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hobby16

Electric Unicycle's BMS problem and solution

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If a total new person may I throw my thoughts in the pot? As a new soon-to-be rider (my TG F3 should be here next week) I have been reading about the various pitfalls of these types of wheels. In all honesty I can see both sides of the issue of battery safety versus fall safety. I feel it would be an acceptable risk to disable it as long as proper overcurrent protection is put in place. In my case I will look for a panel-mount "push-to-reset" thermal-only (not thermal-magnetic to prevent tripping from inrush current) breaker with a weatherproof boot. This would allow mounting so that it could be reset "in-field" if needed.

Is there any information on what the maximum current draw would be for a typical 350 or 500 watt motor? Assuming a 3x draw for instantaneous/burst load a 350-500 watt motor should only draw 18-24 amps. Only downside is that there is a lot of cheap breakers available rated at 50VDC or below, but once you hit 65-80VDC the price jumps and fewer types/configurations are available.

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I don't think the exceeding the voltage ratings a bit is too critical on fuses and breakers at these voltages. The main concern is whether a high voltage may cause flashover or arcing inside the tripped device. My no-name unicycle is using an automotive fuse for protection at 60V, it is only going to come into play for a dead short and I doubt it will flashover in that case. The actual rated voltages of auto fuses seems to be 32V to 80V anyway so using them at 60V isn't a stretch.

As far as the current rating goes, most overcurrent devices can handle loads exceeding their ratings by 2x or 3x for at least a second, even many labeled as "quick acting". I have no idea what the current looks like for these motors though. As a practical matter, it's hard to find automotive fuses over 40A in auto parts stores around here. Assuming a big 1500W motor and 60V, that's 25A, but it seems unlikely the motor pulls that much power for more than a few seconds at a time.

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You are probably right that a 50V breaker would be fine in a 67V system. I am actually quite used to specifying breakers and fuses, just at much higher AC voltages rated 300V or 600V for building electrical systems (and you can't mix-and-match those). Part of that is designing selective versus fully coordinated systems protection to prevent upstream devices from tripping before the downstream devices, limiting the area of power loss. While I typically spec thermal-magnetic I believe most thermal breakers will take 4x rated current for under 1-2 second with no problems (and 10x for less than 0.1 seconds!).

One reason I am considering a breaker versus fusing is that I have never had much luck with blade-type fusing in motorcycles/automobiles. Nothing quite like trying to change out a fuse while your rear is hanging out in the rain. Now that I think about it I have replaced standard fuses with blade-type resetable breakers in the past. Just push the button and away you go. Assuming you fixed what caused it to trip in the first place :)

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@Bigrig

It depends on the battery you choose for the TG. If it's a 130Wh, I doubt your 40A breaker would ever trigger, because the battery won't deliver that peak.

18650 cells have an internal polyswitch (PTC) and some even have an internal permanent fuse. In case of short, the cell simply shutdowns by itself.

Sorry to be stubborn but I still don't see what's worth the hassle of a breaker in a monowheel. Hey, it's essentially a plastic box, the probability of short is minuscule and the consequence is so trivial that is must be considered a very low risk event, not a priority.

Over-engineering the thing would give you only the illusion of safety but no quantifiable benefit (remember, we are dealing with a hypothetical risk with zero occurrence, the guys who THINK shunting is risky have not even tested a single short and have not given a single reason as to why, when, how a short would happen in a wheel  !!!) and inevitable unintended consequences : a breaker triggering in a house or a car would protect you, in a monowheel, it faceplants you !

It's way safer to take elementary precautions as to LiIon charging, like using a charge checker to detect suspicious loss of capacity or unexpected voltages or current, never leave a charger unattended, install a fire alarm, be prepared and ready to extinguish a fire...

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On the eBike forum, there's a thread about the use of CID over PTC. Evidently CID (Current Interrupt Device) is now displacing PTC (Pressure Temperature Current switch) as the defacto physical safety mechanism. While the PTC is reversible, CID is not! Claimed justification away from PTC is that the all important internal resistance is reduced as a result of dispensing with it. 

Image

None of these cells have a PTC. They reduce performance, because it's a temperature sensitive resistor in series with the cell.
The CID is basically an internal pressure tripped fuse.
When the cell is extremely hot and pressure rises (electrolyte boiling) the CID will can cut off the cell, permanently disabling it. 
http://endless-sphere.com/forums/viewtopic.php?f=14&t=70147&start=25

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PTC probably stands for positive temperature coefficient. There are also NTC resistors.

