# [EVDL] Disconnect the battery from all loads before working on a BMS



## EVDL List (Jul 27, 2007)

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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## EVDL List (Jul 27, 2007)

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



> Elithion <[email protected]> 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
> ...


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## EVDL List (Jul 27, 2007)

> Elithion <[email protected]> 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
> ...


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## EVDL List (Jul 27, 2007)

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: [email protected] [mailto:[email protected]] 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 <[email protected]> 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
> ...


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## EVDL List (Jul 27, 2007)

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




> 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.
> ...


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## EVDL List (Jul 27, 2007)

> 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
> ...


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## EVDL List (Jul 27, 2007)

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
>
>_______________________________________________
>| Moratorium on drag racing discussion is in effect.
>| Please take those discussions elsewhere. Thanks.
>|
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## EVDL List (Jul 27, 2007)

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




> 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
> ...


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## EVDL List (Jul 27, 2007)

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
>
>
>


> 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
> ...


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## EVDL List (Jul 27, 2007)

> 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.
> ...


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## EVDL List (Jul 27, 2007)

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


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## EVDL List (Jul 27, 2007)

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





> 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 =>
> ...


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## EVDL List (Jul 27, 2007)

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.



> Martin WINLOW <[email protected]> wrote:
> 
> > Bill,
> >
> ...


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## EVDL List (Jul 27, 2007)

OK, so I am using 136 batts for 380-500v. How do I figure into your
scenario?


> "Martin WINLOW" <[email protected]> wrote:
> 
> > Bill,
> >
> ...


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## EVDL List (Jul 27, 2007)

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


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## EVDL List (Jul 27, 2007)

I have no idea.

MW




> Marcus Reddish wrote:
> 
> > OK, so I am using 136 batts for 380-500v. How do I figure into your
> > scenario?
> ...


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