Disconnect the battery from all loads before working on a BMS

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Disconnect the battery from all loads before working on a BMS

Elithion
All too often a BMS is blown because the user worked on it without first disconnecting the battery from all loads. This video (http://www.youtube.com/watch?v=9RVYLvn-jL8) shows how, when you open a link between two adjacent batteries, without first disconnecting all the loads, the entire pack voltage appears across the gap, frying your BMS.
Davide Andrea
Elithion
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Re: Disconnect the battery from all loads before working on a BMS

David Nelson-5
And that also shows why a BMS has to be able to safely handle a cell
failing open, which can and has happened.

On Sat, Aug 18, 2012 at 12:11 PM, Elithion <[hidden email]> wrote:
> All too often a BMS is blown because the user worked on it without first
> disconnecting the battery from all loads. This video
> (http://www.youtube.com/watch?v=9RVYLvn-jL8) shows how, when you open a link
> between two adjacent batteries, without first disconnecting all the loads,
> the entire pack voltage appears across the gap, frying your BMS.



--
David D. Nelson
http://evalbum.com/1328
http://2003gizmo.blogspot.com
http://www.levforum.com

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Re: Disconnect the battery from all loads before working on a BMS

carrott
In reply to this post by Elithion
On 19/08/12 07:11, Elithion wrote:
> All too often a BMS is blown because the user worked on it without first
> disconnecting the battery from all loads. This video
> (http://www.youtube.com/watch?v=9RVYLvn-jL8) shows how, when you open a link
> between two adjacent batteries, without first disconnecting all the loads,
> the entire pack voltage appears across the gap, frying your BMS.

Been there done that. 10mA from my EVision power supply at 200V
destroyed all of the semiconductors! The BMS fuses didn't blow because
it was current limited by the EVision.

If you aren't wearing gloves you're also at high risk of electrocution
-- before you open the circuit the two sides may be completely safe to
touch (or not if you have a ground fault), but as soon as they separate,
the full pack voltage appears and you get zapped. Last time I did this,
it felt like the EVision power supply takes a sip of power about 10
times per second and it wasn't pleasant.

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Re: Disconnect the battery from all loads before working on a BMS

Lee Hart
In reply to this post by David Nelson-5
Elithion <[hidden email]> wrote:
>> All too often a BMS is blown because the user worked on it without first
>> disconnecting the battery from all loads. This video
>> (http://www.youtube.com/watch?v=9RVYLvn-jL8) shows how, when you open a link
>> between two adjacent batteries, without first disconnecting all the loads,
>> the entire pack voltage appears across the gap, frying your BMS.
David Nelson wrote:
> And that also shows why a BMS has to be able to safely handle a cell
> failing open, which can and has happened.
>  
Yes, full pack voltage can indeed appear across the open! But it seems to me that a BMS should be able to survive a cell or inter-cell jumper failing open. It is a relatively common fault condition.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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Re: Disconnect the battery from all loads before working on a BMS

carrott
On 19/08/12 11:58, Lee Hart wrote:
> Yes, full pack voltage can indeed appear across the open! But it seems
 > to me that a BMS should be able to survive a cell or inter-cell
 > jumper failing open. It is a relatively common fault condition.

Indeed.

Is it possible in with a distributed BMS where each cell's monitor is
powered by the cell? I haven't actually tried introducing enough
resistance to make a crowbar protect the BMS without overheating, but I
guess it could work without affecting voltage measurement accuracy too much.

However protecting the shunt circuit seems more difficult. If the shunt
was turned on it would immediately go overcurrent, if your transistor
was rated for the pack voltage you could turn it off, but how do you
know to turn it off? Watching the cell voltage and shutting down when it
crosses some upper threshold won't work because you have to average out
the charge current pulses to get a good voltage reading.

If you accept blowing the fuse as "surviving", then both these problems
get somewhat easier. If the external connection to the pack will draw
enough current to blow the fuse, your crowbar and shunt circuit only
need to survive long enough to blow the fuse. However the crowbar would
be difficult in my 10mA case, 10mA at 200V is 2W! The BMS is in series
with the EVision, so perhaps half the voltage would be dropped in each,
leaving 1W for the crowbar. 10mA won't blow the fuses.

How do you deal with this? Or a bigger fault, say 100mA? Still not
enough to blow the fuse but perhaps 10W dumped into the protection circuit.

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Re: Disconnect the battery from all loads before working on a BMS

gtyler54
In reply to this post by Lee Hart
I just had a look at a Prius ECU I have lying around, the prius has a link
between pack halves. Each voltage measurement circuit is independent, has a
dual opamp differential input stage, so any disconnection will not worry it.

-----Original Message-----
From: [hidden email] [mailto:[hidden email]] On Behalf
Of Lee Hart
Sent: Sunday, 19 August 2012 11:59 a.m.
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Disconnect the battery from all loads before working on
a BMS

Elithion <[hidden email]> wrote:
>> All too often a BMS is blown because the user worked on it without first
>> disconnecting the battery from all loads. This video
>> (http://www.youtube.com/watch?v=9RVYLvn-jL8) shows how, when you open a
link
>> between two adjacent batteries, without first disconnecting all the
loads,
>> the entire pack voltage appears across the gap, frying your BMS.
David Nelson wrote:
> And that also shows why a BMS has to be able to safely handle a cell
> failing open, which can and has happened.
>  
Yes, full pack voltage can indeed appear across the open! But it seems to me
that a BMS should be able to survive a cell or inter-cell jumper failing
open. It is a relatively common fault condition.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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"Zombie" BMS (was: Disconnect the battery from all loads before working on a BMS)

Bill Dube
In reply to this post by carrott
If a cell goes open circuit, or there is an open circuit introduced
between cells, the pack is no longer functional and needs to be
repaired. Thus, the BMS does not need to survive such an event. You
have repairs to make, so that can include repairing the BMS too. The
cost of repairing the BMS when parts of it are damaged due to this
sort of extreme event is likely much less than would be the cost of
making it robust enough to survive this level of abuse. It would be
nice if the _entire_ BMS did not fry, of course.

