Simple Reliable BMS; flooded Lithiums?

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Simple Reliable BMS; flooded Lithiums?

Mark Hanson-2

Hi Davide etc,
 
Just put a 4.7 ohm resistor in series with the 7.5V zener to limit the current and set the charger to current cut-back to 1A at 3.5VPC (like on my charger.)
 
What we *really* need is flooded Lithiums that would inherently equalize like lead of ni-cads.  I'm not a chemist but it sure would be nice to have a *lightweight* chemistry that didn't have the attracted complexity (and reliability hit) of BMS/regulators/monitors.  I miss my ni-cads but not the weight.
 
Have a renewable energy day,
Mark
www.REEVA.info community service RE & EV's
 
Message: 8
Date: Thu, 3 May 2012 09:10:05 -0700 (PDT)
From: Elithion <[hidden email]>
Subject: Re: [EVDL] Diodes for simple BMS on LiFePO4 batts
To: [hidden email]
Message-ID: <[hidden email]>
Content-Type: text/plain; charset=us-ascii
 
Section 5.2.1.1 "Zener diode" of the Li-ion book discusses this very point.
You can read it for free on Google Books, page 126:
http://books.google.com/books?id=o-QpFOR0PTcC&q=Zener%20diode#v=snippet&q=Zener%20diode&f=false
 
It proposes a 7.5 V Zener every 2 cells to overcome the problems associated
with low voltage Zener diodes.
 
This will only work if the pack is hand balanced (top balance) before the
charger is turned on for the first time ever, and the pack is regularly
charged (such as every night). Once the cells are charged, during the CV
phase, the charging current from the CCCV charger will naturally decay to 0,
such that cell will exceed the max voltage, and no Zener will be damaged.
The Zener diodes will keep the pack balanced from then on, through gentle
action.
 
But, if the pack is ever allowed to drift out of balance (not charged all
the way to the top on a regular basis), the Zener diodes across the most
charged cells will blow up at the next charge cycle.
 
 
-----
Davide Andrea
Elithion
     
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Re: Simple Reliable BMS; flooded Lithiums?

David Nelson-5
On Thu, May 3, 2012 at 12:22 PM, Mark Hanson <[hidden email]> wrote:
> What we *really* need is flooded Lithiums that would inherently equalize like lead of ni-cads.  I'm not a chemist but it sure would be nice to have a *lightweight* chemistry that didn't have the attracted complexity (and reliability hit) of BMS/regulators/monitors.  I miss my ni-cads but not the weight.
>

I think you are missing the benefits that far out weigh the "just over
charge to balance" feature of flooded lead acid and NiCd batteries.
Stay away from the extreme ends of the charge spectrum on LiFePO4
cells and they are significantly less work to maintain than the
flooded cells. Besides, having a charge shuttle reaction in the cell
can decrease the efficiency of the cell. As an example, with my
flooded 6V pack I could go only 4 miles/kWh measured from the wall.
With my LiFePO4 pack and the same driving conditions I can easily go
over 6 miles/kWh. That, along with other benefits, make LiFePO4 packs
easier and better to deal with. The BMS doesn't have to be that
complex, just designed and built well. The BMS boards I used to have
on my pack (I'm running without them right now) only have a 1mA draw
so they don't even discharge the pack in a perceptible way.

My experience and that of others with prismatic LiFePO4 cells is that
the BMS "issues" really are not that big. Don't over charge the cells
and don't over discharge the cells and they will remain happy for a
long time. Oh, and don't undersize your pack either.

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

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Re: Simple Reliable BMS; flooded Lithiums?

Chris Zach
On 5/4/2012 11:47 PM, David Nelson wrote:
> My experience and that of others with prismatic LiFePO4 cells is that
> the BMS "issues" really are not that big. Don't over charge the cells
> and don't over discharge the cells and they will remain happy for a
> long time. Oh, and don't undersize your pack either.

And that's where the cash register goes CHA CHING! Because if I follow
the manufacturer's recommendation for prismatics, recommended discharge
is C/3. Since my car cruises with a 60a draw at 60mph, I would need a
300 volt 180ah pack at the minimum.

Which is obviously insane. So what would be the "right" size pack for a
car that draws 60a at cruise, 120a at moderate acceleration and a peak
of 200a when floored (not often)? Would a pack of 40ah cells suffice?

Chris

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Re: Simple Reliable BMS; flooded Lithiums?

matt lacey-2
My Vectrix has 60Ah cells in it at the moment

My peak discharge is 220A, I draw ~120A under acceleration most of the time,
and holding 60mph takes me 60A, so nearly the same situation as you

I've done 20'000 miles on my pack and it still behaves like brand new

40Ah cells will work aswell, I have a friend who has 40Ah cells in a Vectrix
climbing the hill up to his house everyday takes 80A continuous
hes at 10'000 miles on his pack at the moment, and there hasn't been any
measurable capacity loss

Matt


----- Original Message -----
From: "Chris Zach" <[hidden email]>
To: <[hidden email]>
Sent: Saturday, May 05, 2012 11:31 PM
Subject: Re: [EVDL] Simple Reliable BMS; flooded Lithiums?


> On 5/4/2012 11:47 PM, David Nelson wrote:
>> My experience and that of others with prismatic LiFePO4 cells is that
>> the BMS "issues" really are not that big. Don't over charge the cells
>> and don't over discharge the cells and they will remain happy for a
>> long time. Oh, and don't undersize your pack either.
>
> And that's where the cash register goes CHA CHING! Because if I follow
> the manufacturer's recommendation for prismatics, recommended discharge
> is C/3. Since my car cruises with a 60a draw at 60mph, I would need a
> 300 volt 180ah pack at the minimum.
>
> Which is obviously insane. So what would be the "right" size pack for a
> car that draws 60a at cruise, 120a at moderate acceleration and a peak
> of 200a when floored (not often)? Would a pack of 40ah cells suffice?
>
> Chris
>
> _______________________________________________
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Re: Simple Reliable BMS; flooded Lithiums?

