Comment by 4gotunameagain | Hacker News Reader

Comment by 4gotunameagain | Hacker News Reader

Parent Full thread

Let's check the numbers. They claim :

  Range: 40km
  20 min pedaling charges 50%
  Top speed 25kph

If a 70kg well trained amateur cyclist can output ~200W for an hour [1], so let's guestimate 130 for an average person. That would mean the battery has a capacity of at least: 130W * 2*20min = 86.6Wh. The efficiency of the motor/ESC is not considered.

So at 15kph, it would take 2.6 hours to reach the stated 40km range. 86.6Wh / 2.6hrs = 32.5W

A model based calculator [2] says that with 32.5W you indeed cycle at 15.5kph.

LiFePo batteries have an energy density of 325Wh/L, so we would need a battery volume of: 86.6Wh/325Wh/L = 0.27L, but a quick search for an example online reveals that a commercial 10Ah 12.8V takes up 1L. That would seem to fit within the product.

I was initially doubting the claims, but it seems to check out ?

[1] https://blog.2peak.com/en/what-are-the-average-watts-of-a-go... [2] https://www.gribble.org/cycling/power_v_speed.html

Unusual for ebikes to use lifepo as they are much larger and heavier than standard lithium ion. Especially in cylindrical formats that are needed to make bike friendly battery shapes.

Fair point, then even better for fitting the battery in the case.

Also curious what "50%" means in their specs; the top power for the motor is 250W and top speed of 25kph would take ~100W on flat ground, which would throw off your calculations considerably. (Not that I take issue with your calculations, they just haven't published enough data to know what you'd actually be getting.)

More than happy to learn something through a mistake in my calculations !

Regarding the 50%, I assumed that they stated this number because this is when the battery is in bulk charge and can suck up as much as you can give it.

For the 250W, I hadn't even seen that, good catch. Maybe 250W is the peak power that cannot be sustained for thermal or current rating reasons ? 250W for a 12.8V 10Ah battery is 2C, so that should not be the limiting factor. I do not know about the motor or the ESC

Also, for sure the maximum range is not achieved with the top speed !

Agreed on all points! Very plausible that 250W is peak; other motors on the market vary whether they state peak or sustained so the waters are a little muddy (e.g. I know that the popular BBS02 "750W" motor can put out over 1kW peak).

> with 32.5W you indeed cycle at 15.5kph.

With a perfect bike in a velodrome, maybe.

But on Indias rough roads with an old mountain bike, I suspect this number to be double.

It is a good point, but I highly doubt the stated 40km range was measured with a rusty bike off road. I think it is fair of a seller to state their range in good conditions.

I’m missing something, what’s unrealistic about 15.5kmh? That’s absolutely crawling on a bike.

What he meant was to go 15.5km/h with a good bike on a velodrome might require 32.5W, but it would require quite a bit more power on rough dirt roads with a shittier bike

Reply on news.ycombinator.com