# Li-Ion battery question

Discussion in 'Power Electronics' started by Andy Kay, Oct 3, 2016.

1. ### Andy Kay

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Sep 26, 2016
I'm trying to gain familiarity with Li-Ion battery technology, and I'm perplexed about the following aspect. I read in several places that if the weakest cell in the battery is permitted to over-discharge to depletion, then the fact that the other cells are still pushing current through it destroys it. Can anybody enlighten me as to what the mechanism of destruction is here please?

2. ### Bluejets

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Oct 5, 2014
Excessive heat , usually fire.

3. ### Audioguru

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Sep 24, 2016
Lithium is a metal that is very reactive like magnesium and titanium (they all have "ium") and they burn with a very hot fire. Water on the fire makes it burn hotter.

4. ### Externet

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Aug 24, 2009
I believe your question is aimed to be responded with reverse polarity charging, which destroys the cell chemistry.

(+)------------cell1-----------cell2-----------cell3----------cell4----------(-)

If all cells in the battery above are 4V, the total supplied to a load is 16V.
The electrons flow from the (-) terminal trough whatever be the load connected and continue into the (+) terminal.
The electron flow inside each cell exits its (-) terminal and enters its (+) terminal while powering the load.

If cell 3 gets discharged earlier than the others by whatever reason, the electron flow inside that discharged cell3 recharges it in opposite polarity, electrons flowing into its (+), same as the normal powering the load direction.

That applies reverse voltage to the cell that got discharged first and damages it.

A proper recharging direction would get electrons entering its (-) terminal and exiting its (+)
terminal.

Is this explaining your question ?

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Jan 21, 2010
6. ### Andy Kay

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Sep 26, 2016
@Externet: You hit the nail on the head, thank you. I can see now that the weakest cell goes into a charging phase and is recharged the wrong way round. Incidentally, this piece of knowledge permits me to make more sense of bottom balancing and why there is a strong contingent in favour of it.

@Steve: Very useful site to wander around. Thanks for taking the trouble to let me know about it.