Li-ion protection breakdown voltage?

[Artlav]

Li-ion cells come with undervoltage protection circuits - little integrated thingies that open up the circuit when the voltage across the cell drops below 2.7V or so.
What sort of a switch is that?

I've been thinking, if a cell is inside a large series pack - a dozen of cells for a hobbyist, or a hundred cells for an electric car - won't that switch be subjected to the entire voltage of the pack upon it's opening for the weakest cell?

If so, what is the breakdown voltage of these switches, i.e. in 18650 cells, where can i look it up?
If not, how does that protection work?

I never seen such figures given in battery datasheets, so maybe I've missed the big idea?

 

[meBigGuy]

You do not just stack Li-Ion batteries with single cell protection systems. You need to use a multi-cell over voltage and under-voltage protection system. There are IC's and systems to do this.

http://cds.linear.com/docs/en/article/EDN M Kultgen LTC6802.pdf
http://www.ti.com/lit/an/slua434/slua434.pdf
http://www.intersil.com/content/dam/Intersil/documents/an13/an1333.pdf

 

[Artlav]

Nice. So that is how you do it right.
But if you just stack a set of common, single-protected cells - what would be their limit?

 

[meBigGuy]

Don't.

I've never considered it. The protection circuit design assumes you won't do that.

 

[alphy]

The Li-ion protection breakdown voltage refers to the voltage at which the protection circuit will activate and open the circuit to prevent the cell from over-discharging. This is typically around 2.7V for most Li-ion cells. The switch used in these protection circuits is often a MOSFET (metal-oxide-semiconductor field-effect transistor) which is capable of handling high voltages and currents.

In a large series pack, the weakest cell will indeed be subjected to the entire voltage of the pack when the protection circuit is activated. However, the breakdown voltage of the switch is typically much higher than the maximum voltage of the pack, so it should not be a concern.

The breakdown voltage of the switch can vary depending on the specific cell and protection circuit design, so it may not always be readily available in battery datasheets. However, manufacturers typically test and ensure that the protection circuit can handle the maximum voltage of the pack.

In summary, the protection circuit works by using a switch (usually a MOSFET) to open the circuit when the cell voltage drops below a certain threshold. The breakdown voltage of the switch is typically higher than the maximum voltage of the pack, so it should not be a concern.