What does need to happen is the BMS needs to fail in a predictable,
safe, and _obvious_ manner. The BMS must signal the failure in some
way. If it is damaged, it must _not_ appear to be operational and
healthy. This "zombie" BMS operation after some sort of failure or
abuse is a serious problem. (Probably the root cause of some folks
swearing off BMS use entirely.)

Basically, you need to design it to survive a specific abuse/fault,
or you need it to go "dark" if it is damaged in any way. Not so easy
to do, but "zombie" BMS operation can be a lot like a horror movie. ;-)

Bill D.



At 06:58 PM 8/18/2012, you wrote:

>On 19/08/12 11:58, Lee Hart wrote:
> > Yes, full pack voltage can indeed appear across the open! But it seems
>  > to me that a BMS should be able to survive a cell or inter-cell
>  > jumper failing open. It is a relatively common fault condition.
>
>Indeed.
>
>Is it possible in with a distributed BMS where each cell's monitor is
>powered by the cell? I haven't actually tried introducing enough
>resistance to make a crowbar protect the BMS without overheating, but I
>guess it could work without affecting voltage measurement accuracy too much.

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Re: Disconnect the battery from all loads before working on a BMS

Martin WINLOW
In reply to this post by David Nelson-5
I'm sure it is possible but not wildly likely in the scheme of things. If you are using a distributed system (which I prefer) I would have thought that any fault in the cell sufficient to eventually cause it to fail 'open-circuit' would give oodles of warning.

Regards, Martin Winlow
Herts, UK
http://www.evalbum.com/2092
www.winlow.co.uk


On 18 Aug 2012, at 21:32, David Nelson wrote:

> And that also shows why a BMS has to be able to safely handle a cell
> failing open, which can and has happened.
>
> On Sat, Aug 18, 2012 at 12:11 PM, Elithion <[hidden email]> wrote:
>> All too often a BMS is blown because the user worked on it without first
>> disconnecting the battery from all loads. This video
>> (http://www.youtube.com/watch?v=9RVYLvn-jL8) shows how, when you open a link
>> between two adjacent batteries, without first disconnecting all the loads,
>> the entire pack voltage appears across the gap, frying your BMS.
>
>
>
> --
> David D. Nelson
>





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Re: Disconnect the battery from all loads before working on a BMS

Lee Hart
In reply to this post by carrott
  Lee Hart wrote:
> > Yes, full pack voltage can indeed appear across the open! But it
> > seems to me that a BMS should be able to survive a cell or
> > inter-cell jumper failing open. It is a relatively common fault
> > condition.

Tom Parker wrote:
>  Is it possible in with a distributed BMS where each cell's monitor is
>  powered by the cell?

Yes. The most straightforward way is to have a fuse in series, and a
zener diode or other clamping device across the actual BMS circuitry
itself. If the cell gets reversed, or goes open and the voltage goes
above the zener's breakdown voltage, the zener conducts and blows the fuse.

The fuse and zener need to be properly chosen. The fuse needs a DC
voltage rating at least as high as the pack voltage. The zener needs to
have a zener voltage high enough not to affect normal operation, but low
enough so nothing else on the BMS board will be damaged. The zener also
needs an I^2T rating higher than the fuse,  so the fuse opens before the
zener fails.
 
>  I haven't actually tried introducing enough
>  resistance to make a crowbar protect the BMS without overheating

A crowbar is another approach that could work. They are harder to
design, but have the advantage of faster response and cheaper fuses
(often just a carefully selected piece of wire).

>  I guess it could work without affecting voltage measurement accuracy
>  too much.

A fuse or fusible link wire normally has negligible resistance. I can't
see it affecting accuracy.

>  protecting the shunt circuit seems more difficult.

Not really. The zener or crowbar protects the shunt circuit as well as
it would any measurement circuitry.
 
>  If you accept blowing the fuse as "surviving", then both these
>  problems get somewhat easier. If the external connection to the pack
>  will draw enough current to blow the fuse, your crowbar and shunt
>  circuit only need to survive long enough to blow the fuse. However
>  the crowbar would be difficult in my 10mA case, 10mA at 200V is 2W!

Here is the scenario I think you are describing: You have a 200v pack.
Each cell has a little BMS board connected across it. You also have an
EVision powered by the 200v pack that draws about 10ma. Now suppose a
cell fails open (the cell failed internally, or you have a loose
terminal, or maybe you are deliberately loosening the terminal during
servicing). The circuit now consists of all cells minus one
(200v-3v=197v) in series with a load that passes about 10ma, connected
across that open cell.

If the BMS board draws negligible current, then essentially the full
197v appears across it. BANG!

If the BMS board draws a current similar to the EVision (10ma), then it
forms a voltage divider. About half the voltage is across the EVision,
and the other half across the BMS board (which will probably still
murder it).

If you have a fuse and zener diode as I described above, the current is
too low to blow the fuse. But the zener will clamp the voltage to a safe
value (say, 5v). 5v at 10ma is only 0.05 watts, so the zener won't fail.
Dangerous voltage won't appear across the open cell, the BMS won't be
harmed, and the fuse and zener will also be unharmed.

The fault current would have to exceed whatever the zener can handle on
a continuous basis. A 5.1v 1watt zener can stand 200ma, so you would
pick a fuse rated at less than this. 50ma fuses are available, but a bit
scarce and expensive. Realistically, I would use a 5watt zener which
could handle up to an amp, and a 0.5a fuse which is cheaper and easier
to get.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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Re: Disconnect the battery from all loads before working on a BMS

Bill Dube
A high-voltage DC fuse is not small and not inexpensive. Often these
fuses cost several dollars. On a high voltage pack, this one
component on each cell can add $200 to the cost of the BMS, plus the
mount, plus the assembly, plus the board space.

