they upgrade the packs to use 21700 li-ion cells, more copper components and thicker wires offer less resistance. Now, these packs achieve up to 1,440 watts of power. You get the same exact voltage—but with 80 amps of current. That’s 80% more energy!
Now, double that to cover a 36V system and there are 2,880 watts available in that pack build—more than enough for our 2,200W mower.
Author’s Note: The typical home electrical outlet operates at 120 volts and 15-amps. Do that math, and you can only get 1,800 Watts out of your house outlet. Batteries can now get you further! Run-time remains the only hurdle.
Don’t Forget Battery Capacity
The same equation we use for power works for potential power as well. Just take the nominal battery voltage and multiply by the total amp hours of the pack to get watt-hours. That’s the total amount of energy in the battery.
- 18V x 9Ah = 162Wh
- 36V (40V Max) x 5Ah = 180Wh
- 56V x 2.5Ah = 140Wh
- 72V (80V max) x 2.0 Ah = 144 Wh
It’s actually possible to have more electrical fuel available in a lower voltage system. That’s not always the case, of course, but now you have the key to unlock the real capacity for yourself!
In general, the lower resistance of higher voltage systems makes them more electrically efficient and easier to build. OPE systems that genuinely compete in performance at lower voltages have to lower their resistance through better battery pack design and/or use upgraded lithium-ion cells to do it.
Here’s the thing—on some levels, going with a higher voltage is easier. It stands to reason that you might even get away with lower quality components by going that route. To have enough power to compete with higher voltage systems, lower voltage systems have to build a better battery pack. In all likelihood, they also have to create a higher quality system as a result.
As you look at the available options, keep in mind that voltage isn’t everything. In our experience, cordless OPE products ranging from 36V–54V (40V max–60V max) have plenty of power to get the job done. Higher voltages do well, too.
Just don’t chase the highest number possible.