Firewheel 1000km service

[dmethvin]

After six months of talking about it, I finally got around to adding my second 260Wh battery for a total capacity of 520Wh. The process was made easier because I had the old mainboard and mounting plate that had failed shortly after I received the wheel and was replaced under warranty by Firewheel. After that the wheel was still acting strange so they sent a new battery. After replacing it I did some experimenting and I think what happened is that the battery cells had gotten super unbalanced because I had accidentally shorted them for a brief period (which melted a connector). After a full charge the old battery seemed to behave fine.

Here is the photo album of all the work, including some notes: http://imgur.com/a/8EObi

Also a bonus video: https://youtu.be/GXbKKj5FxhQ

There was an enormous amount of mud and dirt in the wheel! I can only remember a few times when I went through mud so now I am going to be paranoid about it. 

For the previous installment of this saga see here: 

 

[esaj]

Good job on the rebuild!  Interested to hear how the extra packs affect the riding, at least I think mine accelerated a lot better uphill with 3 custom-packs vs. 2 originals.

[DBr]

On ‎17‎.‎02‎.‎2016 at 4:00 AM, dmethvin said:

Here is the photo album of all the work, including some notes: http://imgur.com/a/8EObi

You wrote "I used the sport/comfort switch as a two-level volume control for the speaker,..."
How do you electricaly lower the volume? By switching a resistor in series to the speaker?
If yes, which kind of resistor(s) did you take? 

[dmethvin]

21 hours ago, DBr said:

You wrote "I used the sport/comfort switch as a two-level volume control for the speaker,..."
How do you electricaly lower the volume? By switching a resistor in series to the speaker?
If yes, which kind of resistor(s) did you take? 

Yes, I just have a resistor in series with the speaker, and the sport/comfort switch bypasses that resistor when it's closed. I picked a 20 ohm resistor because it dropped the level by about half as far as I can tell. The "Take care" message is higher volume so it's still possible to hear in low volume mode when you're in quiet places like a nature path.

[DBr]

Ok, thx for the prompt reply.
20 Ohm sounds reasonable to me. In my firewheel a 8 Ohm / 2 watt speaker is build in. So with about 28 Ohm in total we have a bit less than a third of power output in watts, which should be a bit less than half of the volume (in dB). Thus a resistor with a demand (is this the correct english word?) of 1 Watt should be sufficient, even I have no idea about the output of the built-in amplifier. But I doubt that it will deliver more than 1 Watt to the 8 Ohm speaker. Taking this in consideration a 0.5 Watt resistor should be sufficient as well.
How may watts is your 20 Ohm resistor able to carry? 0.25, 0.5 Watts or even more? 

Btw. Hopefully I can continue thread with the mod of my F528 soon. Currently I'm working on a electrical (with LEDs) fuse supervision, because I'm also not able to check my fuses easily. And when one fuse of the two parallel Batteries is blown, you will notice it only by less capacity / milage and if the second is blown as well. :-(  

[dmethvin]

11 minutes ago, DBr said:

How may watts is your 20 Ohm resistor able to carry? 0.25, 0.5 Watts or even more? 

Btw. Hopefully I can continue thread with the mod of my F528 soon. Currently I'm working on a electrical (with LEDs) fuse supervision, because I'm also not able to check my fuses easily. And when one fuse of the two parallel Batteries is blown, you will notice it only by less capacity / milage and if the second is blown as well. :-(  

I have a bunch of 1 watt resistors around so that's what I used. The speaker is very intermittent duty anyway and I don't think it uses too much power. 

With the 40 amp fuse I put in for shunting, there is no "normal" load I can imagine that would blow the fuse. Usually the load will be spread across the two packs. Even if one pack were to become disconnected the second one could only see high loads like that for fractions of a second. The Bussmann fuse I'm using won't blow unless it gets a sustained 100 amp load for 1 second. That is essentially a short!  At a 60 volt battery level a 40 amp load would mean 2400 watts which is way beyond the rated power of the motor.