If you plan to sense voltage while by-pass current is flowing, the
fuse resistance may not be as "negligible" as you might think.

Bill D.


>Yes. The most straightforward way is to have a fuse in series, and a
>zener diode or other clamping device across the actual BMS circuitry
>itself. If the cell gets reversed, or goes open and the voltage goes
>above the zener's breakdown voltage, the zener conducts and blows the fuse.
>
>The fuse and zener need to be properly chosen. The fuse needs a DC
>voltage rating at least as high as the pack voltage. The zener needs to
>have a zener voltage high enough not to affect normal operation, but low
>enough so nothing else on the BMS board will be damaged. The zener also
>needs an I^2T rating higher than the fuse,  so the fuse opens before the
>zener fails.
>
> >  I haven't actually tried introducing enough
> >  resistance to make a crowbar protect the BMS without overheating
>
>A crowbar is another approach that could work. They are harder to
>design, but have the advantage of faster response and cheaper fuses
>(often just a carefully selected piece of wire).
>
> >  I guess it could work without affecting voltage measurement accuracy
> >  too much.
>
>A fuse or fusible link wire normally has negligible resistance. I can't
>see it affecting accuracy.
>
> >  protecting the shunt circuit seems more difficult.
>
>Not really. The zener or crowbar protects the shunt circuit as well as
>it would any measurement circuitry.
>
> >  If you accept blowing the fuse as "surviving", then both these
> >  problems get somewhat easier. If the external connection to the pack
> >  will draw enough current to blow the fuse, your crowbar and shunt
> >  circuit only need to survive long enough to blow the fuse. However
> >  the crowbar would be difficult in my 10mA case, 10mA at 200V is 2W!
>
>Here is the scenario I think you are describing: You have a 200v pack.
>Each cell has a little BMS board connected across it. You also have an
>EVision powered by the 200v pack that draws about 10ma. Now suppose a
>cell fails open (the cell failed internally, or you have a loose
>terminal, or maybe you are deliberately loosening the terminal during
>servicing). The circuit now consists of all cells minus one
>(200v-3v=197v) in series with a load that passes about 10ma, connected
>across that open cell.
>
>If the BMS board draws negligible current, then essentially the full
>197v appears across it. BANG!
>
>If the BMS board draws a current similar to the EVision (10ma), then it
>forms a voltage divider. About half the voltage is across the EVision,
>and the other half across the BMS board (which will probably still
>murder it).
>
>If you have a fuse and zener diode as I described above, the current is
>too low to blow the fuse. But the zener will clamp the voltage to a safe
>value (say, 5v). 5v at 10ma is only 0.05 watts, so the zener won't fail.
>Dangerous voltage won't appear across the open cell, the BMS won't be
>harmed, and the fuse and zener will also be unharmed.
>
>The fault current would have to exceed whatever the zener can handle on
>a continuous basis. A 5.1v 1watt zener can stand 200ma, so you would
>pick a fuse rated at less than this. 50ma fuses are available, but a bit
>scarce and expensive. Realistically, I would use a 5watt zener which
>could handle up to an amp, and a 0.5a fuse which is cheaper and easier
>to get.
>
>--
>Ring the bells that still can ring
>Forget the perfect offering
>There is a crack in everything
>That's how the light gets in -- Leonard Cohen
>--
>Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
>
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Re: "Zombie" BMS (was: Disconnect the battery from all loads before working on a BMS)

Evan Tuer
In reply to this post by Bill Dube
I agree with Bill that it's not something the BMS needs to be tolerant
of and keep working - it just needs to fail in a way that doesn't
cause another problem or hazard.

Since I put my lithium powered EV's pack together 3 years ago, I've
not had cause to dismantle the pack or break the circuit via one of
the 4 inter-pack fuses.   I consider the BMS expendable if something
happened to cause a fuse or other connection to open while the car is
in use (i.e. a crash).

One nice thing about a centralised BMS - if it does fry all the
components, you just plug in another one!



On Sun, Aug 19, 2012 at 4:20 AM, Bill Dube <[hidden email]> wrote:

> If a cell goes open circuit, or there is an open circuit introduced
> between cells, the pack is no longer functional and needs to be
> repaired. Thus, the BMS does not need to survive such an event. You
> have repairs to make, so that can include repairing the BMS too. The
> cost of repairing the BMS when parts of it are damaged due to this
> sort of extreme event is likely much less than would be the cost of
> making it robust enough to survive this level of abuse. It would be
> nice if the _entire_ BMS did not fry, of course.
>
> What does need to happen is the BMS needs to fail in a predictable,
> safe, and _obvious_ manner. The BMS must signal the failure in some
> way. If it is damaged, it must _not_ appear to be operational and
> healthy. This "zombie" BMS operation after some sort of failure or
> abuse is a serious problem. (Probably the root cause of some folks
> swearing off BMS use entirely.)
>
> Basically, you need to design it to survive a specific abuse/fault,
> or you need it to go "dark" if it is damaged in any way. Not so easy
> to do, but "zombie" BMS operation can be a lot like a horror movie. ;-)
>
> Bill D.
>
>
>
> At 06:58 PM 8/18/2012, you wrote:
>>On 19/08/12 11:58, Lee Hart wrote:
>> > Yes, full pack voltage can indeed appear across the open! But it seems
>>  > to me that a BMS should be able to survive a cell or inter-cell
>>  > jumper failing open. It is a relatively common fault condition.
>>
>>Indeed.
>>
>>Is it possible in with a distributed BMS where each cell's monitor is
>>powered by the cell? I haven't actually tried introducing enough
>>resistance to make a crowbar protect the BMS without overheating, but I
>>guess it could work without affecting voltage measurement accuracy too much.
>
> _______________________________________________
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Re: Disconnect the battery from all loads before working on a BMS

Martin WINLOW
In reply to this post by Bill Dube
Bill,

Sorry, but I just see it adding that much.  Why would you need a mount? If you are using surface mount components (as any modern commercial pcb would be, these days) a 250V SM fuse and 5.1V 1W SM zener can be had in single quantity from a good UK supplier (RS) for about US$1.  In the thousands I'm sure it would be considerably less - particularly so in the US.  Extra costs for a bit more PCB real estate and design would be negligible.

