I’ve owned cordless tools of various kinds since I was a teenager and they were always marketed by voltage. In the nineties, 9.6 volt was the big thing. Then in the 00s it was twelve volt, and eventually 24. Now it seems like the larger tools are all 40 volt and above, but not all; my chainsaw is 24 volt and cuts down small trees just fine, and my string trimmer is 40 volt and doesn’t seem to be any more powerful than the chainsaw but the battery lasts longer.
What I don’t understand is why these things seem to be growing in voltage rather than amperage, and in marketing it seems like the voltage is supposed to be a proxy for how powerful it is. I understand the larger tool needs more power but why are the cells wired in serial rather than parallel?
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The higher of voltage you run, the smaller of conductors your need to pass the same amount of power. So 100W of power at 12V is 8.4 amps. But if you jumped it to 24V, it would only be 4.2 amps. And 48V would be 2.1amps. So raising the voltage will let you get the same amount of power out of a tool but not need big copper wiring on the inside.
Power (Watts) is Current (Amperes) times Voltage (V).
So yes, you could get the same power raising the current instead of voltage.
But it’s just easier to raise the voltage. With higher current you need thicker wires and larger contact surfaces everywhere – including in the motor windings and charging equipment.
Power is usually controlled with PWM (pulse width modulation), and electronic components for higher currents are larger and more expensive than components for higher voltage (for the same output power).
TLDR: getting more power with higher voltage is cheaper and smaller than getting the same power with higher current.
One advantage of higher voltage is that conductors for the same amount of power can be smaller and kept cooler. This is one of the reasons we use super high voltages in big transmission lines. In order to get the same amount of power transmitted at a lower voltage requires that the amperage of your power line be higher. Increasing the amps would mean that power lines would have to be far thicker and would be more prone to overheating. Now this may not actually translate to any tangible benefit in something like a power tool. Using lighter weight wiring can save a bit of material cost, but the higher voltage tool is almost certainly running at a lower amperage. So the power it outputs, and therefore it’s ability to drive a screw, might not be different.
The power of an electric motor is voltage * power. So you can either increase voltage or current to have a more powerful motor.
Let’s say we like to increase the power by just changing the voltage or current and keep the other constant.
The current tough a the motor is current = voltage /resistance. I skip the Counter-electromotive force that results in a low effective voltage over the engine. The current is not limited by the batteries or at least that is not the case with the Li-ion cells we used today, it is the motor resistance that limits it.
To increase voltage you just need to have an external voltage source with a higher voltage. The current will also increase because it depends on the voltage and the motor’s resistance. To keep the current the same we can increase the resistance, this can mean thinner wires in the motor so the motor size can shrink. So if we increase voltage we can higher power even if the motor strings in the sizer.
If we on the other hand like to keep the voltage constant and increase the current we need to decrease the resistance. The mean thicker wire so a large had heavier motor.
So for an electric motor of the same size, you can get out more power if the voltage is higher. In hand tools, large motors are quite impractical so the best option to get out more power is to increase the voltage
The main reason: Because the litium-ion battery technology allows us to have compact, yet powerful, batteries with higher voltages.
Back in the days, the 7.2 and 9.6 volt batteries were with 6 and 8 cells, respectively.
Now, we have 24 volts with 6 litium-ion cells. This higer voltage allows much higher power at the same current (and the current is limited by wire cross-section, mostly in the motor).
In addition to higher voltages, modern batteries also store much more energy (for better runtimes). They also do not suffer (as much) from cold than some older batteries (which could only deliver full power when they were warm directly from the charging).
The bigger voltage does not guarantee higher power. The manufacturer can of course just reduce the amount of copper (read: put a much smaller motor in). But, especially the good modern tools with brushless motors (another new thing from this millenia) are way more powerful than their 90s counterparts (and run for far longer).
The 40 volts will likely remain the new maximum for a long while. Now, the limit is about safety (to keep the tools “low-voltage” battery operated, as opposed to corded tools, so, below about 50 volts depending on local regulations).
But, a good 9.6 V, not to mention ~20 V, tool, will likely outperform a cheap 40 V tool. Higher voltage is not magic that guarantees performance.
cordless hand tools seem to be stuck in the 12-14 volt range for smaller things, and 18-20 volt for bigger stuff. but cordless tools for like lawn and garden usage will use 2 or 3 of the big batteries. or whatever the dewilt thing is where a battery is 60 volts and x amphours on a lawmower or weed eater, but 20 volts and y amphours on like a impact wrench. i dunno if we will get a better battery technology than the current li ion 18650s that are series-ed and parallel-ed together in various ways to get the voltages we get now.
battery technology as improved to make higher power in smaller packages. look at how cellphones went from a brick to a thin piece of glass yet it lasts all day.
Not touched on yet is electric motor top speeds as they relate to back EMF and supply voltage. Higher volt motors can spin faster.
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