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I've always learned that when a wire or conductor heats up it'll increase resistance due to the free electrons crashing into other vibrating electrons.

So I'm confused how thermal runaway has its resistance decreased, as temperature increases?

If anyone could explain, I'd be grateful!

Thanks!

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Batteries piles are more thermally insulated from each other than BJT's on a common heatsink which also have a negative tempco (NTC) unlike metal conductors and MOSFETs, which have a PTC.

The problem arises when a short or a load current far exceeds max rated current. The NTC causes the ESR to reduce and thus current to increase in a short or parallel battery shunt operation when mismatched.

If unbalanced then the strongest parallel cell battery, with the lowest ESR supplies more of the current and thus if that mismatch causes that cell to far exceed it's max rated current and temperature continues to rise, the chemistry can reach a temperature in the 250'C range where it becomes an exothermic REDOX reaction and it quickly becomes an explosive event.

I believe new LiPo {and lead acid cells} are matched within <1% for voltage when new and ESR ranges depending on C rate something like from 5 to 20 mOhms for 18650 cells depending on quality.

Large battery arrays have inbuilt fuses for protection such as Tesla batteries.

For Series cells, matching is also important as ESR affects the drop in each cell and heat rise but often the circuit impedance is higher than between two cells put together different charge voltage. In series cells it is the weakest cell ( with highest ESR) that gets the hottest, which helps it perform better but also accelerates aging significantly. e.g. 10A with 50 mOhm is 5 Watts of heat vs a new cell with 10 mOhm and only 1 Watt of heat.

I recall Samsung battery problem was caused by variations in battery physical tolerances and too tight a fit with some thermal expansion.

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  • \$\begingroup\$ Good answer.. Adding... Or in short, batteries produce electricity via a chemical reaction. Like most chemical reactions if you add more heat they reaction happens more quickly... which in the case of a battery, makes move voltage / current .. which makes more heat.... which... well you can figure out the rest. \$\endgroup\$ – Trevor_G Mar 30 '17 at 1:44
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    \$\begingroup\$ yes in short.. no pun intended , exothermic accelerant but triggered by NTC then caused by short or mismatched ESR or shunted mismatched voltages \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 30 '17 at 2:22

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