EVDL Administrator
In reply to this post by Chris Zach
On 5 May 2012 at 11:31, Chris Zach wrote:

> Because if I follow the manufacturer's recommendation for prismatics,
> recommended discharge is C/3. Since my car cruises with a 60a draw at
> 60mph, I would need a 300 volt 180ah pack at the minimum.

Are you sure that isn't 3C?  C/3 seems awfully low.

David Roden - Akron, Ohio, USA
EVDL Administrator

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Re: Simple Reliable BMS; flooded Lithiums?

Voltswagon
They all say charge/discharge recommended at C/3, but many also have a continuous discharge rating of 3C, up to 10 C peak, and are fine charging at at least C.

C/3 is the ultraconservative rating for long life.  No one would seriously use that to size their pack.
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Re: Simple Reliable BMS; flooded Lithiums?

Lee Hart
On 5/5/2012 1:12 PM, Voltswagon wrote:
> They all say charge/discharge recommended at C/3, but many also have a
> continuous discharge rating of 3C, up to 10 C peak, and are fine charging at
> at least C.
>
> C/3 is the ultraconservative rating for long life.  No one would seriously
> use that to size their pack.

The C/3 specification is the recommended maximum *continuous* discharge
current. In other words, discharge the cell over a 3-hour period.

The 3C rating is a *peak* discharge current, sustainable for X seconds.
To be meaningful, you have to know how long X is.

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

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Re: Simple Reliable BMS; flooded Lithiums?

Cor van de Water
In reply to this post by Chris Zach
Hi Chris,
40Ah cells and 200A peak draw might work, but it
sounds like it will put most types of LiFePO4 into
the "danger" zone where the manufacturer optimistically
specs that you should not be too long or too often, but
over which you do not have much control if you find yourself
driving up a steep mountain on the freeway...
Most LiFePO4 seems to be OK with continuous 3C discharge,
I can't remember anyone having a modern Li-Ion battery
that can't deliver 3C continuous, so it seems that a safe
bet is to spring for a 60 or 70Ah string. If you use
110 cells then you have a nominal 352V pack and charging
to 3.5V per cell means a peak 385V charge.
That would certainly make a VERY peppy Prizm.
It would also give you a range comparable to the Leaf.

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 Chris Zach
Sent: Saturday, May 05, 2012 8:32 AM
To: [hidden email]
Subject: Re: [EVDL] Simple Reliable BMS; flooded Lithiums?

On 5/4/2012 11:47 PM, David Nelson wrote:
> My experience and that of others with prismatic LiFePO4 cells is that
> the BMS "issues" really are not that big. Don't over charge the cells
> and don't over discharge the cells and they will remain happy for a
> long time. Oh, and don't undersize your pack either.

And that's where the cash register goes CHA CHING! Because if I follow
the manufacturer's recommendation for prismatics, recommended discharge
is C/3. Since my car cruises with a 60a draw at 60mph, I would need a
300 volt 180ah pack at the minimum.

Which is obviously insane. So what would be the "right" size pack for a
car that draws 60a at cruise, 120a at moderate acceleration and a peak
of 200a when floored (not often)? Would a pack of 40ah cells suffice?

Chris

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Re: Simple Reliable BMS; flooded Lithiums?

AMPhibian
In reply to this post by Mark Hanson-2
Mark Hanson-2 wrote
 
 
What we *really* need is flooded Lithiums that would inherently equalize like lead of ni-cads.  I'm not a chemist but it sure would be nice to have a *lightweight* chemistry that didn't have the attracted complexity (and reliability hit) of BMS/regulators/monitors.  I miss my ni-cads but not the weight.
 
3M has a redox additive for lithium cells that activates at 3.90 V keeping the cells from going to a higher voltage.  I don't know what's involved, or the drawbacks, because I've never seen any cell using it.  Apparently Spinel chemistries have an inherent self balancing mechanism:

"The charge efficiency of the spinel chemistries decreases slightly as it approaches a high SOC. You can see the effect with a CC supply and a temp sensor as plain as day.

This is what causes them to self balance. (and what causes a lead acid to self balance, only they use electrolysis as the energy outlet)

It is not a designed-in feature, simply an inherent property that helps to self correct balance in packs.

In practice, you can start with a pack that is out of balance, run a number of cycles through it, and each time its finishing the CV mode, you see balance creep a little closer each time until it ends up back in roughly perfect balance after enough cycles.

The exceptions of course would be if you have a weak cell, or a very fresh cell with old cells, etc etc."

http://endless-sphere.com/forums/viewtopic.php?f=14&t=27702#p400584
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Re: Simple Reliable BMS; flooded Lithiums?

Morgan LaMoore
In reply to this post by Chris Zach
On Sat, May 5, 2012 at 10:31 AM, Chris Zach <[hidden email]> wrote:
> And that's where the cash register goes CHA CHING! Because if I follow
> the manufacturer's recommendation for prismatics, recommended discharge
> is C/3. Since my car cruises with a 60a draw at 60mph, I would need a
> 300 volt 180ah pack at the minimum.
>
> Which is obviously insane. So what would be the "right" size pack for a
> car that draws 60a at cruise, 120a at moderate acceleration and a peak
> of 200a when floored (not often)? Would a pack of 40ah cells suffice?

Alternatively, you could use the 20Ah A123 pouch cells. They're
available at $1.60/Ah or less (on the grey market), and 40Ah worth
could provide way more current than you need.

They're mechanically more challenging to assemble into a pack, though,
because they're just pouches with large tabs.

-Morgan LaMoore

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