On Determining Actual versus Claimed Capacity of NiMH Cells

[Bones]

Insofar as I'm aware, the IEC approved protocol for determined the capacity of an NiMH cell consists of the following:

• Charge at rate equal to 1/10th of the cell's rated capacity for 16 hours.
• Allow the cell to rest for one hour.
• Discharge at a rate equal to 1/5th of the cell's rated capacity down to 1.0 volts.

The 16 hour charge at 1/10 of a cell's rated capacity is often referred to as a 'standard' charge.

As demonstrated by this image provided by forum member Quension, the IEC protocol may even be referenced in a cell's capacity statement:



Owner's of the Powerex MH-C9000 will note its Break-in mode approximates the IEC protocol.

While other capacity measurement methodologies can be better suited for determining a cell's actual performance under specific conditions, those methodologies are irrelevant insofar as determining a cell's actual versus its rated capacity.

 

[Mr Happy]

Indeed. As I recall, it is also advised to do several of the standard charge/discharge cycles before taking the actual capacity measurement.

 

[TorchBoy]

The only times I've done a couple of (39 hour) break in cycles in a row on my MH-C9000 the second round came out with a lower capacity. That's not quite what I was expecting.

 

[NutSAK]

That's the protocol I'm aware of also.

I believe it's the same method used for determining NiCd capacity also, is it not? I'm testing a Dewalt DW9099 NiCd battery with my Accucel-6 using approximately that method at the moment.

 

[csa]

Yep, that's the standard.

For our purposes, discharging the cell near our intended discharge rate is going to provide more useful numbers for ranking a given set of cells against each other.

In particular, many of us run our cells at closer to 1C.

It would be interesting to see comparisons for cells at that rate. I think we'd probably find that the favorite cells still perform well.

 

[Russel]

One other aspect of the IEC standard is ambient temperature. While it isn't critical for the purpose of checking capacity of your battery inventory, it might be a good idea to make a note of the ambient temperature during the test. Especially if you see extreme seasonal temperature changes.