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Discussion Starter #1
Hi Folk's,

All the lithium battery balancers I've seen (and the one's I use on my LiFePO4 T-Sky's www.evalbum.com/2749 ) have one thing in common, if one dies that battery/cell either goes way high or low (depending on the mode of failure) destroying the cell. So you need to secondarily with *seperate* monitor the cells *also* with a bunch of wires going everywhere like with my scanner www.evdl.org/lib/mh/ (and balancer OV shunt clamp) but to another level of shutting down the charger if anyone of the balancer's or cell voltage goes high. This attracted complexity is why at work we decided to go back to NiMH (and probably why Toyota-Prius & Honda-Insight did too).

I'll probably use LiFePO4 batteries in my www.evalbum.com/1273 when my Ni-Cads go bad (hate the weight and crappy handling of lead) but they require baby sitting and wonder what the long term reliability will be if we see a bunch of new EV's using this complex balancing/monitoring circuitry in production. It would be nice if there was a LiFePO4 or some chemistry that's light and didn't require picky balancing (behaves like lead without the weight) maintaining the KIS principle.

Have a balanced day,
Mark
www.reevadiy.org community service renewable energy & EV's
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Discussion Starter #2
Mark Hanson <[email protected]> wrote:
>
> Hi Folk's,
>
> All the lithium battery balancers I've seen (and the one's I use on my
> LiFePO4 T-Sky's www.evalbum.com/2749 ) have one thing in common,
> if one dies that battery/cell either goes way high or low (depending on
> the mode of failure) destroying the cell.

No, that's doing it wrong. You must monitor the cell voltage and take
action if it's out of bounds. Anyone who claims differently is wrong
and living on borrowed time IMHO :)

See below:


On Tue, Feb 3, 2009 at 3:33 PM, Evan Tuer <[email protected]> wrote:
> On Tue, Feb 3, 2009 at 3:22 PM, Mark Hanson <[email protected]> wrote:
>>
>> Hi,
>>
>> Thanks for the help. I checked my balancer that's opto couplers tied
>> to 100 ohm resistors across each battery (click in divider during float
>> charge 24/7) and some of them weren't making SMT connection
>> although they looked ok. So the batts that didn't have the 100 ohm
>> divider resistor connected floated up to 4.4V and the one's that did
>> sagged to 2.3V while under float charge, should be 3.45V per batt with
>> 48.3V applied for 14 batts. I may also add some 3.7V zeners &
>> bleeder resistors across the batteries to limit the overvoltage for safety.
>
> Hi Mark, a BMS for this type of cell should not only do balancing but
> needs to actively monitor the voltage of each cell and limit charge
> and discharge if the safe limits are exceeded. Unless you are racing
> or something and babysitting the cells by hand, this is absolutely
> essential.
> It's a disaster waiting to happen really :(
>

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Discussion Starter #3
Evan Tuer wrote:

>>
>> All the lithium battery balancers I've seen (and the one's I use on
>> my
>> LiFePO4 T-Sky's www.evalbum.com/2749 ) have one thing in common,
>> if one dies that battery/cell either goes way high or low
>> (depending on
>> the mode of failure) destroying the cell.
>
> No, that's doing it wrong. You must monitor the cell voltage and take
> action if it's out of bounds. Anyone who claims differently is wrong
> and living on borrowed time IMHO :)

I agree with you, Evan, but Mark has a point, just about everything
thats being offered as a "BMS" does just what Mark says.
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Discussion Starter #5
Rod Hower wrote:
> If a BMS module fails, the main controller should respond almost
> immediately and produce an error code and take appropriate measures
> to protect the battery pack.

Yes, it should. But how many of them do?

The error messages should be fail-safe. That is, the BMS modules sends
an "OK to proceed" message almost continuously. *Lack* of a message is
interpreted as an error.

