DIY, Balance board for LiFePO4 cells, with Zener Diode

[Astropower]

Hello,

I have Battery Pack that i build with 16 cells of 15Ah, LiFePO4 (3.2V Nominal 3.65V Maximum).

I dont want to use in BMS, my opinion that i waste of money.

the most two important things to protect the cells, to cut off the battery when voltage too low, my Motor controller have this function.
Second, is to balance the cells while charging.

I found this Balance module :
http://www.batteryspace.com/Balance-Module-for-3.2V-LiFePO4-Battery-Pack.aspx
but it very expensive.

So i was thing to build Balance module to my 16 cells with 32 pcs of Zener diode number: 1N5334B
http://www.onsemi.com/pub_link/Collateral/1N5333B-D.PDF

Nominal Zener voltage 3.6V and 1320mA Balance (with two diode on parallel on each cell 2.64A Balance).

i think it good and cheap solution, to balance while charging.

I would like to get your opinions...

 

[auraslip]

I would be interested in seeing this. I understand it would be a very basic circuit. It would also be very useful.

 

[bigmoose]

Astro be sure that you understand all the subtulties of figure 7 in the data sheet that you referenced. Note that a Zener does not have a sharp 'knee'... it is soft, and varies ... a lot in my experience. Before you go all in, do some experiments with a single cell and a good volt meter. Monitor the current through your zener at the same time if you can.

 

[kortas]

as bigmoose wrote: "Note that a Zener does not have a sharp 'knee'... it is soft, and varies ... "
which will be the problem I think...

I found following scheme which I have not tried, but seems fine:
http://www.zajic.cz/omezovac/omez%20sch.jpg
source: http://www.zajic.cz/omezovac/omezovac.htm (Czech)

It basically does what you said. It should act as ideal zener diode

 

[fechter]

I found following scheme which I have not tried, but seems fine:
http://www.zajic.cz/omezovac/omez%20sch.jpg
source: http://www.zajic.cz/omezovac/omezovac.htm (Czech)

It basically does what you said. It should act as ideal zener diode

Yes, that circuit will work much better. Personally I'd place a resistor in series with the main pass transistor to limit the current.

 

[Njay]

Related thread:

http://endless-sphere.com/forums/viewtopic.php?f=2&t=37978

 

[texaspyro]

the most two important things to protect the cells, to cut off the battery when voltage too low, my Motor controller have this function.

LVC at the pack level is pretty useless to protect the pack. The "runt" cells in the pack will drop below their safe LVC level before the pack voltage drops below the sum of the LVCs of all the cells. This winds up killing those cells. And if they wind up getting reverse voltage across them you can wind up with a bad case of KFC (Kentucky fried crotch)

 

[auraslip]

A very conservative pack level lvc works fine if youre positive your cells are healthy.