The information I've seen on typical 18650 Li-Ion cells is that the continuous discharge rate should be held to less than 2x capacity. So for the 260Wh pack that would be about 9 amps for one pack or 18 amps for two packs in parallel. Since the Firewheel motor is called "800 watts" it shouldn't draw more than 14 amps but it's always hard to decode those specs and know the "real" numbers. If those are roughly correct though, it seems like a 520Wh battery would be enough to prevent either overloading the 18650 cells and/or causing voltage sag that might affect the responsiveness.

[esaj]

4 minutes ago, dmethvin said:

Since the Firewheel motor is called "800 watts" it shouldn't draw more than 14 amps but it's always hard to decode those specs and know the "real" numbers.

Is that from an official source? So far the only specs I've seen for the Firewheel motor were 550W continuous / 1350W peak (somewhere here in the forums). But AFAIK, the "ratings" of the motors are all over the place, more like best guesses (calculated with some formulas?) and not that clear cut... It's not like it's going to blow at 1351W or couldn't draw even more if "demanded", another thing how long it can take it

[dmethvin]

31 minutes ago, esaj said:

Is that from an official source? So far the only specs I've seen for the Firewheel motor were 550W continuous / 1350W peak (somewhere here in the forums). But AFAIK, the "ratings" of the motors are all over the place, more like best guesses (calculated with some formulas?) and not that clear cut... It's not like it's going to blow at 1351W or couldn't draw even more if "demanded", another thing how long it can take it

Somehow I had it in my head that it is 800W, but I cannot find a reference now. As far as the maximum current draw I suppose that is the resistance of the wiring if it was fed the battery voltage at a 100% duty cycle . Not quite a short circuit but also not likely to be sustainable for long!

[esaj]

1 minute ago, dmethvin said:

Somehow I had it in my head that it is 800W, but I cannot find a reference now.

I'm not sure on my numbers either, so could be 800W too.

 

1 minute ago, dmethvin said:

As far as the maximum current draw I suppose that is the resistance of the wiring if it was fed the battery voltage at a 100% duty cycle . Not quite a short circuit but also not likely to be sustainable for long!

Yeah, I was talking about the stated peak (maximum) wattage of the motor... I'd expect the motor to burn if fed constantly as much as the batteries can deliver

Somewhat related, I've been on and off looking for a straight answer to a couple of other things related to the motors, but never got anything that really sounded clear or sure:

- Is the motor (coil) wiring the only resistance in the path after the bridges (ie. how do the coil inductances / electromagnetic fields / eddy currents / what have you affect it)

-Can the motor actually be driven at 100% duty cycle (as the gate voltage on high-side of the half-bridge should be something like 5-10V above the source-voltage for the mosfet to fully conduct, and the charge-pumps seem to use the PWM-cycle to work, ie. they must go "on and off", unless a driver IC can handle this?)

-If the motor can be driven at or near 100% duty cycle, and the motor reaches the speed where the back-EMF is almost at that voltage, what happens if you do a sudden acceleration and the voltage sags? The motor brakes? How is this handled in current controllers, the motor speed actually never goes above a speed where the back-EMF would become larger than the minimum battery voltage? Just wondering how I could get similar speeds from the Firewheel with either full or almost empty battery... (no matter if the battery display shows 99% or 20% or 0% during acceleration).

Ie. as an example with numbers pulled from a hat:

-Riding at some speed where the motor back-EMF would be, 50V and the duty cycle would be 90% = around 55,55V from battery direction
-I'd accelerate faster, taking the back-EMF to 53V, but the battery voltage would dip below that (and for example this was on a slight decline or the momentum from the acceleration would speed my slightly above that, so it wouldn't prevent the acceleration from taking place, but then hit level ground)
Should the wheel now tilt forwards as the voltage from the battery direction couldn't keep the speed up, or does the firmware logic prevent you from going above such speeds that any "normal" voltage sag could cause this, or is there something happening in the motor that prevents this in the first place...