MW


On 19 Aug 2012, at 19:56, Bill Dube wrote:

> A high-voltage DC fuse is not small and not inexpensive. Often these
> fuses cost several dollars. On a high voltage pack, this one
> component on each cell can add $200 to the cost of the BMS, plus the
> mount, plus the assembly, plus the board space.
>
> If you plan to sense voltage while by-pass current is flowing, the
> fuse resistance may not be as "negligible" as you might think.
>
> Bill D.
>
>
>> Yes. The most straightforward way is to have a fuse in series, and a
>> zener diode or other clamping device across the actual BMS circuitry
>> itself. If the cell gets reversed, or goes open and the voltage goes
>> above the zener's breakdown voltage, the zener conducts and blows the fuse.
>>
>> The fuse and zener need to be properly chosen. The fuse needs a DC
>> voltage rating at least as high as the pack voltage. The zener needs to
>> have a zener voltage high enough not to affect normal operation, but low
>> enough so nothing else on the BMS board will be damaged. The zener also
>> needs an I^2T rating higher than the fuse,  so the fuse opens before the
>> zener fails.
>>
>>> I haven't actually tried introducing enough
>>> resistance to make a crowbar protect the BMS without overheating
>>
>> A crowbar is another approach that could work. They are harder to
>> design, but have the advantage of faster response and cheaper fuses
>> (often just a carefully selected piece of wire).
>>
>>> I guess it could work without affecting voltage measurement accuracy
>>> too much.
>>
>> A fuse or fusible link wire normally has negligible resistance. I can't
>> see it affecting accuracy.
>>
>>> protecting the shunt circuit seems more difficult.
>>
>> Not really. The zener or crowbar protects the shunt circuit as well as
>> it would any measurement circuitry.
>>
>>> If you accept blowing the fuse as "surviving", then both these
>>> problems get somewhat easier. If the external connection to the pack
>>> will draw enough current to blow the fuse, your crowbar and shunt
>>> circuit only need to survive long enough to blow the fuse. However
>>> the crowbar would be difficult in my 10mA case, 10mA at 200V is 2W!
>>
>> Here is the scenario I think you are describing: You have a 200v pack.
>> Each cell has a little BMS board connected across it. You also have an
>> EVision powered by the 200v pack that draws about 10ma. Now suppose a
>> cell fails open (the cell failed internally, or you have a loose
>> terminal, or maybe you are deliberately loosening the terminal during
>> servicing). The circuit now consists of all cells minus one
>> (200v-3v=197v) in series with a load that passes about 10ma, connected
>> across that open cell.
>>
>> If the BMS board draws negligible current, then essentially the full
>> 197v appears across it. BANG!
>>
>> If the BMS board draws a current similar to the EVision (10ma), then it
>> forms a voltage divider. About half the voltage is across the EVision,
>> and the other half across the BMS board (which will probably still
>> murder it).
>>
>> If you have a fuse and zener diode as I described above, the current is
>> too low to blow the fuse. But the zener will clamp the voltage to a safe
>> value (say, 5v). 5v at 10ma is only 0.05 watts, so the zener won't fail.
>> Dangerous voltage won't appear across the open cell, the BMS won't be
>> harmed, and the fuse and zener will also be unharmed.
>>
>> The fault current would have to exceed whatever the zener can handle on
>> a continuous basis. A 5.1v 1watt zener can stand 200ma, so you would
>> pick a fuse rated at less than this. 50ma fuses are available, but a bit
>> scarce and expensive. Realistically, I would use a 5watt zener which
>> could handle up to an amp, and a 0.5a fuse which is cheaper and easier
>> to get.
>>
>> --
>> Ring the bells that still can ring
>> Forget the perfect offering
>> There is a crack in everything
>> That's how the light gets in -- Leonard Cohen
>> --
>> Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
>>
>> _



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Re: Disconnect the battery from all loads before working on a BMS

Bill Dube
Looking in Digikey, a fast-blow 250 VDC surface mount fuse costs
$0.75, or more. For a 250 volt pack, this works out to 70+ fuses =>
$52. These are 11m long, and 4.6mm wide.

Also, pack voltages are getting closer to 400 volts these days. If
you want to span the full market, your BMS must span the full
traction voltage range. They don't make surface mount fuses rated
over 250 VDC, that I am aware of. You have to go with a larger
through-hole fuse that typically costs over $2 each. (Only one listed
at $1.47.)  At $2 each, this adds $140 to the 250 volt BMS, and $230
to the 400 volt BMS.

At $52 to $230 for the fuses, it is cheaper to just "let the smoke
out" of an abused cell module than it is to effectively protect them
all from open-circuit damage. If you go without the fuse holder, you
still must re-work/replace the cell module. Not user serviceable
without a fuse holder so there is no difference from the customer's
perspective.

Bill D.


