What capacity is "too low to use"?

What cell capacity is "Too low to use"?

I know that many people aim for 2000mah, but why is that the target?

Different people will have different aims or concerns.

safety:
eg "Under XXXXmah is old and I tihnk they may be dangerous"

time spent on building/reliability/downtime/maintenance:
eg "Under XXXXmah takes too long to include for what it contributes and/or will make my system less reliable"

design considerations:
eg, "I have many, many cells in parallel. so currents each cell provides are very small, so small capacity cells will still contribute well" , or
"I cant get many cells, so I have to use as many as possible", or
"I have only got space for XXXX number of cells, so I want them to be the best I can get"

Why do you have a cut off and what is the cut off capacity?

It's more that the cells once they lose a certain amount of their original capacity, they go downhill quick after that. Also, there is a lot of labour testing, assembling packs, and some people are also space limited.

As an experiment I made a block of 20 really low capacity (circa 1000mah cells) and added them to an existing pack as an experiment. Boosted capacity and has been in service just fine for maybe 6-7 months. But I would expect the capacity of this add-on pack to degrade before my other packs as the cells are more 'tired'.

If the original capacity was 1000mah on those cells, they might still be ok, but they were mostly 2400-2600mah cells. I think most people (like me), can't be bothered looking up every cell and just set a 2000mah (or so) cutoff.

mbrunton said:

It's more that the cells once they lose a certain amount of their original capacity, they go downhill quick after that.
[...]

Click to expand...

This is the main criteria I sort the cells. Most cells go "downhill", when they have between 60-40% of the original capacity. So if you use one that has only 50% SoH (state of health ie. capacity now vs original capacity), then the capacity will probably decrease very fast. And if you use one with 70% SoH it will work fine for a time.

I decided to generally use all above 80% SoH and all which are above 70% SoH and have a capacity over 2000mAh.

An other reason is the use-case. If you want your battery to be as small as possible you should aim for high-capacity cells. If size does not matter even a 100mAh cell could benefit you. But as stated, the main reason is the "age" of the battery. After a certain amount of circles they loose more and more capacity and it is not always rational to keep them.

I use all cells above 70% nominal capacity regardless of their current capacity.

I shoot for 90% low as the majority of the pack - say 55 of a 60cell pack and sprinkle in 5 or so down to 80%. Below 80% I save and have used here and there for skunk projects.
I do this and limit DOD average (for the year) to 50% with an eye to a 20yr life span for my solar battery bank where its still overall above 70% of original capacity.

The only 'fact' I have is a battery university chart showing 87% capability after 5000 cycles using the 75-25% SoC curve.

I'd be interested in any info on expected life span of 80% cells vs 90% cells over time at the 75-25 SoC curve.

Let me try to give an answer to all of the issues.

There is no "too low to use" capacity.
I consider the above 2000mAh recovered cells as premiums (red level), because they survived a while and are still very good.
I use these for high capacity projects that need a lot of capacity and power.
The 2000mAh is probably something historically related too, since the spread massive of Li-ion in laptops and other devices started with the 2000mAh cells.
That said, the 1000-2000mAh (yellow level) cells I consider universally usable for many applications. Give that they are obtained for free or nearly free, they can cover a wide range of applications that do not need premiums.
The 500-1000mAh range (green level) I consider as low-mid segment. They have limited use, but still good for many applications like LED strings/lights.
The 100-500mAh range (cyan level) I already consider junk (along with all ultrafire-like fakes). Those have limited use but are still good for things like single cell flashlights.
The below 100mAh (blue level) are considered near-death and unusable for most things. But they still can be used for low power lasers, which only drain a few mW of power, so I don't throw them away either.
So you see, each capacity range is usable for something of you don't have a very specific application in mind.

From a safety perspective, the capacity is not that relevant, except at very low levels when it is insufficient to cause any damage. Any other than recycled-level capacity can do damage if the cell is improperly used.

As a rule for series-parallel thing from the perspective of the capacity, there's only one actual rule: make sure that what you put in series has the same capacity or nearly the same. You can make a 3S by using a 3000mAh, 2x1500mAh (2nd pair) and 3x1000mAh (3rd pair). It will work and it will balance fine if you don't exceed the spec's current.

Cut off: I stick with 3.3V. No less that that. That is the original Li-ion specification and I prefer sticking with that if I want long lasting cells. If there's an emergency, I'll use them until dead, but generally no.
Many manufacturers today can provide the spec's current even at as low as 2.5V, but that does not mean it's something good for a cell's life.

No such thing as a cut-off capacity. Capacity means what is between where you cut off and you fully charge.
Officially, that is 3.3V-4.2V. Unofficially, all smart-phone battery manufacturers CHEAT and overcharge their batteries to 4.35. Well, that works on 18650s too quite well, but don't expect too much life out of them if you decide to do this.