> A properly designed BMS should fail in
> a manner that does not continuously drain the cell. I don't think
> this is an imposable task

No, it certainly can be done. Even my stupid zener-lamp regulator does
this. No parts failing open or shorted will load the cell to death.
--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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Discussion Starter #6
Jack Murray wrote:
> If I recall, Lee's Battery Balancer for Lead adds another 25% cost on
> top of the batteries, so while its good, it can be argued its not
> cost effective.

It's about $500 for the control board (supports up to 80
cells/batteries), plus $150 per relay board (which support up to 8
cells/batteries each). For 16 batteries, that's about $50 per battery.
For 64 cells, that's about $25 per cell. If you build it yourself, you
can do it cheaper.

It's worth it if it doubles the life of your pack, because it's cheaper
than a second pack. I got 10 years out of my set of Concorde GPC-1295
lead-acid AGMs; this is a battery that only lasts 3 years for most people.

> I think the current Lithiums are now good enough in weight and power
> and durability (if they last as expected)...

Ah, but that's the $20,000 question, isn't it?

Most batteries don't die of old age -- they are MURDERED by their owners
through ignorance and abuse. A (good) BMS is an insurance policy against
this.

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

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Discussion Starter #7
Jack Murray wrote:
> OK thanks for the actual number. So for a 144v pack of 6v cells, its
> $950. 24 T-125's at $150 ea, $3,600, so my guess of 25% was close.
> So add another $150 battery instead, seems like a better option?

It depends on your goal. Another battery is heavier, needs more room,
increases the total energy stored, and affects the charger and
controller. The balancer has a different purpose; it's job is to monitor
and extend the life of the pack.

> Double the pack life? Sounds like a battery salesman... ;)

No; a BMS salesman. I don't sell batteries. :)

> Recall how we discounted anecdotal evidence when it came from a
> certain someone else.

Yep. :) Though I'm providing actual data. The specific batteries, the
specific EV they were used in, how they were used, and what results were
obtained.

You should always distrust anecdotal data. It usually contains no
verifiable facts. "This here XYZ product worked good, it worked real
good!" says Joe X of Alaska.

Instead, look for verifiable facts. Real people, locations, dates, data
taken, etc.

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

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Discussion Starter #8
And people wonder why some of us are simply running with no BMS at all.
Seems to me a BMS is a bandaid for cells of different capacity and different
resistance. The correct answer would be better cell grouping. No reason
cells couldn't be grouped at the factory with the same actual AH capacity
and same internal resistance. Get them close enough and stay away from the
ends of the charge/discharge curve and you'll likely be fine. Even with my
pack varying in capacity of 4 ahs and different resistance I'm not seeing a
noticeable change in the last 6 months of driving, even when going pretty
close to 100% DOD a few times.


Bill Dube wrote:
>
> The better systems report a failed cell module or cell group module,
> but some do not, just as you have said.
>
> You mentioned the two most common modes of failure; load open and
> load closed. There are _many_ modes of possible failure, one of which
> could be a module that is reporting that it is heathy, but in fact it
> is not. Another mode of failure is a module reporting that some
> problem with it or its cell has occurred, when nothing is really wrong.
>
> When you add in redundancy, and with it increased complexity, you
> increase the modes of failure. This can make the problem worse
> instead of better. The complicated BMS can generate a seemingly
> continuous stream of fault signals, preventing the vehicle from being
> operated in a useful manner. Yes, the cells are protected, but you
> can hardly drive the car.
>
> A consensus type system comes to mind, but the costs go out of control.
>
> I don't know if there is any perfect answer. One extreme puts the
> cells at risk, another gets too costly to be practical, while another
> is too eager to stop the car.
>
> Bill D.
>
>
>
>

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Discussion Starter #9
So maybe doing what I'm doing is sort of insurance. My BMS modules
have an HVT of 4.00vpc and LVT of 2.93vpc. I'm charging to just under
3.5vpc so I'll be able to monitor pack balance and catch a low or high
cell. Now, I just have to get that AC relay installed so that when I'm
not baby sitting the charging my charger will get shut off if a cell
hits HVT.

