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Discussion Starter · #2 · (Edited)
The dendrite formation is also voltage sensitive. As you get to the top of the charge, the rate increases. Overcharging one cell out of the pack increases it's formation rate exponentially.
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"The difficulty, though, lies in recharging those batteries. When lithium ions combine with electrons at the anode, they should ideally arrange themselves in neat, thin layers of atoms that optimize the limited volume available inside a sealed battery cell. But for a variety of reasons, lithium does not naturally settle into such an orderly configuration, especially during faster charging. When a battery charges too quickly, or overcharges, the metal piles up on the surface of the anode and begins to form dendrites. As the dendrites grow, Archer says, those porous, chunky structures provide more and more surface area for chemical transformations that inevitably lead to the proliferation of dendrites. There’s a razor-thin tipping point, too: The potential difference between storing lithium and plating it on the anode surface is just 200 microvolts"
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Interesting, I'd love to see the experiment performed on a bunch of different li-ion battery chemistries to see which ones are better/worse.
 

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Interesting, I'd love to see the experiment performed on a bunch of different li-ion battery chemistries to see which ones are better/worse.
Yes. One thing that tends to be overlooked in these discussions is how different chemistries can behave very differently.
 

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Discussion Starter · #6 ·
Both of these findings are consistent with our batteries in the Bolt (and all of the current NMC and NCA vehicle batteries). The higher the State Of Charge/ higher cell voltage, the greater the chance of dendrites bridging past the unprotected area of a folded separator. The higher the current and charge rate the higher the chance of dendrite bridging. A complete charge session from a very low SOC to a high voltage at 100% SOC, the greater the chance of dendrite bridging. The effects of which apparently tend to be cumulative when the pack is habitually run down to 10% and charged in one session back to 100% over it's lifetime.
 

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A 10X increase in charge rate produced a 40X increase in dendrite formation rate.
I've mentioned in other threads those who are demanding faster-faster charge rates, in the immediate future, we can expect difficulties. I predict current and near-future vehicles with very fast charge rates are going to show unintended consequences which will make the few Bolt fires look like the ant-shite they are. Would love to be wrong about this, but the path of progress is seldom smooth and seldom uninterrupted.

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..."There’s a razor-thin tipping point, too: The potential difference between storing lithium and plating it on the anode surface is just 200 microvolts"...
That works both ways. In theory, limiting maximum charge by as little as 0.001 V has a 50x safety margin (200 microvolts = 0.0002 V).
 

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Where did you get the 50x for the tipping point of over charging?
Maybe I'm not understanding the 200 microvolts reference. Is that the difference in the maximum charging voltage between forming dendrites and not forming dendrites (ie 4.2002 vs 4.2000)? Or is it variation in charging voltage that causes dendrites (ie, charging voltage must stay within 3.7999 and 3.8001 when charging at 3.8 V)?
 

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Discussion Starter · #12 ·
Maybe I'm not understanding the 200 microvolts reference. Is that the difference in the maximum charging voltage between forming dendrites and not forming dendrites (ie 4.2002 vs 4.2000)? Or is it variation in charging voltage that causes dendrites (ie, charging voltage must stay within 3.7999 and 3.8001 when charging at 3.8 V)?
They are apparently talking about if you exceed the safe maximum cell voltage during charging. 4.25v or whatever. Which any good BMS program will stay far away from and cut the charge when highest cell approaches the limit.
 

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They are apparently talking about if you exceed the safe maximum cell voltage during charging. 4.25v or whatever. Which any good BMS program will stay far away from and cut the charge when highest cell approaches the limit.
OK, so that should be an easy limit to establish. If charging at 4.2502 results in dendrites, while charging at 4.2500 does not (0.0002 V difference, or 200 microvolt difference), then limiting the charging voltage to 4.249 (0.001 V difference) should be plenty of safety margin.
 

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Discussion Starter · #14 ·
4.1v would be even better to make sure. Whatever. Keeping the battery in the middle of the charge is the best for longevity. And for safety with these unknown faulty packs. This is not the only thing that causes the dendrities since they are always forming to one extent or the other. Forget the cascading overvoltage statement because the properly designed BMS will keep everything away from those voltages. Reread the rest of the papers. It also seems that using the minimum charge rate that will give you what you need in the time that you have would be the safest. Contrary to the no overnight charging edict.
 

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Discussion Starter · #16 ·
That is what I am getting at. Most of the fires occurred in the morning after charging all night. Which has nothing to do with night. Obviously. And I believe has to do with a 10 hour charge when that is from 10% to 100%. Habitually. Not because of the 10 hours. Habitually fast charging 10-100% should be even worse. I am banking on L1 at 8 amps 40-65%, twice a day as being the safest since I can plug into an outlet at work.
 
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