# [EVDL] Diodes for simple BMS on LiFePO4 batts



## EVDL List (Jul 27, 2007)

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> Upon testing the two problems I found with this simple approach for
> voltage clamping is the leakage current at 3.3V and the thermal
> runaway if the charger current is too high or one battery is out of
> balance (you put a charged battery replacement in a partially
> discharged pack).
> 
> 3.2V = 10ma 3.3V = 18ma 3.5V = 55ma 3.6V = 90ma 3.74V = 200ma -
> point where thermal runaway was observed, after 50 seconds went up
> to 2A supply current limit & self heated. 3.8V 284ma start (then to
> the moon) 4.0V 1.01A start (then up)
> 
> possibly a HER604 (6A) or so diode would have less thermal runaway
> at higher currents. It would be nice if there was an axial diode
> at higher 10-20A current rating then it wouldn't matter if the
> charger didn't cut back precisely at the right time.
> 
> Any other simple BMS ideas?
> 

How about a Zener diode with ~3.5v breakdown running in reverse? Any
better performance? (Can you find one in the 3-6 A range with a 3.5v
breakdown?)

Jay
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## EVDL List (Jul 27, 2007)

> Jay Summet <[email protected]> wrote:
> > How about a Zener diode with ~3.5v breakdown running in reverse? Any
> > better performance? (Can you find one in the 3-6 A range with a 3.5v
> > breakdown?)
> ...


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## EVDL List (Jul 27, 2007)

Elithion wrote
> 
> such that cell will exceed the max voltage, and no Zener will be damaged.
> 

Sorry, I skipped a word: "such that _no_ cell will exceed the max voltage"


-----
Davide Andrea
Elithion 
--
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## EVDL List (Jul 27, 2007)

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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