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BMS Discharge bypass / charge only bms


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It's my opinion that a lot of our unexplained cutouts are bms related.  I've experienced bms related cutoffs in both my Onewheels and a couple of EUC's.  I had one of the first V11's and it had a faulty left side battery pack that caused a cutout.  A little about me.  I've been into the EUC / Onewheel arena for two years now.  I'm 57 and have been an electronics tech for the last 28 years and I'm also an EV enthusiast.  Not counting the dozen EUC's and Onewheels I've owned, I've also owned 6 full size EV's.  Two of those were conversions one of which I completely rebuilt and upgraded from lead to lithium batteries.  And yes I'm a very big Tesla fan although I haven't had the pleasure of owning one yet. : /  And shhh, don't tell anyone, I admit when I'm wrong and don't know everything. : p

I've attached a pic of a pretty standard bms design used in many EUC's, Eskates, etc.  The first bms shows a standard wiring config.  The only item worthy of noting not in the pic of this bms is a battery temp sensor which is usually installed.  The 2nd bms shows a bms discharge bypass config.  The bypass makes this a charge only bms.  From research online, this appears to be a fairly common practice in the e skate and other communities.  The idea behind this is to bypass the discharge section of the bms to get full power from the battery.  They can use a bms that has a lower amp rating but still get the higher amps desired because of the bms bypass wiring.  Also an added benefit is the ability to avoid the bms shutting off power and thereby avoiding the not so pleasurable faceplant. The bms balancing function remains when charging and notification of fault conditions, depending on the features inherent to the bms, still function.

I believe that the "normal" config, running the battery power exclusively through the bms, poses a flaw.  It is a single point of failure.  When some fault condition appears whether valid or not, the bms cuts power = faceplant.  Yes I do understand the function of a bms and that it's designed to protect the battery/circuitry but this is done at the expense of the rider.  And please let's not get into an ethical debate on whether you should or shouldn't have a bms.  Omg, not here lol.

So, whew, this is a discussion about this bms bypass topic and also to see if anyone has done this kind of mod to their EUC.  I had been researching and modding Onewheels since I started riding them but hadn't gotten to researching the config of the bms in EUC's.  I ran across this info a couple months ago and have been researching it.  I plan to pull the battery on my V10f or buy one, and dissect it to see how it's configured.  I've already done basic circuit analysis on this mod and there's also a number of people already running this config so I don't personally have to do a bunch of field testing.  Buuuttt, I love electronics so can't help getting in there and doing my own easter egging.

986111232_BMSBypass.thumb.png.fba83a141e895c93b9ee06a366b0f9e2.png.

Anyway, what's your opinion on this bypass topic.  And by the way the text in the middle of the bms pics, "My thinking..."  was part of the pic and not my entry.

 

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Posted (edited)

I wanted to add that the pics were pulled from the internet at this URL.  You know, the whole providing references dealio. : p

https://forum.esk8.news/t/charge-only-bms-wiring-advice/14288

Edited by Senior Coffee
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2 hours ago, Senior Coffee said:

Anyway, what's your opinion on this bypass topic.

Just don't do it with (modern) EUCs!

It was a big topic in the beginning when ebike bms were used which cut off power at single cell undervoltage! @hobby16 had a great topic regarding how to bypass this protection for the different wheels.

Nowadays the bms have "just" a short circuit protection and fuses at (near) the motherboard. They do not hinder normal operation or endager us riders. If these protections are triggered it was in the aftermath of some accident or the wheel was just "in progress of destroying itself" anyhow.

Battery currents are much lower than motor currents! One does nit need 200A of battery current as it is reported as motor current for shermann/gw wheels!

Such used fuses with roughly ?30-50A? per battery pack are sufficient. Maybe sherman already has some slightly stronger ones?

There is a chance that one of the reasons of gotway leading fire reports could be the missing short curcuit protection/fuse? Beside water ingress, QC, beeing one of the most powerful wheels...