IIRC the main drawback of high performance PTC (ie robust and of very low resistance) is it's a heavily patented device, thus quite expensive, then a no-go in high volume, low cost battery cells. So installing a fuse (heck, "current interrupt device" sounds so much more trendy) instead is a good way to drive down costs, that counts too. Anyway, there goes the reasonning some years ago, maybe things have changed.

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 I wonder if they could be paralleled.

Nice find dmethvin !

Yes you can. Any thermally positive coefficent component can be paralleled, like mosfets, and of course PTC fuses. Two paralleled PTC must just be enough space to have the double power dissipation if you want to double the ratings (if you stick them together, power dissipation is not at its best => you won't maximize the rating of each PTC).

I must think to install one of these some day to give them a try. Three should probably ok on a Firewheel.

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Someone could help me to shunt the Airwheel q3 with 260wh lion battery. ( there are 2 controller bms because the batterys  are 2 in parallel). I need to shunt both?

in this photo you can see my  fingers indicated the bypass (b- to p-) is the right ?

image.jpg

 

This is another picture

image.jpg

Edited by peppuzzo

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Someone could help me to shunt the Airwheel q3 with 260wh lion battery. ( there are 2 controller bms because the batterys  are 2 in parallel). I need to shunt both?

in this photo you can see my  fingers indicated the bypass (b- to p-) is the right ?

1) No, shunt just one pack, it's enough to prevent faceplant.

2) yes, that's it. b- and p- correspond to where your 2 fingers are. Connect them and shunt done.

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Thansk hobby16, 

for fail safe i have shunted both. Now seem works fine. I hope also in future !!!

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Hi guys im.new in this forum..  Can i please get some opinions,  my wheel cutted off while I was riding and I felt on the floor. Before it use to do it after a bit of riding it, when i turned off and then tried to turn it on, it started beeping and losing stability that I couldn't get on it to ride it. 

But now it does it while im riding it. 

 

Should I shunt the BMS or is it a main board problem? 

@hobby16

Hi guys im.new in this forum..  Can i please get some opinions,  my wheel cutted off while I was riding and I felt on the floor. Before it use to do it after a bit of riding it, when i turned off and then tried to turn it on, it started beeping and losing stability that I couldn't get on it to ride it. 

But now it does it while im riding it. 

 

Should I shunt the BMS or is it a main board problem? 

@hobby16

Hi guys im.new in this forum..  Can i please get some opinions,  my wheel cutted off while I was riding and I felt on the floor. Before it use to do it after a bit of riding it, when i turned off and then tried to turn it on, it started beeping and losing stability that I couldn't get on it to ride it. 

But now it does it while im riding it. 

 

Should I shunt the BMS or is it a main board problem? 

 

_20150724_171034.JPG

Edited by Aurita Fragale

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Should I shunt the BMS or is it a main board problem? 

@hobby16

Hi,

Even if it were a mainboard problem, your BMS still needs to be shunted. Problem solving, one step at a time.

Shunt to two (not three) midle mosfet like this http://www.wepix.net/image/nfQXYZwk/image1.png

Then let see if the problem persists (be prudent while testing, please).

What is the model of your wheel ? If you have more pictures, I am interested.

Edited by hobby16

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Hi,

Even if it's a mainboard problem, your BMS needs to be shunted. One step at a time.

Shunt to two (not three) midle mosfet like this http://www.wepix.net/image/nfQXYZwk/image1.png

Then let see if the problem persists (be prudent while testing, please).

thanks for your quick reply @hobby16,  but my battery has 4 mosfets.. Which one do I need to shunt?.  i have posted the picture above.. Thanks a lot 

Hi guys im.new in this forum..  Can i please get some opinions,  my wheel cutted off while I was riding and I felt on the floor. Before it use to do it after a bit of riding it, when i turned off and then tried to turn it on, it started beeping and losing stability that I couldn't get on it to ride it. 

But now it does it while im riding it. 

 

Should I shunt the BMS or is it a main board problem? 

 

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thanks for your quick reply @hobby16,  but my battery has 4 mosfets.. Which one do I need to shunt?.  i have posted the picture above.. Thanks a lot

 

Shunt the two in the middle

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Shunt the two in the middle

sorry to ask again @hobby16 but, I'm a bit confused cuz in the forum the ia this pic that is exactly like my battery and they shunt 3 mosfets and not the last one on the right..  should i do the 2 in the middle o 3 like this picture?  

