Understanding voltage and watt hours

[TexasElectric]

Hi,

I see a lot of YouTube videos and manufactures talk about the "watt hours" of their electric scooters/eBikes batteries.

If I have a 48v electric motor and I use 200 watts in one hour. Am I correct in saying that I would need a 4.17 amp hour battery? (4.17 amps * 48 volts = 200 watts?)

So here's my thought... I am an avid cyclist. I own, what's called a power meter. Power meters for bicycles are strain gauges that calculate the amount of torque the rider is applying based on cadence (rpm) of pedaling and the "strain" or flex on the power meter. These power meters are generally expensive ($xxx to $x,xxx) and very accurate.

With that being said... When I am riding down a flat road I can average 175-225 watts and maintain 18mph.

Can I assume (in a perfect world not taking into account mechanical and electrical efficiency losses) that if I want an electric motor to power me along at 18mph, using 200 watts of power; a 48v motor receiving 4.17 amps for 1 hour should do the trick?


Thank you, and I appreciate and feed back from you guys.

 

[wesnewell]

Yes, that's correct, but in real life it will take more than 200wh.

 

[LockH]

Hi Texas.

Folks tend to tap in electrical/energy terms in "minimum"... and "continuous"... and "average"... and "peak" where the goal is always (?) to minimize energy consumption? BUT, for one example, a "regular" route might be to daily workplace that's 200 feet higher than home, such that commutes TO work is "up" hill/against gravity, then return is "down" hill/with gravity... Maybe there's some prevailing wind where rides are usually (but not always) against some wind one way, then with the wind(s) on return? MAYBE the trip is to a store, and you're laden with "stuff"/heavier on the return trip?

In summary there's LOTS of variables re energy consumption. (How "fresh" IS your battery?) Same as two years ago? (Doubt it.) Many/any stop-and-goes on your trips?

So hope for "averages" BUT factor in some "overheads"... Figure on any "system" being able to handle "peak" energy loads... comfortably (without "stressing" components like the battery itself, etc).

Have fun.

 

[Chalo]

Your arithmetic is correct, but you can't disregard the things you're choosing to disregard. 200W out of the battery in no way represents 200W at the wheel, and an e-bike won't go as fast on the same power as your road bike. 48 volts nominal is not the same thing as 48 actual volts. On top of that, it's generally bad practice to run a battery flat; it will last longer and work better if you don't drain it all the way.

It's true that 200W of electrical power drawn from a constant 48 volt supply for an hour represents 4.17 amp hours of charge. It's not true that it will result in you going 18 mph.

 

[Drunkskunk]

Other things to consider:

Power Meters measure power to the road, not input power. If your Power Meter reads 200 watts, YOU are actually generating around 800 watts.

Ebike motors have variable efficiency, depending on RPM and load. Ballpark figure, about 75% at cruise. to get 200w output, you would use 250w. but as you slow down, encounter hills, or hit a headwind, that efficiency changes and your average power consumption will jump up a great deal.

Batteries degrade over time. 5%-10% per year is normal. So if you need a battery that can deliver 500 watts, and you want it to still be useful in 3 years, better get one 30% larger than you need right now.

Your 200w output is accurate for someone riding a roadbike in a tuck at 18mph. if you're using motor power alone, that's not going to be a comfortable position. If you sit comfortably on a bike, such as a hybrid or a mountain bike, your power use will jump almost 25%, and if you jump the speed to 20mph, there is another 25% power increase.

try this simulator, it may help you understand some of what is going on with an ebike's power use:
http://www.ebikes.ca/tools/simulator.html