Just watch any video of a sherman motherboard frying - imho very impressive. Even with fuses and short circuit protection - consider this with just molten battery wires stopping the desaster...

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2 hours ago, Chriull said:

Such used fuses with roughly ?30-50A? per battery pack are sufficient. Maybe sherman already has some slightly stronger ones?

For info my very recent Sherman came with 30A fuses for each side.

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1 hour ago, Planemo said:

my very recent Sherman came with 30A fuses for each side.

Only 30A?

Each side is 5P and GA cells are rated for 10A continuous discharge... that pack is safe at 50A.

Fusing for 30A seems like a desperate attempt to reduce incidence of arcing during board failure.
I'd rather "fuse to protect the pack" with the benefit of more headroom during normal riding, than risk a nuisance blow and cutout crash (e.g.: if one pack became open-circuit, a single 30A circuit may be too little to sustain 45mph).

p.s.- Does Sherman produce an error when one pack is unplugged?

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I have to take a look "under the skirt" as the saying goes.  Very interested to see what kind of BMS these modern wheels have.  I've been inside every Onewheel made and all of the power runs thru those bms's.  Doing the bypass on my Onewheels netted a much better riding experience in increased power, range and peace of mind knowing that I wasn't going to get a random shut off.  A quiet ride is how I've dubbed it.

I saw some V10 packs on eBay.  After researching the bms I'll wire up the wheel so I can use the extra battery pack in parallel with my V10F.

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I really wonder if this is what happened to me on my V8 Friday afternoon, the battery wasn't low, but if one cell briefly dipped below the minimum voltage, the BMS could have cut the power on me. I'm not sure what exactly caused my cutout, but I have the roadrash to remember it by! 😅

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4 hours ago, Senior Coffee said:

all of the power runs thru those bms's.

Running through the BMS isn’t in itself a bad thing. As @Chriull wrote, this topic would’ve been very current and relevant some 5 years ago. Now I don’t know the exact protection behaviors of the current BMSs, but for example the V11 has an unique system where the two packs aren’t even hard wired together. If one pack goes bad or even just drops in voltage, the wheel continues to operate with one battery only.

And in other wheels as well, if a single BMS cuts off due to a battery cell issue, it does it so that the other pack can continue normal operation of the wheel. But a single cell shorting to 0V doesn’t even yet launch that. My 16S (2017) had a cell group fall down to 0V at 4000km ridden. The wheel continued normal operation. It should’ve of course warned the rider that the available peak power has been decreased, which the V11 finally does. But cutting off the whole wheel it did not do.

2 hours ago, FlyingWigs said:

but if one cell briefly dipped below the minimum voltage, the BMS could have cut the power on me.

I’m pretty sure the V8 doesn’t do that either. IIRC the Ninebot E-series was the last one that had a BMS that would cut out from a single cell undervoltage.

 

EDIT: The modern BMSs have a communication line that can cutoff the charge portion of both BMSs in case a single cell group goes above 4.25V. But I don’t think that cuts off the discharge sections either. Not perfectly sure about that though.

Edited by mrelwood
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14 hours ago, RagingGrandpa said:

Only 30A?

Yep, definately. I must admit I was expecting to see 50A, but only because I thought I had read (some time back) that they were.

14 hours ago, RagingGrandpa said:

Each side is 5P and GA cells are rated for 10A continuous discharge... that pack is safe at 50A.

Yep, although I don't know 100% that Veteran (at least the latest ones) are using Sanyo GA's. Do you know if someone who has stripped the packs?

Either way, I am aware the pack is probably good for 50A, but do they really need a fuse to match? I'm not convinced.

14 hours ago, RagingGrandpa said:

 (e.g.: if one pack became open-circuit, a single 30A circuit may be too little to sustain 45mph).

OK, but in your example above, would a single '50A' 5P pack be enough to sustain 45mph anyway, at what would be a sudden and immediate draw without the slightest dip....? I'm not going to be the guinea pig :)

14 hours ago, RagingGrandpa said:

p.s.- Does Sherman produce an error when one pack is unplugged?

