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I was looking at 'power bank' circuit boards on Aliexpress, the kind that you add to a 18650 cell to make a basic 5V in/out supply. I noticed they all seemed to be lacking any kind of temperature sensing.

Most serious BMSes come with some kind of temperature sensor, so they can kill the charge current if the temperature rises too much. Is this necessary for a single lithium ion 18650?

I know about failure modes of packs where a cell goes short circuit in a parallel group, causing the other cells to discharge through it. Or a cell in a series group has a bit less capacity and ends up reverse biased. These events might cause damage if the charger continues to pump current through the battery.

But neither single-cell 18650 charging boards, nor 18650 protection circuits, appear to have temperature sensing. So is it safe to run a single cell without it?

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With a decent battery and charge-protect circuit it should be fine, since both the single-battery protection (using, for example, the popular DW01 chip) and charging IC (like TP4056) already limit the current through the 18650 cell; the current has nowhere else to flow, unlike the case with serial/parallel configurations.

If you look at the DW01 datasheet and it's typical application circuit (below), you can see that it actually monitors the current already and can completely disconnect the battery using M1 and M2 transistors in case of a failure. In parrallel and serial connected cells the IC does not monitor the currents between different cells and can't disconnect them from each other in case of a failure.

The temperature sensor, however, is still recommended for additional safety and redundancy, that is why some LiPo cells include a temperature sense output in their integrated protection circuit: for example, an external short circuit directly between the cell contacts situation could only be handled by a temperature sensor. This situation, however, could easily be avoided by designing a decent PCB and enclosure.

DW01 connection circuit

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