Subclause 15.4.3 and Manganese-Lithium coin cells

[eldercosta]

While reviewing subclause 15.4.3 I had the following question regarding the subclause below.

15.4.3.4 Lithium batteries
Primary lithium batteries shall comply with the requirements of IEC 60086-4. Secondary lithium batteries shall comply with the requirements of IEC 62133 or IEC 62133-2. See also 7.3.3.

NOTE Batteries includes both single cells and assemblies of cells, i.e. battery packages.

Compliance is checked by inspection of the battery design documentation or by performance of the tests identified in IEC 60086-4.4 for primary lithium batteries and IEC 62133 IEC or 62133-2 for secondary lithium batteries.

I have two Manganese Lithium batteries (size 621) in my design. They are only used as backup power of RTCs/CMOS RAMs. The cells are soldered on board and the charging circuit follows the manufacturers guidelines (charging voltage 3.3V through a small signal schottky diode and proper series resistor to avoid overcharge).

Though technically these coin cells would fall under the secondary lithium definition (according to the subclause above), I am under the impression these cells are out of the scope. Two manufacturers (Panasonic and Seiko-Epson) do not state IEC 62133(-2) compliance (at least I could not find any reference to this compliance) which suggests it is not required for the typical application of these cells. FWIW, the amount of Lithium in these cells is very low (2mg according to Panasonic, 3.2mg according to Seiko), so I guess most of the concerns of Li-Ion cells do not apply either.

I don't have the full 62133(-2) standard here I only saw the samples of it, including the scope and the index and they did not provide enough info to determine it is applicable or not. I may order if it turns out to be really necessary.

So, do these cells fall in the scope of IECthese standards?

Thanks in advance.

 

[Peter Selvey]

The critical point is the threshold above which protective features are needed, in which case those features should be verified in design and production. There are several features both inside the cell (relief vent, pressure switch, PTC device for overcurrent) as well as external (current limit resistors, reverse charge diodes, voltage monitoring, overcurrent and overcharge monitoring .... )

My gut feeling, with out any reference (sorry) is that these cells are sooooo small (about 5mAh) that none of the internal features are required (or even practical), and protection is just the current limit resistor and diode which can be verified by the end use manufacturer, and even then risks are so low that it's just going through the motions. Unfortunately, I don't know of any literature to support this, and it may be necessary to hound Panasonic or Seiko sales people to get someone to confirm.

 

[eldercosta]

Hello, Peter.

Thank you very much for your comment. It looks like we share a similar view on this matter.

To me, this looks like one of the cases in the Standard where the rulers intents go in one way (secondary batteries/cells with high power density) but if we stick to letter, it applies to these small cells as well, as the 60601-1 itself does not provide a way out; the rationale is not of much help either, unlike the rationale of other (sub)clauses.

I will contact the manufacturers about this matter and purchase the 62133-2 standard to see if I find a way to deal with it via risk management (which I think it is the way to realistically go) along with bench tests to check if, for instance, a short circuit on the cells terminals produces any potentially harmful condition.

Best Regards.

 

[eldercosta]

Hello, Peter, just a follow-up post, for the benefit of others that may have the same question in the future.

I had a response from the manufacturer's representative and he told me their coin cell does not need to be compliant with IEC 62133-2 because its internal resistance is higher than 3 Ohms (actually, it is 80 Ohms).

I bought the standard to have a more solid base for the rationale of the subclauses in the risk management file.

The safety related clause of the 62133-2 (#5) does not apply to coin cells and the clause that specifies type tests and sample size (#6) does exclude cells with resistance higher than 3.0 Ohms from being tested by the methods of clause #7.

Annex D indicates how to measure the internal AC impedance that correlates with resistance; this Annex may be usefull as well, as not every manufacturer states the internal resistance (though it must be in the same range).

Best Regards.