Not sure, I haven't tried it. I don't think so but would be interesting to find out. Maybe I will sometime.

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18 hours ago, RagingGrandpa said:

Only 30A?

3 hours ago, Planemo said:

Yep, definately. I must admit I was expecting to see 50A, but only because I thought I had read (some time back) that they were.

The fuses when originally added as a protection to the Sherman, following Marty's OHH burned board and eWheels intervention, were indeed 50A up until a recent decrease down to 30A by Veteran.  I know this to be a fact following my 2x control board replacements that were both sent out including fuses to retrofit/replace the existing 50A my wheel already had fitted. 

The first board arrived in January with 50A, the second in April with 30A so the change was put in place sometime between January and April, although still having No clear direction from Veteran on the reasoning behind this, I have opted to stick with the 50A fuses at this stage!

Edited by fbhb
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1 minute ago, fbhb said:

I have opted to stick with the 50A fuses!

I think I would as well tbh. If a battery short is gonna blow a 30A fuse, it will blow a 50A just as easily, given the sheer power the packs can put out! If I could find the 50's anywhere I would swop them out, but they seem to be a slightly unusual design. I could of course just change the whole fuse/box so that might be the option.

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11 minutes ago, Planemo said:

If I could find the 50's anywhere I would swop them out, but they seem to be a slightly unusual design.

You could try and send @Marty Backe a PM and ask him if he has a spare set of 50's.  I'm sure he had some sent to him by Veteran and opted Not to fit them IIRC, so it could be worth hitting him up.  Sure he would put something that small and light in the post from the States to you, if you ask nicely :D

Edited by fbhb
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Not sure I could ask that tbf, even if he were to let them go for free, shipping would be at least 10USD I would think. That said, prob still cheaper than trying to find something else so I guess I could always paypal him whatever costs he needed.

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18 minutes ago, Planemo said:

That said, prob still cheaper than trying to find something else so I guess I could always paypal him whatever costs he needed.

Yeah, yeah I did mean for you to cover his shipping if required, but if he does have them I'm sure Marty would let you have them for Free.  It's got to be worth sending him a PM to find out for sure.

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On 6/10/2021 at 1:12 AM, Planemo said:

I am aware the pack is probably good for 50A, but do they really need a fuse to match?

That's 50A continuous.
Like: sustained for 15 minutes, after which the cells will be depleted.

100A pulsed current (e.g.: 100ms) would be fine too, for each 5p pack.

Fusing for 50A certainly protects the pack; fusing with even lower values adds additional protection from board arcing, but go too low and you're risking cutout...
More discussion on fuse values here.

Veteran seems to recommend 2x30A now, which I think is perfectly appropriate.
Older models had 2x50A, which seems slightly high but still useful.

Given all the other risks involved with reworking your EUC's wire harness, I think it's better for owners to "leave your wiring alone" rather than mess with it, regardless of which fuse your Sherman has.

Each cell is capable of flowing 40A+ through a bolted short (200A per pack), and that's the most important thing to avoid.
Any fuse value of [60A per pack] or below, should be effective at this minimum level of cell protection.

 

Edited by RagingGrandpa
Analysis led me to conclude 30A fuses are great!
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17 hours ago, mrelwood said:

Running through the BMS isn’t in itself a bad thing. As @Chriull wrote, this topic would’ve been very current and relevant some 5 years ago. Now I don’t know the exact protection behaviors of the current BMSs, but for example the V11 has an unique system where the two packs aren’t even hard wired together. If one pack goes bad or even just drops in voltage, the wheel continues to operate with one battery only.

And in other wheels as well, if a single BMS cuts off due to a battery cell issue, it does it so that the other pack can continue normal operation of the wheel. But a single cell shorting to 0V doesn’t even yet launch that. My 16S (2017) had a cell group fall down to 0V at 4000km ridden. The wheel continued normal operation. It should’ve of course warned the rider that the available peak power has been decreased, which the V11 finally does. But cutting off the whole wheel it did not do.

