NBraun said:
My goal is to have a bike that can cruise 30-35 with a top speed of 50-55, and can have a range of 30-40 miles.
At what speed? It takes quite a lot more battery capacity (and a better-quality battery) at higher speeds. Even more if there's a lot of stops and starts, ane even more if it's heavy.
I'll guess at 30mph you'll use 30-40wh/mile+ just cruising around. If the battery is the one you list below, that's 74vx20ah = 1480wh (assuming you actually get full capacity out of it). So you could theoretically get almost 40-50 miles out of it, as long as you're in that actual range of wh/mile. If it takes more then you'll get less range.
I understand why a 50 mph bicycle isn't a good idea, I plan to run moped tires, and don't plan on using the cheapest flimsiest bike frame.
Brakes are also important, but they only have to be good enough to skid the wheel (no more braking power at that point).
I want to run a 72 volt 20ah battery, either LifePo4 or LiPo. I have a feeling that the Lifepo4 will be cheaper by the time I build a lipo battery, bulk charger, and buying a balance charger.
It'll also be significantly heavier, and not necessarily much (if any) safer if it's a cheap one (because it may be built badly or built out of junk or cheap cells, etc). It probably will also only provide a max of 20-40A, as most of the cheap LiFePO4 only does 1C-2C. That's probably not enough current to run your setup at higher speeds, or allow good acceleration from a stop.
If you have experience with LiPo and know how to care for them to minimize fire risks, they'll perform MUCH better.
This is what i'm thinking for a LifePo4 battery.
https://www.alibaba.com/product-detail/72-Volts-20Ah-LiFePO4-NMC-Lithium_60592363545.html?spm=a2700.7724838.2017115.46.3c93b095OzSBwv
The specs on that one indicate a few things, especially that the company hasn't got a clue what they're selling, and I wouldnt' trust them (this is true of most of the companies selling batteries out there):
Model Number:
26650-20S4P
Cell Specification:
Li 26650 5000mAh 3.7v
It's made of cylindrical 26650 cells, 20 in series and 4 in parallel. but it isn't LiFePO4, because those wouldn't be 3.7v, they'd be 2.8v average, and 3.6v full. LiCo (and similar types) are usually listed as 3.7v (average voltage), and 4.2v full.
It could just be that the company has no idea what numbers to actually use--they may not know anything about batteries other than how to assemble them (if that).
20s "72v" nominal should indicate non-LiFePO4, as it'd be 74v at average voltage of 3.7v x 20. At a 4.2v/cell full charge x 20, it'd be 84v, which matches the charger. Definitely not a LifePO4 battery. What chemistry it actually is, there's no way of finding out (the company obviously has no idea).
Size:
120*140*290mm
(4.72" x 5.51" x 11.41"
This is a fairly large size block you'll have to find room for on the bike. Inside the triangle of the frame is the best place, but this may not fit there. You might have to use a pannier on one side of the rear, around axle level. (I wouldn't put it up on a rear rack).
Since the pack may not be able to handle the power draw you will need, you might have to find room for two of them (paralleled at the output).
BMS System Inside:
72V 20S 40A BMS
Continuous Discharge:
40A
Peak Discharge:
120A
There's a fair likelihood that it will have significant voltage sag at the higher currents, especially as it gets closer to empty on each discharge cycle. That affects how much power you actualy get out of it.
If you really got 120A at 74v (the average voltage), it'd be nearly 9kw. But it'd probably sag a lot in voltage--how much you won't know without testing it. If you needed that power for more than a few seconds, it would probably trip the BMS (if it has overcurrent protection) or damage it (by overheating the output FETs if it doesn't have protection).
40A at 74v is 3Kw, which isn't too bad, but depending on the motor/controller setup and wheel size, it may or may not have quick acceleration, if you need that.
Charge Model:
CC/CV 84V 2A
This also indicates non-LiFePO4, for a 20s pack.
Packaging Details
72V NMC Lithium Ion Battery 20Ah
Dimension: 120*140*290mm (Can be customized)
Weight: 8.5 kgs
This at least has a chemistry name that's more likely to be really it, but NMC usually charges to 4.1 or 4.15v, not 4.2, so the charger that comes with this might be overcharging it a bit (reducing it's lifespan). It's average voltage is lower, to, at 3.6-3.65v, not 3.7v/cell, so a bit less Wh. But generally it's considered "safer" than LiCo (RC LiPo). It's C-rate still isn't much more likely to be higher than any other cheap battery, though.
However, I wouldn't trust that this is actually what chemistry it is, and they probably don't actually know what it is either. (or anything else they sell).