Not that I'm expecting you to know the answers either, more like "thinking out loud" (or in text )

[dmethvin]

Yeah, I'm no expert on motor control either, I was wondering about how it would behave in those edge cases. I think that is one of the things that makes every EUC so flakey when you get past the speeds where you should be, all the nice assumptions they are making don't apply anymore. I just imagine some poor guy in the factory who's in charge of this firmware trying to ride it and simulate the crazy things that might happen. 

[Keith]

12 hours ago, esaj said:

Ie. as an example with numbers pulled from a hat:

-Riding at some speed where the motor back-EMF would be, 50V and the duty cycle would be 90% = around 55,55V from battery direction
-I'd accelerate faster, taking the back-EMF to 53V, but the battery voltage would dip below that (and for example this was on a slight decline or the momentum from the acceleration would speed my slightly above that, so it wouldn't prevent the acceleration from taking place, but then hit level ground)
Should the wheel now tilt forwards as the voltage from the battery direction couldn't keep the speed up, or does the firmware logic prevent you from going above such speeds that any "normal" voltage sag could cause this, or is there something happening in the motor that prevents this in the first place...

Not that I'm expecting you to know the answers either, more like "thinking out loud" (or in text )

@esaj, Just me thinking out loud as well but surely, that is the key issue for why faceplanting happen, the maximum speed a motor is capable of for a given applied voltage is the point where back EMF + losses (resistance, voltage drop through drivers, etc, etc ) = applied voltage. At that point torque = zero. If, under any circumstances you approach that point in use (I.e. Well before you actually reach it!) you are going to faceplant as the wheel can no longer support you.

Should you be going down hill such that back EMF + losses > applied voltage I think you will have already fallen as you will have had to pass through the zero torque point already, of course if that resulted in regenerative breaking it would have helped you on your way through the air; although I'm not convinced the voltage would pass backwards through the drivers that are trying to accelerate you.

Of course, when you are actually braking, you do, at that point have back EMF+losses > applied voltage. However, at that point you do have the drivers trying to drive you in the opposite direction and the torque is trying to lift the back of the peddles resisting your weight on the rear of the peddles because you are trying to slow down.

theoretical maximum power from a wheel is surely measured by measuring the resistance of any one phase in the wheel. Maximum torque and power will occur at zero RPM at which point it is simply V=IR. Of course whether the gauge of wiring used will handle that current for any appreciable time is another matter. Don't forget that to keep the number of turns down (to get the required kV (RPM/Volt), whilst filling the space with as much copper as possible, the windings are multi strand so total copper area Is the sum of the individual strands.

I am afraid, my electronics knowledge breaks down somewhat when considering how close to the battery voltage the MOSFets manage to reach. I'll have to scope one of my model aircraft motors to see, I'd always assumed it got quite close?

[esaj]

4 hours ago, Keith said:

@esaj, Just me thinking out loud as well but surely, that is the key issue for why faceplanting happen, the maximum speed a motor is capable of for a given applied voltage is the point where back EMF + losses (resistance, voltage drop through drivers, etc, etc ) = applied voltage. At that point torque = zero. If, under any circumstances you approach that point in use (I.e. Well before you actually reach it!) you are going to faceplant as the wheel can no longer support you.

Should you be going down hill such that back EMF + losses > applied voltage I think you will have already fallen as you will have had to pass through the zero torque point already, of course if that resulted in regenerative breaking it would have helped you on your way through the air; although I'm not convinced the voltage would pass backwards through the drivers that are trying to accelerate you.

Yes, it would make sense that the wheel would tilt forward and you'd faceplant, but the reason I've been wondering about what the actual maximum back-EMF (ie. motor speed) is, is that I've been able to ride around 30km/h on the Firewheel with full & almost empty battery. When I had the (battery) voltage-meter attached, I could see that before the out-of-battery -warning triggers, the voltage must sag all the way to around 47 volts or so (don't remember the exact value, it was something between 46 and 48V, and of course the display might not be that accurate), which has lead me to believe that the maximum back-EMF must be even lower than that or otherwise I would have faceplanted already. At rest the voltage sits somewhere around 52-54V at that point, but sags really fast with any acceleration or hill climbing. Of course, compared to most riders, I'm lightweight, so it could be different with a heavier rider.