At 09:31 AM 8/20/2012, you wrote:

>Bill,
>
>Sorry, but I just see it adding that much.  Why would you need a
>mount? If you are using surface mount components (as any modern
>commercial pcb would be, these days) a 250V SM fuse and 5.1V 1W SM
>zener can be had in single quantity from a good UK supplier (RS) for
>about US$1.  In the thousands I'm sure it would be considerably less
>- particularly so in the US.  Extra costs for a bit more PCB real
>estate and design would be negligible.
>
>MW
>
>
>On 19 Aug 2012, at 19:56, Bill Dube wrote:
>
> > A high-voltage DC fuse is not small and not inexpensive. Often these
> > fuses cost several dollars. On a high voltage pack, this one
> > component on each cell can add $200 to the cost of the BMS, plus the
> > mount, plus the assembly, plus the board space.
> >
> > If you plan to sense voltage while by-pass current is flowing, the
> > fuse resistance may not be as "negligible" as you might think.
> >
> > Bill D.
> >
> >
> >> Yes. The most straightforward way is to have a fuse in series, and a
> >> zener diode or other clamping device across the actual BMS circuitry
> >> itself. If the cell gets reversed, or goes open and the voltage goes
> >> above the zener's breakdown voltage, the zener conducts and
> blows the fuse.
> >>
> >> The fuse and zener need to be properly chosen. The fuse needs a DC
> >> voltage rating at least as high as the pack voltage. The zener needs to
> >> have a zener voltage high enough not to affect normal operation, but low
> >> enough so nothing else on the BMS board will be damaged. The zener also
> >> needs an I^2T rating higher than the fuse,  so the fuse opens before the
> >> zener fails.
> >>
> >>> I haven't actually tried introducing enough
> >>> resistance to make a crowbar protect the BMS without overheating
> >>
> >> A crowbar is another approach that could work. They are harder to
> >> design, but have the advantage of faster response and cheaper fuses
> >> (often just a carefully selected piece of wire).
> >>
> >>> I guess it could work without affecting voltage measurement accuracy
> >>> too much.
> >>
> >> A fuse or fusible link wire normally has negligible resistance. I can't
> >> see it affecting accuracy.
> >>
> >>> protecting the shunt circuit seems more difficult.
> >>
> >> Not really. The zener or crowbar protects the shunt circuit as well as
> >> it would any measurement circuitry.
> >>
> >>> If you accept blowing the fuse as "surviving", then both these
> >>> problems get somewhat easier. If the external connection to the pack
> >>> will draw enough current to blow the fuse, your crowbar and shunt
> >>> circuit only need to survive long enough to blow the fuse. However
> >>> the crowbar would be difficult in my 10mA case, 10mA at 200V is 2W!
> >>
> >> Here is the scenario I think you are describing: You have a 200v pack.
> >> Each cell has a little BMS board connected across it. You also have an
> >> EVision powered by the 200v pack that draws about 10ma. Now suppose a
> >> cell fails open (the cell failed internally, or you have a loose
> >> terminal, or maybe you are deliberately loosening the terminal during
> >> servicing). The circuit now consists of all cells minus one
> >> (200v-3v=197v) in series with a load that passes about 10ma, connected
> >> across that open cell.
> >>
> >> If the BMS board draws negligible current, then essentially the full
> >> 197v appears across it. BANG!
> >>
> >> If the BMS board draws a current similar to the EVision (10ma), then it
> >> forms a voltage divider. About half the voltage is across the EVision,
> >> and the other half across the BMS board (which will probably still
> >> murder it).
> >>
> >> If you have a fuse and zener diode as I described above, the current is
> >> too low to blow the fuse. But the zener will clamp the voltage to a safe
> >> value (say, 5v). 5v at 10ma is only 0.05 watts, so the zener won't fail.
> >> Dangerous voltage won't appear across the open cell, the BMS won't be
> >> harmed, and the fuse and zener will also be unharmed.
> >>
> >> The fault current would have to exceed whatever the zener can handle on
> >> a continuous basis. A 5.1v 1watt zener can stand 200ma, so you would
> >> pick a fuse rated at less than this. 50ma fuses are available, but a bit
> >> scarce and expensive. Realistically, I would use a 5watt zener which
> >> could handle up to an amp, and a 0.5a fuse which is cheaper and easier
> >> to get.
> >>
> >> --
> >> Ring the bells that still can ring
> >> Forget the perfect offering
> >> There is a crack in everything
> >> That's how the light gets in -- Leonard Cohen
> >> --
> >> Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
> >>
> >> _
>
>
>
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Re: Disconnect the battery from all loads before working on a BMS

Lee Hart
Bill Dube wrote:

> Looking in Digikey, a fast-blow 250 VDC surface mount fuse costs
> $0.75, or more. For a 250 volt pack, this works out to 70+ fuses =>
> $52. These are 11m long, and 4.6mm wide.
>
> Also, pack voltages are getting closer to 400 volts these days. If
> you want to span the full market, your BMS must span the full
> traction voltage range. They don't make surface mount fuses rated
> over 250 VDC, that I am aware of. You have to go with a larger
> through-hole fuse that typically costs over $2 each. (Only one listed
> at $1.47.)  At $2 each, this adds $140 to the 250 volt BMS, and $230
> to the 400 volt BMS.
>
> At $52 to $230 for the fuses, it is cheaper to just "let the smoke
> out" of an abused cell module than it is to effectively protect them
> all from open-circuit damage. If you go without the fuse holder, you
> still must re-work/replace the cell module. Not user serviceable
> without a fuse holder so there is no difference from the customer's
> perspective.
>  
If folks don't want to do something, they can always find excuses;
there's no room, it costs too much, it takes too much time, etc.
Besides, it will never fail, or if it does fail, nothing bad will
happen. That's human nature.

Most of the time, you get away with it. You get lucky, and nothing bad
happens. As far as I know, Bill has never had a serious fire. But some
people are not always lucky, and things go seriously wrong (like Olly's
recent experience). That tends to make one much less likely to blow off
safety devices as being unnecessary!

I've been unlucky before. I learned my lesson, and now don't leave out
fuses and other safety devices just to save money. I'd rather spend $50
on fuses than risk destroying a $10,000 EV!

