How Much Power Do I Need?

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fechter
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How Much Power Do I Need?

Post by fechter » Jul 12, 2007 3:24 pm

When considering any kind of EV, one of the first steps is to determine how much power your vehicle will need to attain the desired speed.

The power requirement is mainly a function of speed, wind resistance and hill climbing power. Keep in mind that wind resistance is both headwind and speed added.

As speed increases, the wind resistance increases exponentially. Below 10mph, wind resistance can be largely ignored.

Hill climbing power requirements are a function of total vehicle weight and grade (steepness). These relationships are linear.

Below is a chart based on "typical" values. If your weight is substantially more, the power requirements will be more.

You can use one of the many EV calculators to punch in your specific numbers. Here are a few:

http://www.kreuzotter.de/english/espeed.htm

http://www.ebikes.ca/simulator/

http://www.geocities.com/CapeCanaveral/ ... vcalc.html

https://endless-sphere.com/forums/viewtopic.php?t=538

The power requirements are "at the wheel" with most of these calculators. This is ususally how most electric motors are rated.

To find the power requirements of the batteries, we must factor in efficiency. The batteries must supply more power than what you get at the wheel due to losses. In some cases the losses can be huge. At best, with typical bike motors, you'll be in the 80-85% range.

Climbing hills where the motor bogs down, or running too high of a gearing can cause the efficiency to drop below 50%. This is bad. Not only are we wasting half the power under these conditions, the half we're wasting goes into heating up the motor, which can overheat or destroy it.
"One test is worth a thousand opinions"

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TylerDurden
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What is POWER?

Post by TylerDurden » Aug 19, 2007 10:38 pm

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What the heck is POWER anyway?
Generally speaking, power is the force that makes your electric vehicle go. The force at the wheel is transferred from the force from the motor , which is supplied by batteries through a controller module.


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Power is also the electric force that makes the electric motor turn. It is the product of the volts and the amps delivered by the batteries and controller. Gasoline motors usually are described or "rated" in horsepower, but small EV motors are usually "rated" in watts: 746 watts = 1 horsepower.


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When a motor is described as being "250W motor", it is rated to be able to deliver about 1/3hp continuously without stalling or overheating. Its rating will specify what voltage and amperage it needs to deliver the power at a specific rotational speed, usually described as revolutions per minute ( RPM). Of course, you cannot expect that 250W motor to continuously handle a task or "load" that requires more than 250W of continuous power; any more than you would expect a single-engine to power a multi-engine airplane for very long, if at all.

So, when you consider power for an EV, you will need to consider how much power at the wheel will be required to go the speed you want: on flat roads, up hills and with how much cargo. It all takes power. Faster, higher and heavier all take more power (see chart in post below). When you determine your requirements for power at the wheel, you can consider the power a system can deliver.

There is no 100% efficient system: all systems have components with inherent losses that diminish the output of power. Add them all up, and you may find total system efficiency and total power output is much lower than you expected. If you need 500W at the wheel, the losses from wind resistance, tire resistance, motor heat, controller losses , total weight and battery voltage-sag may require you to use a 750W motor, bigger controller and more batteries.

There are trade-offs all along the line: you may want higher voltage, but lower amps... or the opposite. You may want a heavier bike, but lighter batteries. Each component you consider will have advantages AND limitations... The good news is: there is plenty of power available for just about any setup you might want. What you choose for your setup depends on your goals, your efforts and your finances.
Last edited by TylerDurden on Sep 12, 2009 9:10 am, edited 1 time in total.
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TD

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TylerDurden
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Re: How Much Power Do I Need?

Post by TylerDurden » Sep 03, 2009 7:50 pm

This simple chart (mentioned above) illustrates some typical power requirements on an e-bike. The watts calculated are "at the wheel"; meaning you would need to calculate the efficiency of your system and de-rate to achieve the final output power.
ebikeWatts.gif
ebikeWatts.gif (27.5 KiB) Viewed 8368 times
(Estimate accordingly for your rider & bike profile: riding position, tires, weight, etc.)

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