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Can someone please explain what I am losing/gaining in the trade-off between Volts and Amp-Hours when buying a piece of equipment such as a chainsaw.

I'm currently comparing two items. I'm making this concrete with actual products because I think this will draw attention to what we (the layman) are struggling with. These are both Battery cordless chainsaws

  • DeWalt 20V Lithium-Ion with 5.0 Ah Battery
  • Ryobi 40V with 4.0 Ah Battery

As pointed out in the comments those are Amp Hours and not Amps. So let's pivot this question: How does someone with almost zero knowledge of battery chainsaws work out what Voltage is appropriate for their usage?

Or maybe a better question is: What do I need to understand about Volts in this context to make a better decision on a purchase like this based on just that information? i.e. if everything else was equal, how do Volts enter into the equation based on the application?

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    $\begingroup$ Which is the higher quality product? $\endgroup$
    – Solar Mike
    Sep 21 at 19:51
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    $\begingroup$ hat is not amps. That is amp-hours. Your question is like asking what performs better? A gas chainsaw with a smaller fuel tank or chainsaw with a larger fuel tank? You cannot discern performance in that case because you have no useful information about the engine. Similarly, all you know is that the Dewalt battery stores 100 Watt hours and the Ryobi stores 160 Watt-hours of energy. Whether one runs longer is or is more powerful cannot be discerned because you have no info about the motor running off the battery. $\endgroup$
    – DKNguyen
    Sep 21 at 21:32
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    $\begingroup$ @DKNguyen - thanks for pointing out my mistake. I've changed the question based on that. $\endgroup$
    – Guy
    Sep 21 at 21:52
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    $\begingroup$ The edits don't change anything because, for the same power, you can design a motor to perform the same for 40V or 20V. $\endgroup$
    – DKNguyen
    Sep 21 at 22:23
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    $\begingroup$ Side note: if the Ryobi is not a Li-ion battery, that's a drawback. Li batteries hold their charge pretty much forever, so you never have to worry about using a tool that's been sitting on a rack for 6 months. $\endgroup$ Sep 22 at 13:09
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It is hard to be certain, but higher voltage tools generally perform better.

What matters in most power tools is power. Power = volts times amps. We can get power using lower voltage, but the higher amps that would be required is problematic. The motor would need to be physically larger because the current draw would be more severe. As a result, manufacturers tend to instead create higher-voltage systems for higher power tools. These tools can increase power without the motor size problems of a lower voltage system. We can see that manufacturers' top product lines are at higher voltages.

The other difference is the battery capacity. Between your two examples, the 40V 4 a-H battery holds significantly more power than the 20V 5 a-H one. Again, this is because power equals volts times amps. This means that the 40V tool will likely be more powerful, and may also last longer in use.

What you give up in the higher voltage tool is generally money. This isn't certain because build quality and longevity affect the tool's cost. One tool may be built for a contractor and the other for a homeowner, so it's hard to say for sure. What you give up in the lower voltage tool is generally performance - both power and battery life.

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    $\begingroup$ Thank you for taking the time to explain that! $\endgroup$
    – Guy
    Sep 23 at 1:54
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I will try to add something Tiger Guy's answer, which I think is spot on.

Ah and V

For the DeWalt 20V Lithium-Ion with 5.0 Ah Battery, the 5Ah Battery means that is can provide energy equal to 1A for 5h (or 5A for 1h). However you don't know the actual discharge rate. So, you only know the capacity of the battery (same as knowing the size of the fuel tank of a vehicle).

If you multiply Volts with Amps you get Wh. So

  • DeWalt : 100 Wh
  • Ryobi : 160 Wh

From the comparison the Ryobi has a larger capacity (so I would expect the batteries to be bulkier).

Voltage and motors.

This is the tricky part. Regarding what people in general refer to as a powerful motor/hand tool is (IMHO) the torque that it produces. I.e. if a tool produces more torque then it is more "powerful".

The torque of a motor is strongly related to the motor (and more specifically its characteristics) and the current flowing through it. Typically nowadays (I hope I am not that outdated), power tools use brushless DC Motors.

enter image description here

Figure: Brushless DC Motor (Source: youtube video)

However, (in most DC motors) - for a given configuration (e.g. number of windings, thickness and resistance of wire) if you increase the voltage, the current also increases and therefore you increase the torque. Brushless motors in particular have a KV rating which shows the rpm per voltage applied.

However, the configuration of the motor is not fixed, so you can have different motors at each product, with different power requirements. Unless you actually go into the datasheet of the power tool and maybe even the motor, its not easy to understand which is more powerful.

Final thoughts

(I don't expect this will help you directly with your decision which chainsaw to buy, but) keep in mind that numbers next to technical quantities are often used (almost) out of context for marketing purposes from companies. So, (usually) you get what you pay for.

Also, I'll close with continue the analogy from vehicle: The Battery is the fuel tank and the motor is ... the vehicle engine (and the drivetrain to some extent). You can have a more powerful engine which means that you can go faster and accelerate more (but the fuel consumption is greater so your run out of fuel faster. Or with a smaller engine, the acceleration and the speed is less, but the fuel tank lasts more hours. However, unless you pop the hood, or start up the vehicle, you don't really know what engine or the drivetrain is underneath.

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    $\begingroup$ Thanks jonathan $\endgroup$
    – NMech
    Sep 22 at 13:20
  • $\begingroup$ Awesome answer and info @NMech - I wish I could mark both yours and Tiger Guys answers as correct. Thank you! $\endgroup$
    – Guy
    Sep 23 at 1:54
  • $\begingroup$ I am glad it was helpful. Tiger guy nailed it, I was providing additional info. $\endgroup$
    – NMech
    Sep 23 at 2:33

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