Digikey is an expensive place to buy fuses. Go to someplace like
fusesunlimited.com for more reasonable pricing.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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Re: Disconnect the battery from all loads before working on a BMS

Bill Dube
The cell module need not survive, but as I said quite plainly
earlier, it needs to fail safely.

Adding the components to make each and every BMS module survive is
not cost effective, but you still would have to make sure it failed
safely. Plenty of inexpensive ways to do that. All you need is to be
a bit clever and to test a bit. A thin "meander" trace will do the
trick if designed carefully and tested rigorously.

At 03:35 PM 8/20/2012, you wrote:

>Bill Dube wrote:
> > Looking in Digikey, a fast-blow 250 VDC surface mount fuse costs
> > $0.75, or more. For a 250 volt pack, this works out to 70+ fuses =>
> > $52. These are 11m long, and 4.6mm wide.
> >
> > Also, pack voltages are getting closer to 400 volts these days. If
> > you want to span the full market, your BMS must span the full
> > traction voltage range. They don't make surface mount fuses rated
> > over 250 VDC, that I am aware of. You have to go with a larger
> > through-hole fuse that typically costs over $2 each. (Only one listed
> > at $1.47.)  At $2 each, this adds $140 to the 250 volt BMS, and $230
> > to the 400 volt BMS.
> >
> > At $52 to $230 for the fuses, it is cheaper to just "let the smoke
> > out" of an abused cell module than it is to effectively protect them
> > all from open-circuit damage. If you go without the fuse holder, you
> > still must re-work/replace the cell module. Not user serviceable
> > without a fuse holder so there is no difference from the customer's
> > perspective.
> >
>If folks don't want to do something, they can always find excuses;
>there's no room, it costs too much, it takes too much time, etc.
>Besides, it will never fail, or if it does fail, nothing bad will
>happen. That's human nature.
>
>Most of the time, you get away with it. You get lucky, and nothing bad
>happens. As far as I know, Bill has never had a serious fire. But some
>people are not always lucky, and things go seriously wrong (like Olly's
>recent experience). That tends to make one much less likely to blow off
>safety devices as being unnecessary!
>
>I've been unlucky before. I learned my lesson, and now don't leave out
>fuses and other safety devices just to save money. I'd rather spend $50
>on fuses than risk destroying a $10,000 EV!
>
>Digikey is an expensive place to buy fuses. Go to someplace like
>fusesunlimited.com for more reasonable pricing.
>
>--
>Ring the bells that still can ring
>Forget the perfect offering
>There is a crack in everything
>That's how the light gets in -- Leonard Cohen
>--
>Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
>
>_______________________________________________
>| Moratorium on drag racing discussion is in effect.
>| Please take those discussions elsewhere.  Thanks.
>|
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Re: Disconnect the battery from all loads before working on a BMS

Cor van de Water
Bill,

True in principle,
not so easy in practice.
You'd be surprised how many *amps*
even a thin trace can carry, in a previous
company we tried to design-out a fuse by
using a PCB trace but quickly went back to
populating a fuse when we found how unpredictable
PCB traces can be with just a slight variation of
width, thickness or even presence of ground plane
(sucking the heat away so it does not fail until
a *much* higher current)

Besides, at 400V DC, how long will the arc grow
and vaporize the trace before it extinguishes?

I had a mishap with an old meter in line when I
was "bad boy" charging my 312V pack and the
inside of that old analog needle-meter (used to
check current into the pack) was completely
blackened with traces burned off the PCB until
finally the arcing stopped (took less than a sec
but it completely destroyed the meter)
It took a few more minutes for the bright blue
spark to disappear from my retinas and I could
see again...

I think it was discussed before - use "safety"
resistors that are designed to fuse if you want
to combine the function on BMS and have no
separate fuse. You can use them for the "bypass"
load resistor and if they ever get a much higher
voltage than a single cell, they fail open.
All the rest of the BMS monitoring should draw
microamps and can have high value resistors
between pack and BMS without affecting the
measurement much.
Use a resistor at each tap point to avoid that
a single failure can still cause a short circuit,
this would also be adviseable if you need to have
it UL certified. In case of doubt, use two resistors
in series to allow any voltage peak to divide over
2 resistors to stay below its max voltage rating.

Regards,

Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
Email: [hidden email]    Private: http://www.cvandewater.com
Skype: cor_van_de_water     XoIP: +31877841130
Tel: +1 408 383 7626        Tel: +91 (040)23117400 x203

-----Original Message-----
From: [hidden email] [mailto:[hidden email]] On
Behalf Of Bill Dube
Sent: Monday, August 20, 2012 2:55 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Disconnect the battery from all loads before working
on a BMS

The cell module need not survive, but as I said quite plainly earlier,
it needs to fail safely.

Adding the components to make each and every BMS module survive is not
cost effective, but you still would have to make sure it failed safely.
Plenty of inexpensive ways to do that. All you need is to be a bit
clever and to test a bit. A thin "meander" trace will do the trick if
designed carefully and tested rigorously.

At 03:35 PM 8/20/2012, you wrote:

>Bill Dube wrote:
> > Looking in Digikey, a fast-blow 250 VDC surface mount fuse costs
> > $0.75, or more. For a 250 volt pack, this works out to 70+ fuses =>
> > $52. These are 11m long, and 4.6mm wide.
> >
> > Also, pack voltages are getting closer to 400 volts these days. If
> > you want to span the full market, your BMS must span the full
> > traction voltage range. They don't make surface mount fuses rated
> > over 250 VDC, that I am aware of. You have to go with a larger
> > through-hole fuse that typically costs over $2 each. (Only one
> > listed at $1.47.)  At $2 each, this adds $140 to the 250 volt BMS,
> > and $230 to the 400 volt BMS.
> >
> > At $52 to $230 for the fuses, it is cheaper to just "let the smoke
> > out" of an abused cell module than it is to effectively protect them

