# [EVDL] Simple Reliable BMS; flooded Lithiums?



## EVDL List (Jul 27, 2007)

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 <[email protected]>
Subject: Re: [EVDL] Diodes for simple BMS on LiFePO4 batts
To: [email protected]
Message-ID: <[email protected]>
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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## EVDL List (Jul 27, 2007)

On Thu, May 3, 2012 at 12:22 PM, Mark Hanson <[email protected]> wro=
te:
> 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 hav=
e a *lightweight* chemistry that didn't have the attracted complexity (and =
reliability hit) of BMS/regulators/monitors. I miss my ni-cads but not t=
he 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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## EVDL List (Jul 27, 2007)

> 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
> ...


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## EVDL List (Jul 27, 2007)

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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## EVDL List (Jul 27, 2007)

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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## EVDL List (Jul 27, 2007)

> Chris Zach <[email protected]> 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
> ...


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