 

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Of course, when you are actually braking, you do, at that point have back EMF+losses > applied voltage. However, at that point you do have the drivers trying to drive you in the opposite direction and the torque is trying to lift the back of the peddles resisting your weight on the rear of the peddles because you are trying to slow down.

From what I've gathered about regenerative braking so far, it might not be this simple, actually it seems like regenerative braking uses a whole different commutation sequence than "normal" driving. When normally driving of the motor, one half-bridge is high and another one is low (and the third one's not conducting), driving current through two phases. During braking, two phases must be both low, so that there's a short-circuit between the phases, which actually causes the motor to slow down and the energy to build up in the coils, and then one (or both?) of those is switched high to dissipate the energy from the coils to the battery (otherwise it would start to burn off as heat in the coils & wiring & bridge transistors?). Or maybe I've just understood wrong   Based on this: http://electronics.stackexchange.com/questions/56186/how-can-i-implement-regenerative-braking-of-a-dc-motor  (Although it talks about a single-phase motor with single half-bridge, or H-bridge with the other side always low).

One another thing that puzzles me in the regenerative braking of our wheels is how it works when the batteries are more full (but not completely full). Like for example, if the battery voltage is at 60V, the voltage from the motor would have to be higher than that for the current to flow towards the batteries. Maybe it's due to the inductive spikes (ie. the voltage builds up to higher value when the phases are shorted), but of course there must be also some upper limit (jolting the batteries with something like several hundred volts doesn't seem feasible, the short-lived spikes can be 10 times higher than the "normal" back-EMF), although I believe there are transient suppressor diodes ("avalanche" diodes) inside the motor to eat the spikes during normal operation, otherwise the bridges would blow all the time? I tested with a small 12V 3-phase motor I have and a cheapo DIY-oscilloscope and got 50-60V spikes out of the motor just by flicking it with my fingers:

 

 

Quote

theoretical maximum power from a wheel is surely measured by measuring the resistance of any one phase in the wheel. Maximum torque and power will occur at zero RPM at which point it is simply V=IR. Of course whether the gauge of wiring used will handle that current for any appreciable time is another matter. Don't forget that to keep the number of turns down (to get the required kV (RPM/Volt), whilst filling the space with as much copper as possible, the windings are multi strand so total copper area Is the sum of the individual strands.

I know it's nitpicking, but shouldn't that be 2 phases, as the current is always passing through two of the phases at a time?   Anyway, you could very well be right, I really have no idea how the manufacturers come up with the motor wattages. What I've heard is that people in the e-bike communities have "overvolted" their motors and they still have worked nicely with "above spec" voltages & wattages, but wouldn't try it on a $1000 wheel

 

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I am afraid, my electronics knowledge breaks down somewhat when considering how close to the battery voltage the MOSFets manage to reach. I'll have to scope one of my model aircraft motors to see, I'd always assumed it got quite close?

Could be, I don't know really. I built the motor driver for the 12V 3-phase motor based on the e-bike schematics Mystamo (or was it someone else?) posted in the Firmware-topic. It uses a charge pump on the high-side of the bridges to drive the voltage high enough to keep the mosfets conducting, but won't work at 100% duty cycle, because the "pumping" action needs the signal to go high and low (charge -> discharge), otherwise it won't work. But it could be that the wheels use mosfet-driver ICs that have some separate charge pump / voltage multiplier-thingamabob inside them that works separately from the PWM duty-cycle.

[dmethvin]

On 2/17/2016 at 6:27 AM, esaj said:

Interested to hear how the extra packs affect the riding, at least I think mine accelerated a lot better uphill with 3 custom-packs vs. 2 originals.

