$62 e-bike

[hayden55]

Originally Posted by redpoint5

I'm at the point now that unless I can easily sell something for more than $100, it's best to just drop it off at Goodwill and take the tax deduction.

I like that idea.

I think i found my backup idea. Limebike donated all of their bikes to engineering for hpv. I'm gonna get one of them for free and try it out.

[hayden55]

Bike has been acquired. Brand new 26" Walmart Special 21-speed full suspension mtb with an aftermarket adult sized seat. $40 total off of fb marketplace.
I've been looking at a lot of odds and ends but it looks like my limiting factor will be how to figure out the rpm/volt of these hub motors. Seems like a lot are limited to x rpm at 36/48v to keep them at level 1,2,3 etc...
I don't want that. That's no fun.
Here is what I'm seeing Right now:
26" wheel kit.
wheel circum x rpm:
100 RPM - 8 mph
200 RPM - 15 mph
300 RPM - 23 mph
400 RPM - 31 mph
500 RPM - 39 mph
582 RPM - 45 mph
Anybody know how to figure out zero load rpm on these kits?
Looking like I will need to order a motor and over volt it to do what I want to do.

[hayden55]

Calculations are in:

Battery planned: 14s10p
Cells: NOS LG ICR18650B4
Cell spec: ~99% life 3.6V 2600 mAh
Pack Watts: 1310.4 Wh
Usable: 1179 Wh @ 88% dod (94%->6%)
Life cycles : 7500 = basically forever for me
75c/cell -> pack cost before assembly : $105 plus tax and shipping
Approx assembled cost: $100/kWh
Avg Range calculation for work commute: 33 miles (25 mph, 30 mph, 45 mph)
* how far bike can travel given my commute

kit:
1500w 48v kit (63v capacitors)
1500/48 = 31.25A output
Assuming 1500w output, 78% motor efficiency,
-> 31.25/.78 = 40A controller input consumption
94% battery voltage : 4.116
94% series voltage: 57.624
Power Output /Input at 94% : 1800W / 2308W

Cells per parallel are 5A sustained
Pack max sust amperage = 50A
Safety Factor: 50/40 = 25%

Hoping all in should be $400 max including the bike purchase.

*To add:
1. BMS to cost
2. Charger to cost
3. The voilamart 1500w 48v kit on ebay is 35A so I'll redo my calc's.
So nominal power output to the ground at 78% eff is actually 1375 Watts.
Max power is 1600 Watts. All calculations at 14s config.

[Stubby79]

Where did you get the cycle life # from?

Watch that they're not passing off peak power/amps for rated. (Rated tends to be half of the amps listed on the controller)

[hayden55]

Originally Posted by Stubby79

Where did you get the cycle life # from?

Watch that they're not passing off peak power/amps for rated. (Rated tends to be half of the amps listed on the controller)

Its a generic 1c 18650 chart.
Its something like:
Lower Upper Cycles Capacity
0 100 500 100
6 94 7500 88
8 92 15000 84
10 90 25000 80
20 80 35000 60
30 70 40000 40

Yeah honestly I hear those controllers are all over the place. I'm sure its peak and probably stabilizes down lower.

[Grant-53]

Take your VOM and measure the resistance of the motor. That will help in the calculations. Pay attention to how the hub mounts in the frame. You may need to fabricate brackets. Be sure to wire for dynamic braking. Cheap brakes will not be enough. See the other builds from the past posted here. When you get the system dialed in and the riding position fine tuned then work on the aero fairings.

[hayden55]

Originally Posted by Grant-53

Take your VOM and measure the resistance of the motor. That will help in the calculations. Pay attention to how the hub mounts in the frame. You may need to fabricate brackets. Be sure to wire for dynamic braking. Cheap brakes will not be enough. See the other builds from the past posted here. When you get the system dialed in and the riding position fine tuned then work on the aero fairings.

How are you doing regen braking? The $190 kit on ebay doesn't have it but I have a couple ideas to ghetto rig it.

