After I disassemble the entire motorcycle while filming in stop motion, I then give a talk about battery chemistry, size, voltage, and other concepts.
I said earlier that the lithium chemistry of the battery cell is like the type of fuel (Lithium Ion is to gasoline as Lithium Iron Phosphate is to diesel) and the size of the battery, measured in Amp Hours, is like the size of the gas tank. But there's one more part to the equation that doesn't really crossover into this metaphor: the discharge rate of the battery. Maybe it could be thought of as the fuel pressure?
The discharge rate is how much current (amps) the battery can provide - instantaneously, not over time which is the capacity (amp hours). This is how strong your battery pack is and will ultimately determine how well the bike can accelerate or go up hills. And there's two measurements: continuous discharge rate and burst discharge rate.
The larger the gas tank (battery capacity, in this metaphor) is, the higher the discharge rate is. Which is why this metaphor doesn't make sense. And Li-ion and LiFePO4 have very different discharge rates - Lithium Ion being better, especially as a ratio to weight and volume. Li-ion cells are more powerful yet smaller and lighter, however they don't last as long and are more expensive than LiFePO4.
But for now let's just focus on what discharge rate I need to achieve out of my battery pack.
What has already been decided is the motor size. It is rated for 4,000 watts continuous. At 72 volts, that is 55 amps (4000/72=55). So if I want my battery pack to be matched to the motor I selected, then I need a pack that can continuously discharge 55 amps.
(I'm trying to explain it in a way that makes sense to folks who have never built an e-bike, like myself, but I hope I'm not sounding like I know what I'm talking about too much! I'm not even getting into the motor controller rating, i.e. phase amps vs battery amps, which just boggles my mind)
28 авг 2024