> > all from open-circuit damage. If you go without the fuse holder, you

> > still must re-work/replace the cell module. Not user serviceable
> > without a fuse holder so there is no difference from the customer's
> > perspective.
> >
>If folks don't want to do something, they can always find excuses;
>there's no room, it costs too much, it takes too much time, etc.
>Besides, it will never fail, or if it does fail, nothing bad will
>happen. That's human nature.
>
>Most of the time, you get away with it. You get lucky, and nothing bad
>happens. As far as I know, Bill has never had a serious fire. But some
>people are not always lucky, and things go seriously wrong (like Olly's

>recent experience). That tends to make one much less likely to blow off

>safety devices as being unnecessary!
>
>I've been unlucky before. I learned my lesson, and now don't leave out
>fuses and other safety devices just to save money. I'd rather spend $50

>on fuses than risk destroying a $10,000 EV!
>
>Digikey is an expensive place to buy fuses. Go to someplace like
>fusesunlimited.com for more reasonable pricing.
>
>--
>Ring the bells that still can ring
>Forget the perfect offering
>There is a crack in everything
>That's how the light gets in -- Leonard Cohen
>--
>Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
>
>_______________________________________________
>| Moratorium on drag racing discussion is in effect.
>| Please take those discussions elsewhere.  Thanks.
>|
>| REPLYING: address your message to [hidden email] only.
>| Multiple-address or CCed messages may be rejected.
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Re: Disconnect the battery from all loads before working on a BMS

Lee Hart
In reply to this post by Bill Dube
Bill Dube wrote:
> The cell module need not survive, but as I said quite plainly
> earlier, it needs to fail safely.
>  
I agree completely.
> Adding the components to make each and every BMS module survive is
> not cost effective.
That depends entirely on how you do it. Some designs are intrinsically
safe simply by having large enough value resistors so they won't be
damaged by 300vdc inputs. Others will fail safe by simply burning a
resistor open, which is easy to replace. The designer just has to use
resistors that will safely fail open without arcing over or starting a fire.
> you still would have to make sure it failed safely. Plenty of inexpensive ways to do that. All you need is to be a bit clever and to test a bit. A thin "meander" trace will do the
> trick if designed carefully and tested rigorously.
>  
That's true; you can always build your own fuse. You just have to test
it to be sure it works! For example, that "blowing" it at 250vdc doesn't
set fire to the board or something. You buy commercial fuses so that
someone *else* has done this testing for you.

I just want to point out that if one takes the attitude that safety
isn't cost-effective, and therefore leaves it out, the poor user of that
product is in for some tragic surprises if/when something goes wrong!

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

_______________________________________________
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|
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Re: Disconnect the battery from all loads before working on a BMS

Rick Beebe
In reply to this post by Bill Dube
On 8/20/2012 1:47 PM, Bill Dube wrote:
> At $52 to $230 for the fuses, it is cheaper to just "let the smoke
> out" of an abused cell module than it is to effectively protect them
> all from open-circuit damage. If you go without the fuse holder, you
> still must re-work/replace the cell module. Not user serviceable
> without a fuse holder so there is no difference from the customer's
> perspective.

I generally agree with that but I'd worry that in 5 or 6 years when the
event happens and the module blows that a replacement may no longer be
available. Longevity of design doesn't seem to be a current strong-suit
of the BMSs that are available today.

--Rick

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Re: Disconnect the battery from all loads before working on a BMS

Martin WINLOW
In reply to this post by Bill Dube
Bill,

I don't want to drag this on but...

Hi Bill,

These from Newark (who are known as Farnell over here and between them and Radio Spares are the main suppliers of bulk electronic components in the UK)...

http://www.newark.com/on-semiconductor/1n5338brlg/zener-diode-5w-5-1v-017aa/dp/10N9622?in_merch=Popular%20Products
5.1V 5W zener (tho not SM) 1000+ US$0.15

http://www.newark.com/multicomp/mst-500ma-250v/fuse-pcb-500ma-250v-slow-blow/dp/95M6790
250V 0.5A (again thru hole) 1000+ US$0.165

Yes, 450V are more exy - over a dollar each but combined the diode and the fuse is not going to be anywhere near your figures - and most DIY packs are at teh usual 120 or 144V mark in which case you would be looking at a mere $15 more at most.

Anyway, if people want it I guess the BMS people will supply it... and it won't be free, that's for sure!  MW



On 20 Aug 2012, at 18:47, Bill Dube wrote:

