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Suppose a cheapish lipo/18650 has a typical protection circuit in place, which, e.g., shuts off the battery when voltage gets too low, shuts off the battery when the current exceeds 4A, and prevents over-charging.

If the battery becomes old and begins to fade, and the charging device draws more current than the battery offers, could excessive heat be generated without causing an under voltage, and thus, getting through the protection circuit? Generating heat without the voltage dropping too low could be an unpreventable hazard, right?

Thanks

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Your cell will heat by dissipating I^2R power through its' internal resistance. Since the resistance is inside the cell and your protection circuit is measuring the voltage at the terminals, the circuit will always see this as a voltage drop. So, at the very least, there is a sign that all is not well inside.

As for your question, it depends on a lot of things, perhaps most importantly on whether your protection circuit also has a temperature sensor. :) For example, if you wrapped your cell in lots of insulation and then used it at its' current limit, there's a decent chance it would overheat, regardless of its' state-of-health.

Bottom line? A well-constructed (and designed) solution used as specified in said design should not fail, regardless of age. Since you mentioned cheap-ish in your question, I'd keep some fire safety equipment handy. :)

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