Li-ion protection breakdown voltage?

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Discussion Overview

The discussion revolves around the protection mechanisms in lithium-ion (Li-ion) cells, particularly focusing on undervoltage protection circuits and their implications when cells are arranged in series. Participants explore the potential issues related to the breakdown voltage of these protection switches in the context of multi-cell configurations.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant notes that Li-ion cells typically include undervoltage protection circuits that activate when the voltage drops below approximately 2.7V, questioning the type of switch used in this context.
  • The same participant raises concerns about the implications of using single-cell protection in a series configuration, suggesting that the switch may be exposed to the entire pack voltage when it opens for the weakest cell.
  • Another participant emphasizes that stacking Li-ion batteries with single cell protection is not advisable, recommending the use of multi-cell protection systems instead, and provides links to relevant resources.
  • A subsequent reply acknowledges the correct approach but questions the limits of using common, single-protected cells in a series setup.
  • Another participant advises against using single-protected cells in series, indicating that the design of the protection circuit assumes such configurations are not employed.

Areas of Agreement / Disagreement

Participants generally agree that using single-cell protection in a series configuration is problematic, but there is no consensus on the specific breakdown voltage of the switches or the implications of using such configurations.

Contextual Notes

There are limitations regarding the assumptions made about the protection circuits and the lack of specific breakdown voltage figures in battery datasheets, which remain unresolved in the discussion.

Artlav
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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?
 
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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?
 
Don't.

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

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