> Looking in Digikey, a fast-blow 250 VDC surface mount fuse costs
> $0.75, or more. For a 250 volt pack, this works out to 70+ fuses =>
> $52. These are 11m long, and 4.6mm wide.
>
> Also, pack voltages are getting closer to 400 volts these days. If
> you want to span the full market, your BMS must span the full
> traction voltage range. They don't make surface mount fuses rated
> over 250 VDC, that I am aware of. You have to go with a larger
> through-hole fuse that typically costs over $2 each. (Only one listed
> at $1.47.)  At $2 each, this adds $140 to the 250 volt BMS, and $230
> to the 400 volt BMS.
>
> At $52 to $230 for the fuses, it is cheaper to just "let the smoke
> out" of an abused cell module than it is to effectively protect them
> all from open-circuit damage. If you go without the fuse holder, you
> still must re-work/replace the cell module. Not user serviceable
> without a fuse holder so there is no difference from the customer's
> perspective.
>
> Bill D.
>
>
> At 09:31 AM 8/20/2012, you wrote:
>> Bill,
>>
>> Sorry, but I just see it adding that much.  Why would you need a
>> mount? If you are using surface mount components (as any modern
>> commercial pcb would be, these days) a 250V SM fuse and 5.1V 1W SM
>> zener can be had in single quantity from a good UK supplier (RS) for
>> about US$1.  In the thousands I'm sure it would be considerably less
>> - particularly so in the US.  Extra costs for a bit more PCB real
>> estate and design would be negligible.
>>
>> MW
>>
>>
>> On 19 Aug 2012, at 19:56, Bill Dube wrote:
>>
>>> A high-voltage DC fuse is not small and not inexpensive. Often these
>>> fuses cost several dollars. On a high voltage pack, this one
>>> component on each cell can add $200 to the cost of the BMS, plus the
>>> mount, plus the assembly, plus the board space.
>>>
>>> If you plan to sense voltage while by-pass current is flowing, the
>>> fuse resistance may not be as "negligible" as you might think.
>>>
>>> Bill D.
>>>
>>>
>>>> Yes. The most straightforward way is to have a fuse in series, and a
>>>> zener diode or other clamping device across the actual BMS circuitry
>>>> itself. If the cell gets reversed, or goes open and the voltage goes
>>>> above the zener's breakdown voltage, the zener conducts and
>> blows the fuse.
>>>>
>>>> The fuse and zener need to be properly chosen. The fuse needs a DC
>>>> voltage rating at least as high as the pack voltage. The zener needs to
>>>> have a zener voltage high enough not to affect normal operation, but low
>>>> enough so nothing else on the BMS board will be damaged. The zener also
>>>> needs an I^2T rating higher than the fuse,  so the fuse opens before the
>>>> zener fails.
>>>>
>>>>> I haven't actually tried introducing enough
>>>>> resistance to make a crowbar protect the BMS without overheating
>>>>
>>>> A crowbar is another approach that could work. They are harder to
>>>> design, but have the advantage of faster response and cheaper fuses
>>>> (often just a carefully selected piece of wire).
>>>>
>>>>> I guess it could work without affecting voltage measurement accuracy
>>>>> too much.
>>>>
>>>> A fuse or fusible link wire normally has negligible resistance. I can't
>>>> see it affecting accuracy.
>>>>
>>>>> protecting the shunt circuit seems more difficult.
>>>>
>>>> Not really. The zener or crowbar protects the shunt circuit as well as
>>>> it would any measurement circuitry.
>>>>
>>>>> If you accept blowing the fuse as "surviving", then both these
>>>>> problems get somewhat easier. If the external connection to the pack
>>>>> will draw enough current to blow the fuse, your crowbar and shunt
>>>>> circuit only need to survive long enough to blow the fuse. However
>>>>> the crowbar would be difficult in my 10mA case, 10mA at 200V is 2W!
>>>>
>>>> Here is the scenario I think you are describing: You have a 200v pack.
>>>> Each cell has a little BMS board connected across it. You also have an
>>>> EVision powered by the 200v pack that draws about 10ma. Now suppose a
>>>> cell fails open (the cell failed internally, or you have a loose
>>>> terminal, or maybe you are deliberately loosening the terminal during
>>>> servicing). The circuit now consists of all cells minus one
>>>> (200v-3v=197v) in series with a load that passes about 10ma, connected
>>>> across that open cell.
>>>>
>>>> If the BMS board draws negligible current, then essentially the full
>>>> 197v appears across it. BANG!
>>>>
>>>> If the BMS board draws a current similar to the EVision (10ma), then it
>>>> forms a voltage divider. About half the voltage is across the EVision,
>>>> and the other half across the BMS board (which will probably still
>>>> murder it).
>>>>
>>>> If you have a fuse and zener diode as I described above, the current is
>>>> too low to blow the fuse. But the zener will clamp the voltage to a safe
>>>> value (say, 5v). 5v at 10ma is only 0.05 watts, so the zener won't fail.
>>>> Dangerous voltage won't appear across the open cell, the BMS won't be
>>>> harmed, and the fuse and zener will also be unharmed.
>>>>
>>>> The fault current would have to exceed whatever the zener can handle on
>>>> a continuous basis. A 5.1v 1watt zener can stand 200ma, so you would
>>>> pick a fuse rated at less than this. 50ma fuses are available, but a bit
>>>> scarce and expensive. Realistically, I would use a 5watt zener which
>>>> could handle up to an amp, and a 0.5a fuse which is cheaper and easier
>>>> to get.
>>>>
>>>> --
>>>> Ring the bells that still can ring
>>>> Forget the perfect offering
>>>> There is a crack in everything
>>>> That's how the light gets in -- Leonard Cohen
>>>> --
>>>> Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
>>>>
>>>> _
>>
>>
>>
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Regards, Martin Winlow
Herts, UK
http://www.evalbum.com/2092
www.winlow.co.uk




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Re: Disconnect the battery from all loads before working on a BMS

Morgan LaMoore
That fuse has a breaking capacity of only 100A @ 250VDC. An EV battery has
much more than 100A available in a short circuit.

That said, I'm more interested in fuses to protect against wiring shorts
than to specifically protect the BMS.

On Tue, Aug 21, 2012 at 10:24 AM, Martin WINLOW <[hidden email]> wrote:

> Bill,
>
> I don't want to drag this on but...
>
> Hi Bill,
>
> These from Newark (who are known as Farnell over here and between them and
> Radio Spares are the main suppliers of bulk electronic components in the
> UK)...
>
>
> http://www.newark.com/on-semiconductor/1n5338brlg/zener-diode-5w-5-1v-017aa/dp/10N9622?in_merch=Popular%20Products
> 5.1V 5W zener (tho not SM) 1000+ US$0.15
>
>
> http://www.newark.com/multicomp/mst-500ma-250v/fuse-pcb-500ma-250v-slow-blow/dp/95M6790
> 250V 0.5A (again thru hole) 1000+ US$0.165
>
> Yes, 450V are more exy - over a dollar each but combined the diode and the
> fuse is not going to be anywhere near your figures - and most DIY packs are
> at teh usual 120 or 144V mark in which case you would be looking at a mere
> $15 more at most.
>
> Anyway, if people want it I guess the BMS people will supply it... and it
> won't be free, that's for sure!  MW
>
> ...
>
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