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Goin down a hill with a wheel


Zenko

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45 minutes ago, Zenko said:

I heard a lot about this balance cells thing, it didn't really meet a real consensus that it's bad to not fully charge the wheel from time to time no ? Is there more information / facts about it to confirm what you say ? Like a topic or article or review... Only fact i'm sure of is to try to keep the battery over 30% as much as possible.

No articles I'm afraid. Battery University may have one but I've not seen it yet. I only have the cumulative knowledge of lots of posts on here by people who know a lot more than me about battery management systems. What has been said makes sense.

A weaker cell, with maybe a higher resistance than those around it, will not take up energy as readily as others and so when a charge is ended early it will have a lower voltage than the others. This not only reduces the potential capacity of the pack but also may eventually cause a premature protective shutdown of the pack due to a cell undervolting (very bad for Li-ion). By leaving the charger connected during the saturation stage the BMS shunts power around the cells at full capacity but allows power through the weaker cell to top it off and bring it in line with the rest. If this is not done occasionally then with each partial charge the weaker cell will fall further and further out of line with the rest making the problem bigger and bigger until it is reset with a balancing charge. That is the theory as I understand it. Make sense. I'm going with it.

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10 hours ago, Zenko said:

My small report with wheelog -

  • ...
  • Current - wheelog is a mess with this, can't really make any statement, it measure upside down it seems... because it's constantly stuck around 64/67 (not sure anymore exactly). When I look into the graph, I see some huge spikes even one got to 70A, not really sure if it's possible ._. I did mess it a bit by accelerating braking hard all the way down the hill... But the high current is probably due to acceleration, not braking...

 

I assume you use an old wheellog version – it showed for negative currents 70. this is repaired (at least for kingsongs). The new version can be found in the wheellog topic here in the forum.

5 hours ago, Zenko said:

When wheel push you => power consumption

When wheel is rolling down => generate energy

„Rolling down“ is a bit inprecise...

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So when I brake in descent, I generate energy, I get back my battery full. Therefore warning bips start coming precisely at this time.

Full is very relative. Charge is „determined“ by battery voltage and beeping, tiltback and finally bms cutoff happen at certain voltages. (Btw: Beeps and tiltback depend on the total pack voltage, bms cutoff comes once the first cell reaches the threshold! So if one has an (very) unbalanced pack the cutoff could theoretically maybe happen without any warning!)

this measured battery voltage goes not only up by the battery getting more charge while regenerative braking, but also by the current flowing through the internal resistance of the cells! Since regenerative braking can be quite a fast charging this voltage drop over the internal resistance can be quite big. Also by some „internal chemical processes“ the cell voltage additionally rises temporarly. (Imho like the internal resistance rises and the drops again by the higher currents)

so if you start down an incline with about 90%, brake hard until it beeps (96%?) and stop again, and wait  a bit you‘ll see the 90% (or 91 or so) shown again...

this should be quite comparable to cell behaviour while the cell is under (heavy) load. One has the cell dropping voltage by loosing some charge, a (higher) voltage drop over the internal resistance and some additional voltage drop by this „internal chemical processes“ which recovers again once the cell is under no or less load again.

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I don't really know why, but you're theory and statement is wrong (got bashed in french forum for it). Hard braking does not generate lots of power, somehow....

Imho efficiency of regenerative braking is very low ( read once a master thesis of an electric trike which reached something around 20% efficiency with extra super capacitors to recharge the battery!). The harder one brakes, the higher the charging current for the battery and the lower the efficiency should get.

if they have some proof/theory/knowledge in the french forum about non regenerative braking occuring in EUCs i would be very interested in this! I just found some hints in wheellog logs the non regenerative braking could have happened at lower speeds, but these were not a real proof...

Btw; hard braking generates exactly the same power as hard accelerating needs. Energy can not vanish! Its just the question were the power goes to/is dissipated!

ps:

power supplied by the battery for accelerating

- power lost by the „internal chemical processes“ of the cells

- power dissipated by the internal resistance of the cells

- power dissipated at connectors/wires/mosfets...

- power dissipated at the motor (coils/magnetic losses/internal friction...)

- power needed to overcome the wheel friction/deformation

- power needed to overcome the air drag

= power to accelerate rider + wheel

= power from braking rider + wheel

- power provided by air drag

- power provided by wheel friction/deformation

- all the other above mentioned internal losses

= power to recharge the battery

 

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2 hours ago, Chriull said:

Rolling down“ is a bit inprecise...

Wow i wrote that, my bad, was probably tired at this time. Should be more simple statement like "wheel is pulled by some force and try to hold it"

I did not understand half of your following statements, as I'm not really that familiar with electric stuff. I'll read again with a clear mind, again, tired a little bit right now.

2 hours ago, Chriull said:

Imho efficiency of regenerative braking is very low ( read once a master thesis of an electric trike which reached something around 20% efficiency with extra super capacitors to recharge the battery!). The harder one brakes, the higher the charging current for the battery and the lower the efficiency should get.

if they have some proof/theory/knowledge in the french forum about non regenerative braking occuring in EUCs i would be very interested in this! I just found some hints in wheellog logs the non regenerative braking could have happened at lower speeds, but these were not a real proof...

One of them used to take a small descent many times, like 5 times in a row, around 5 meters, and checked the effect, concluding he charged around 1/2% of his battery with this. I don't really know  the details, but that's how he made some assumptions saying i could charge 20% of my battery with my road. What i'm sure of is that tomorrow, i'm going with a 60% battery, and i'll see what i'll get at the end, with a soft ride (10-15km/h). Doing that consistently will clearly tell me how much i generate at which speed ! (yeah yeah i know, and the weight, the wind, blablabla).