[Stubby79]

Originally Posted by hayden55

Its a generic 1c 18650 chart.
Its something like:
Lower Upper Cycles Capacity
0 100 500 100
6 94 7500 88
8 92 15000 84
10 90 25000 80
20 80 35000 60
30 70 40000 40

Yeah honestly I hear those controllers are all over the place. I'm sure its peak and probably stabilizes down lower.

https://www.batteryspace.com/prod-specs/5457_B4.pdf

Cycle life, according to that, is 300, though that is charging it to a full 4.2 volts. Yes, you'll get more than that, keeping it to ~4.05 volts and not discharging it past 20% remaining, but I doubt it'll be over 1000 cycles. Of course, if you don't use it all the way down to 20% capacity, it'll help. As will keeping the load light on each cell so that it doesn't heat up, which would kill them quicker than rated.

Unless your controller has no speed limit, you will only really see 1500w on hills. The motor will also limit it with the back EMF (voltage the motor generates as it spins), so unless the rim is much too large for it, the voltage being fed in to it will limit the top speed, at which point the power draw will drop off. (though chances are this is pretty high and wind resistance might be enough that you WILL pull 1500w continuously). Oh and don't forget that you'll spend a lot of time accelerating and decelerating, unless its one long stretch all the way to your destination.

Dynamic braking could mean dumping the energy to a dump-load rather than trying to push it back in to the batteries. Would be cheaper and simpler than trying to pump it back in without a regen controller. You could try dumping it in to a voltage booster, which could then push it back in to the batteries, but I'd try a dump-load (large resistor) first. I haven't gotten around to testing this yet.

I've got a 3000w motor off of ali-express. Other than not being well balanced, it is built well enough to handle the power. I'd not put it on any bike that wasn't built like a tank. Your $62 might have been better spent on a used, decent-quality bike.

Hmm. That's all my thoughts for now. Not trying to discourage, looking forward to seeing your budget build.

[hayden55]

Yup cycle life is a tough one. It looks like the LG cells may get 60% of the cycle life of the Panasonic cells that the table is based on. Also, those numbers are with mild temps ,0.5c charge, and 1.0c discharge.
I finally bought my batteries last night. Going to build a 15s7p pack. Cells ended up being $90.54 (86 cents each).
I definitely say I will get above 1000 cycles, but it may be more based on this chart:
Capacity End V Start V Cycles Usable kWh
100 3 4.2 300 982.8
88 3.072 4.128 4500 864.864
84 3.096 4.104 9000 825.552
80 3.12 4.08 15000 786.24
60 3.24 3.96 21000 589.68
40 3.36 3.84 24000 393.12

I am interested in building my own programmable BMS with an Arduino and some relays, temp sensors, and an ammeter. I would be able to program in all my limits on voltage, amps, and could try out the dump load as well. I can even do some dip switches that swap the program to/from each capacity level for prolonging life. (ie: switch combo 3 = 30to70% capacity range limit) Dump load probably just to do the braking and not charging though. Maybe later.
But we are discharging harder in a hotter environment. Past that I only have a lot of fuzzy math since I don't have any motor curves to work with. I have noticed the motor only achieves so many RPM/Volt so it will be interesting to see how fast we go and how far we go. I ended up spending $40 on the bike so If I decided to sell the bike I wouldn't lose much money if it ends up being lame and slow. Then I could keep my battery for other projects, and sell the rest.
*also the motor has a limiter disable wire.

So far:
$40 - bike
$90.54 - battery cells
$196.15 - 1500w voilamart kit 26" rear wheel

By the way I appreciate all the feedback and advice everyone. Having a lot of fun so far doing calculations and planning config/testing.

Hoping this will turn into a 40 mph e-bike on a new frame with all the safety hardware to be legal.

[Should be biking]

If you’re planning to ride this on the street I’d recommend switching to some puncture resistant tires with minimal tread so you’ll have more grip. The puncture resistance will be an added bonus. I have to brake pretty hard just to slow down from 25mph on my ebike. Since the bike is rim brake that might be one argument for getting regen braking working. Looking forward to how the build works out.