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I will be using a LiPo battery for a project, and I want to make certain that I fully understand the safety features of the Protection Circuit Module included with the battery, so that I have a safe design.

When looking at the datasheet for the battery it includes details of the PCM, and lists the limits for when the PCM protects the battery. One of the features is the short-circuit protection, which I interpret as the protection against having a too large current rushing into the battery.

The battery I am using is very small, 25 mAh, but the PCM is listed that the current limit for when the short-circuit protection starts working is 0.7A, and that the short-circuit detection time is 7.2-11 ms.

Here's my question:

  • How can this be safe?
  • What if I have a charger that malfunctions and does not regulate the charging current correctly, but has a current limitation of 0.5A. Could I not then have a constant 0.5A going into my battery for which the short-circuit protection does not protect for, and that my small battery would most likely not survive?

The details of the battery PCM is included below.

Details of battery PCM

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    \$\begingroup\$ The PCM usually protects against OUTPUT short circuit current. You have quoted the overcurrent delay time - the short cct delay is 380 uS max. || For overcurrent 2.1A x 11 mS x say 4V -> Joules = V x I x t = 4 x 2.1 x 0.011 ~~= 0.1 Joule - = 100 mW. seconds . It seems "rather unlikely" that that amount of energy would do significant damage. If you REALLY want a faster and/or lower current PCM you could easily enoughbuild one with a suitable fast comparator. || Do you REALLY have a 25 mAh LiPo - that's tiny - what is the application? \$\endgroup\$ – Russell McMahon Dec 6 '19 at 11:46
  • \$\begingroup\$ Consider the consequences of charging it at 0.5A. Its charge voltage will increase by 0.5A * cell internal resistance, and the over-charge protection will cut in a bit early. If the cell is designed to survive charging at 20C, it is safe. Otherwise you will need additional charge current limitation outside the protection module, which is normally not the job of the PCM. (And Russell is of course right - over-current protection usually refers to discharge protection) \$\endgroup\$ – Brian Drummond Dec 6 '19 at 11:53
  • \$\begingroup\$ Where did you get the 25mAh battery? Can you provide a link to the datasheet? \$\endgroup\$ – Bruce Abbott Dec 6 '19 at 19:41
  • \$\begingroup\$ @BruceAbbott You can see the battery here: honcell.com/products/models/id/1667.html \$\endgroup\$ – Lars Petersson Dec 7 '19 at 16:22
  • \$\begingroup\$ @RussellMcMahon Yeah, its a really cute little battery, the application is an IoT product with really low consumption, but where a fast charge (1C) rate is required. \$\endgroup\$ – Lars Petersson Dec 7 '19 at 16:23
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What if I have a charger that malfunctions and does not regulate the charging current correctly, but has a current limitation of 0.5A. Could I not then have a constant 0.5A going into my battery for which the short-circuit protection does not protect for, and that my small battery would most likely not survive?

Lipo batteries generally have higher rated discharge current than charge current. The PCM current limit is supposed to protect against over-current during discharge. It is not designed to prevent charging at a lower current that might damage the cell.

Charging at a current over the cell's rating probably won't destroy it, but the cycle life will be reduced. The greater danger with Lipos is charging over 4.2V, as this will blow it up! Your PCM cuts at 4.27~4.28V, so it should prevent this catastrophe if the charger malfunctions.

For such a small cell the PCM's over-current rating may be too high to protect it in all circumstances, however the safety risk is also low for a battery this size. The PCM has a separate 'short circuit' protection that should protect against very high current draw (a useful feature, because it's very easy to accidentally short the leads when working with these tiny cells!).

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  • \$\begingroup\$ Ok, so just to clarify so I understand, the short circuit protection protects against a too high current draw OUT of the battery, and the overcurrent protection protects against a too high current going into the battery? Also, you say that the risk of destroying the battery by having a too high charge current is low, would there be any way of motivating that, or is it just engineering judgement from your side? \$\endgroup\$ – Lars Petersson Dec 11 '19 at 8:06
  • \$\begingroup\$ As I understand it the current limit applies to both charging and discharging, while the short circuit protection (which uses low battery voltage to detect the short) only applies to discharging. I'm not saying high charge current won't damage the battery, only that it is probably safe because such a small battery doesn't have enough active material to 'cook off' like a larger battery might. Also, my experience with commercial devices suggests that fast charging doesn't 'destroy' them, though it definitely reduces their cycle life. \$\endgroup\$ – Bruce Abbott Dec 11 '19 at 9:18

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