Screenshot_2015-07-24-18-16-18~2.jpg

Edited by AURITA

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Hi,

Even if it were a mainboard problem, your BMS still needs to be shunted. Problem solving, one step at a time.

Shunt to two (not three) midle mosfet like this http://www.wepix.net/image/nfQXYZwk/image1.png

Then let see if the problem persists (be prudent while testing, please).

What is the model of your wheel ? If you have more pictures, I am interested.

i have a Weerda wheel with a battery: Samsung  VC3 176WH 2900mAH.. Here u have the pic @hobby16

_20150724_172002.JPG

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sorry to ask again @hobby16 but, I'm a bit confused cuz in the forum the ia this pic that is exactly like my battery and they shunt 3 mosfets and not the last one on the right..  should i do the 2 in the middle o 3 like this picture? 

There are 4 mosfets in each case but the 2 boards are not "exactly" alike.

So shunt just the 2 in the middle of your 4 mosfets. If in doubt, make the shunt (you risk absolutely nothing) take a picture and ask confirmation.

BTW, thank you for your picture, I didn't know the "weerda". The pedals seem huge !

Is it a 14" or 16" wheel ? Does the cover of the battery compartment open from outside or you must open the 2 half housings to have access to it ?

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There are 4 mosfets in each case but the 2 boards are not "exactly" alike.

So shunt just the 2 in the middle of your 4 mosfets. If in doubt, make the shunt (you risk absolutely nothing) take a picture and ask confirmation.

BTW, thank you for your picture, I didn't know the "weerda". The pedals seem huge !

Is it a 14" or 16" wheel ? Does the cover of the battery compartment open from outside or you must open the 2 half housings to have access to it ?

it is a 14" wheel and the pedal re very comfortable..  yes, The only annoying thing is that to open the battery compartment you must open the 2 half..  

Have problem of the cutt off so couldn't really try for a long distance..  But it is very nice to ride it. @hobby16

1437766350799-600856722.jpg

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Hi Peppuzzo, maybe you shoul think about redoing the soldering! Its not to be seen cleary on the foto, but it could be a bad solder joint ( https://learn.adafruit.com/adafruit-guide-excellent-soldering/common-problems )

I known,

Unfortunately my soldering iron is a crap. I will  buy a professional one  as soon as possible.

Thanks for the guide!

Edited by peppuzzo

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it is a 14" wheel and the pedal re very comfortable..  yes, The only annoying thing is that to open the battery compartment you must open the 2 half..  

Have problem of the cutt off so couldn't really try for a long distance..  But it is very nice to ride it

Does the cutoff occur always like that since the beginning or after some use ?

A frequent cut off may have 2 causes :

1) a crappy BMS => the shunt will solve the problem, entirely

2) one or several defective cells => the shunt will just make the ride safer but your range will take a hit.

After shunting, check as accurately as possible the mileage possible with a full charge (of course, with a Charge Doctor, it would be much easier to diagnose). If you have about 12Wh/km (meaning you can ride about 15km with your 176Wh battery), the battery is fine, you have case 1, problem solved !

If you have 2), well, ask the guaranty...

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Does the cutoff occur always like that since the beginning or after some use ?

A frequent cut off may have 2 causes :

1) a crappy BMS => the shunt will solve the problem, entirely

2) one or several defective cells => the shunt will just make the ride safer but your range will take a hit.

After shunting, check as accurately as possible the mileage possible with a full charge (of course, with a Charge Doctor, it would be much easier to diagnose). If you have about 12Wh/km (meaning you can ride about 15km with your 176Wh battery), the battery is fine, you have case 1, problem solved !

If you have 2), well, ask the guaranty...

after 4 days of receiving it new from the seller, after some use i noticed that there were 2 lights of battery left out of 4.. And it starting doing the cutt off when I stopped to ride after a while of training,  then i turned off and i wanted to ride again soo turned on and it started doing that..

Will do the shunt today and will let u know how is it..

@hobby16

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after 4 days of receiving it new from the seller, after some use i noticed that there were 2 lights of battery left out of 4.. And it starting doing the cutt off when I stopped to ride after a while of training,  then i turned off and i wanted to ride again soo turned on and it started doing that..

Will do the shunt today and will let u know how is it..

@hobby16

hi.. Done the shunt today..  Will try to charge the battery now..  Is okay like this?  @hobby16

14378300855051602880810.jpg

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