Thanks for your answer !

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Disclaimer: I'm not an expert, and there are several people on this thread that know a lot more about this than I do. This is just my guessing, and what I would do.

If you have never done a balancing charge and the issue appeared a while go, it could be that the battery cells are indeed out of balance. What I would do is leave the wheel in the charger for a long time, like 9 hours after the charger led has turned green. Then go ride, but NOT downhill first! I'd ride to the other way, or just accelerating around the yard first.

Next time you go down the same hill at 90% battery, it might not give overvoltage warning.

 

My guess is that the pedals felt soft because tiltback while braking is such a rare situation, or the firmware tried to limit overcharging and therefore applied less braking power.

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For comfortable riding on varied terrain, a smaller wheel  or a v shaped belt are ways we acan achieve this end. When riding up or down a hill the contact point of the wheel moves forward or backwards. This causes surprises. Especiallygoing down hill wh all the wheight is on the heels suddenly and sometimes goes too far back so you might be flipped off.  My vote is for a timy, but wide wheel with shock absorbers.

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On 10/12/2017 at 2:38 AM, Zenko said:

But constant speed isn't going to charge even more battery ? so when i finally need to brake, it's gonna be even more close to a cut off ? Carving is my second life saver for it, that's for sure (I like carve-braking, I find it more efficient than straight classic brake).

In my case the downhill eventually transitions into a slight uphill so I don't need to brake.  

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On 15.10.2017 at 3:39 PM, exoplanet said:

What is a good way to discharge excess energy in down hills?a heating element and a fan?

Halogen bulps seems to be a nice solution. Once a member posted here a diy solution. But imho no follow up after the first tests...

Edit: found it:

 

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On 10/12/2017 at 6:39 PM, Chriull said:

if they have some proof/theory/knowledge in the french forum about non regenerative braking occuring in EUCs i would be very interested in this! I just found some hints in wheellog logs the non regenerative braking could have happened at lower speeds,

We do a lot of physics in french high school : ) there is basically 3 possibilities : 1) immobility or very slow speed, the wheel is working against gravity. It is not a non regenerative breaking but a real work to hold you back. 2) At faster speed, the wheel breaks (regenerative, voltage rise in the log) so that there is no acceleration. Accelerating is first letting the weight doing its job then put some work if you want to go faster (3rd possibility). It all depends on how steep is the hill, and because there is friction, it also depends on the weight of the rider+wheel and the surface of the rider.

Otherwise you would be like this guy.

 

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2 hours ago, jbwheel said:

We do a lot of physics in french high school : ) there is basically 3 possibilities : 1) immobility or very slow speed, the wheel is working against gravity. It is not a non regenerative breaking but a real work to hold you back. 2) At faster speed, the wheel breaks (regenerative, voltage rise in the log) so that there is no acceleration. Accelerating is first letting the weight doing its job then put some work if you want to go faster (3rd possibility). It all depends on how steep is the hill, and because there is friction, it also depends on the weight of the rider+wheel and the surface of the rider.

Otherwise you would be like this guy.

 

Wow!:blink1: 

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9 hours ago, jbwheel said:

We do a lot of physics in french high school : ) there is basically 3 possibilities : 1) immobility or very slow speed, the wheel is working against gravity. It is not a non regenerative breaking but a real work to hold you back. 2) At faster speed, the wheel breaks (regenerative, voltage rise in the log) so that there is no acceleration. Accelerating is first letting the weight doing its job then put some work if you want to go faster (3rd possibility). It all depends on how steep is the hill, and because there is friction, it also depends on the weight of the rider+wheel and the surface of the rider.

Yes, one needs the same energy to break as it was build up by the system. As the kinetic energy goes with the square of the speed its "much less" at lower speeds and zero at immobility.

By regenerative braking real work is done! Beside the real heat losses (power dissipation) there is real power consumed by the battery for charging. And as the voltage generated by the motor is transformed "upwards" by the mosfets in combination with the motor coils, the braking power ("current flowing into the battery") can be nicely controlled.

With normal (bike) brakes you also do not need to put energy (besides pressing the brake shoes) into the system for braking. No matter at which speed.

So these are imho no arguments against regenerative braking possubilities.

The main difference with electric motor braking by putting a load at the motor output (regenerative braking is just a case of this) is that only an asymptotic approach to a speed of zero is possible.

Beside this intrinsic limitation the voltage transformation will not be feasable below some specific speed - there is just not enough voltage generated by the motor to transform it up to a little bit above battery voltage.

Then there is just another mode of motor braking left, that switches the polarity of the voltage put on the motor. But this needs quite some power from the battery and leads to high power dissipation in the motor coils. So this cannot be used at higher speeds. (Or speaking in terms of foc (field orientated control) putting the vector of the commuted magnetic field "behind" the vector by the motor generated field - very imho, one of the firmware guys would have to confirm this...)

In my opinion this last kind of braking just happens at the very very very end of a braking process and is "hidden by/transforms smoothly into"(1) the normal balancing process that happens continously.

Unfortionately the sampling resolution of the data sent by the wheels and logged by wheellog is not high enough to clearly see/follow/analyse this "mode" switching (or at least i have not found an real unambigous proof of this in my logs.

Edit (1): while at higher speeds balancing just needs some slight speed(acceleration) changes, at very low speeds balancing has to change the direction of the wheel and by this at some point this "polarity change braking" has to occur?

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