It definitely feels better when climbing hills. Before, I could get up hills but I had to lean much more carefully because it would dip forward and feel soft. Now I can go up a hill much more aggressively. It also feels a little more balanced because there is equal weight on both sides of the shell. 

I also like the additional range. The weather hasn't been nice enough that I wanted to do a really long ride yet, but based on the battery level vs distance I would think I could get more than 20 miles (32km). Tomorrow is supposed to be a nice day so I may try doing a long trip, which will also be the first long trip with the new Big Apple tire.

The only downside is the extra weight. I can still pick up and carry the wheel for short distances, but the extra weight makes it less comfortable than before.

[dmethvin]

Followup on the battery life after a few trips. It took several days of riding and I finally got the battery down to where it was reading low single digits at rest, and almost immediately going to 00 while riding. I wasn't getting tilt-back yet, but I had to ride 4 miles to pick up my wife's car at the shop so I didn't want to risk running out on the trip. So on the single charge I got about 38 miles or 61km!  I think I will get a Charge Doctor so I can preserve the battery a bit by not charging it to 100%.

[DBr]

Wow, 61km, amazing! Did you rode on flat terrain only? And what was you average speed? Did you get a ‚power is low warning‘ at the end and wheel gets very slow? That’s what I got, if battery is quit low. Riding the last ~4-5 km(s), before the wheel starts shaking (caused by low power), the display showed 0%. I expected even some minus values. 

[dmethvin]

It was mostly flat, but I did go up and down some small hills at times. I had not gotten the "power is low" warning yet but I just couldn't take a chance it would stop on the next trip. I was amazed! The second pack I put in had been sitting on my workbench at nearly full charge for 8 months, so I was afraid it might have degraded. But it must be in pretty good shape! I wonder how accurate the GPS distance is on phones, maybe it is possible it is too high 10 or 20 percent? Even 50km is pretty impressive. 

[esaj]

17 minutes ago, dmethvin said:

It was mostly flat, but I did go up and down some small hills at times. I had not gotten the "power is low" warning yet but I just couldn't take a chance it would stop on the next trip. I was amazed! The second pack I put in had been sitting on my workbench at nearly full charge for 8 months, so I was afraid it might have degraded. But it must be in pretty good shape! I wonder how accurate the GPS distance is on phones, maybe it is possible it is too high 10 or 20 percent? Even 50km is pretty impressive. 

One thing that probably has (at least some) effect is the fact that you rode the distance in multiple trips (over several days), allowing the battery to recuperate in between. Still, about 50km in one go is what I'd expect, considering that I use pretty much 10-11Wh/km with the 260Wh ("only" 32 cells) battery in fairly fast riding (not off-road) & good conditions (no strong head wind & not too cold).

[dmethvin]

This definitely wasn't a very good science experiment, too many variables changing at once. Since I hadn't shunted the original pack I never felt comfortable going below 25% battery, so I don't have any max range data for the 260Wh configuration. After upgrading to 520Wh I rode for a short while with the OEM tire, but then switched over to the Schwalbe Big Apple. Plus I lost some weight recently. 

Subjectively I feel like the new tire could be helping here. The EUC feels a lot more responsive, especially with the higher tire pressure, which seems like it would cause less drag. 

[dmethvin]

Some more interesting information from the Schwalbe site that may affect results, especially since I increased the pressure before doing the long rides.

http://www.schwalbetires.com/tech_info/rolling_resistance

Why do Pros ride narrow tires if wide tires roll better?

Wide tires only roll better at the same inflation pressure, but narrow tires can be inflated to higher pressures than wide tires. However, they then obviously give a less comfortable ride. In addition to this, narrow tires have an advantage over wide ones at higher speeds, as they provide less air resistance.

Above all, a bicycle with narrow tires is much easier to accelerate because the rotating mass of the wheels is lower and the bicycle is much more agile. At constant speeds of around 20 km/h, the ride is better with wider tires. In practice, the energy saving is even greater than in theory as the elasticity of the tires absorbs road shocks, which would otherwise be transferred to the rider and so saves energy.