I’m pretty sure the V8 doesn’t do that either. IIRC the Ninebot E-series was the last one that had a BMS that would cut out from a single cell undervoltage.

 

EDIT: The modern BMSs have a communication line that can cutoff the charge portion of both BMSs in case a single cell group goes above 4.25V. But I don’t think that cuts off the discharge sections either. Not perfectly sure about that though.

I had a V11.  One of the battery packs didn't work at all, no voltage when I checked.  It cutout on me going very slow and only 7 miles on the wheel.  Like I said I'm looking forward to dissecting one of the modern battery packs and seeing what type of BMS they're using and it's design.

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43 minutes ago, Senior Coffee said:

I had a V11.  One of the battery packs didn't work at all, no voltage when I checked.  It cutout on me

I remember reading about your case. You had one of the first V11’s IIRC.

I have disassembled my V11 numerous times, and on many of those occasions I have checked the operation by connecting one battery only. As expected, the wheel functions normally, other than the alert “please repair”.

If the battery’s output voltage or individual cells’ voltage drops dangerously low, the pack’s BMS should disconnect the output. Measuring 0V at the connectors in that situation makes perfect sense, even if the pack itself had some voltage left.

 If one pack’s BMS disconnects the battery, any wheel from the last 4 or so years should remain operational with a single pack. The max power of the wheel of course drops drastically, but the wheel itself should not turn off.

Both of these point in the direction of your V11 having had another issue that would’ve been the actual cause for the cutoff. Assuming of course that you didn’t overlean the crippled wheel. A bug or an early version oversight in the FW can’t be excluded. I think.

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2 hours ago, mrelwood said:

If one pack’s BMS disconnects the battery, any wheel from the last 4 or so years should remain operational with a single pack.

Maybe. I had the unfortunate experience where a new pack experienced failure during charging and the pack signaled something that instantly shut down the control board. I don't have schematics or firmware flow diagrams, but in this case whatever the bad pack told the control board was interpreted as "turn off NOW". No lights, no bluetooth but also no fire, no smoke, and no damage so all was good in the end (after replacing the duff pack).

While the wheel (16x) runs just fine on 1 battery, I'm of the mind that a BMS signal can indeed turn a 16x off.

Edited by Tawpie
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12 hours ago, mrelwood said:
 

I remember reading about your case. You had one of the first V11’s IIRC.

I have disassembled my V11 numerous times, and on many of those occasions I have checked the operation by connecting one battery only. As expected, the wheel functions normally, other than the alert “please repair”.

If the battery’s output voltage or individual cells’ voltage drops dangerously low, the pack’s BMS should disconnect the output. Measuring 0V at the connectors in that situation makes perfect sense, even if the pack itself had some voltage left.

 If one pack’s BMS disconnects the battery, any wheel from the last 4 or so years should remain operational with a single pack. The max power of the wheel of course drops drastically, but the wheel itself should not turn off.

Both of these point in the direction of your V11 having had another issue that would’ve been the actual cause for the cutoff. Assuming of course that you didn’t overlean the crippled wheel. A bug or an early version oversight in the FW can’t be excluded. I think.

Yeah I didn't overlean for sure.  My riding weight is 220#.  Because of that I always ride cautiously and take it very slow on hills and there are a lot of them here in N. Virginia.  I literally was going about 7 or 8 mph and wasn't accelerating.  I agree with you that there was another issue in the wheel.  The single battery should have plenty of capacity for the conditions I was in at that moment.  I was only 7 miles into the very first ride and was riding very slowly and adjusting to the different riding characteristics.  The app showed the packs at around 90% charge which should have been a clue.  10% discharge after only 7 miles.  Anyway I'll never own another V11.  Not because of my event but because I really didn't find it all that appealing.

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6 hours ago, Senior Coffee said:

10% discharge after only 7 miles. 

That would total almost 70 miles! That’s a perfectly normal discharge rate. Perhaps even a bit slow.

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