FWIW, my pack balance over the last two months has gone from a max
difference of 0.029v (at 25.6C) to 0.072v (at 18.0C) at the end of
charge. The highest cell has changed twice and the lowest cell has
changed once except that on the last check the lowest cell was matched
by the original lowest cell. It will be interesting to see what
happens over time. I have over 4K mi on this pack since January.

AMPhibian <[email protected]> wrote:
>
> And people wonder why some of us are simply running with no BMS at all.
> Seems to me a BMS is a bandaid for cells of different capacity and differ=
ent
> resistance. The correct answer would be better cell grouping. No re=
ason
> cells couldn't be grouped at the factory with the same actual AH capacity
> and same internal resistance. Get them close enough and stay away from=
the
> ends of the charge/discharge curve and you'll likely be fine. Even wit=
h my
> pack varying in capacity of 4 ahs and different resistance I'm not seeing=
a
> noticeable change in the last 6 months of driving, even when going pretty
> close to 100% DOD a few times.
>
>
> Bill Dube wrote:
>>
>> The better systems report a failed cell module or cell group module,
>> but some do not, just as you have said.
>>
>> You mentioned the two most common modes of failure; load open and
>> load closed. There are _many_ modes of possible failure, one of which
>> could be a module that is reporting that it is heathy, but in fact it
>> is not. Another mode of failure is a module reporting that some
>> problem with it or its cell has occurred, when nothing is really wrong.
>>
>> When you add in redundancy, and with it increased complexity, you
>> increase the modes of failure. This can make the problem worse
>> instead of better. The complicated BMS can generate a seemingly
>> continuous stream of fault signals, preventing the vehicle from being
>> operated in a useful manner. Yes, the cells are protected, but you
>> can hardly drive the car.
>>
>> A consensus type system comes to mind, but the costs go out of control.
>>
>> I don't know if there is any perfect answer. One extreme puts the
>> cells at risk, another gets too costly to be practical, while another
>> is too eager to stop the car.
>>
>> Bill D.
>>
>>
>>
>>
>
> --
> View this message in context: http://electric-vehicle-discussion-list.413=
529.n4.nabble.com/Lithium-Battery-Balancers-Fatal-Flaw-tp2992128p2993040.ht=
ml
> Sent from the Electric Vehicle Discussion List mailing list archive at Na=
bble.com.
>
> _______________________________________________
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> | Multiple-address or CCed messages may be rejected.
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>



-- =

David D. Nelson
http://evalbum.com/1328

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Discussion Starter #10
> OK thanks for the actual number. So for a 144v pack of 6v cells,
> its $950. 24 T-125's at $150 ea, $3,600, so my guess of 25% was close.
> So add another $150 battery instead, seems like a better option?

Then your argument is moot. BMS doesn't have much "cost-benefit ratio"
if you are running floodies, since you can "just add water" - doing an
equalizing charge with an average (not-too-dumb) charger isn't very
risky and has been successful for over a century in EVs. You don't
have the option of electrolyte replacement in the SLAs Lee was using
or *anything* made with lithium.

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Discussion Starter #11
Bill Dube <[email protected]> wrote:

> I don't know if there is any perfect answer. One extreme puts
> the cells at risk, another gets too costly to be practical, while
> another is too eager to stop the car.

I mis-read Mark's original comment - sorry. He was complaining about
the complexity and reliability of the monitoring circuits, not the
necessity of using them.

Anyway, I would say that it doesn't have to be perfect, it just needs
to fail safe, and not fail too often. We have that already. There are
cell level (or at least group level) battery monitoring systems in all
hybrid cars, all laptops. Very few of them even approach perfect, but
in the main they are good enough. People use hybrid cars and laptops.

Even my own home-built BMS has so far (one year, 11,000 miles) been
good enough for the non-technical person using the car. It has locked
up and prevented the car from starting once in that time. Main switch
off and on, and it worked again. I've enabled the watchdog now so
next time it will log the problem and reset itself.

There does seem to be a lot of "fear uncertainty and doubt" about the
whole subject though. The suggestion that "all available BMSs" are
worse than having no BMS still seems completely ludicrous.

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