This is part one of a two-part post.
This rating causes more discussion and disagreement than all of the others combined.
The manufacturer’s current rating, or ratings, are not the level at which the cell will explode. They are also not the numbers to start operating at, going higher and higher if nothing bad happens right away.
In my opinion the manufacturer’s rating is the number they feel will give you decent performance for the rated number of discharge/charge cycles (cycle life) with a reasonable degree of safety. It’s not some kind of hard limit. It is their recommendation for how hard you can use the cell to get the cycle life it’s rated for.
If you operate at above the rating, perhaps using a 20A cell at 25A, then you get worse performance (more voltage sag), shorter overall (cycle) life, and increased risk of trouble as the cell ages.
If you operate at below the rating then the opposite happens. You get better performance, longer cycle life, and decreased risk.
How much these things get better, or worse if above the rating, is too hard to quantify without testing. But the further above the rating you go the worse things can get and the further below the rating you go the better things can get.
This lack of a hard limit, a single do-or-die number you should never exceed, is what causes so many problems. Depending on your tolerance for risk, your performance requirements, and your cycle life requirements, you can have a current rating for a cell that spans a wide range of numbers.
For example…
If a cell has a 20A rating from the manufacturer for reasonable performance then perhaps it would have a 300 cycle life before it lost 20% of its capacity. You can use the cell beyond this (if otherwise it’s still in good condition) but a 20% loss of capacity is a common end-of-life point for a cell.
The manufacturer could give that same cell a 30A rating and it would be a perfectly legitimate rating. But the performance could be crap and the cell might only last 100 cycles. The risks of using that cell could also significantly increase if it was not kept from reaching high temperatures.
The manufacturer could also give that cell a legitimate 10A rating. It would operate very efficiently (good performance) and have a very long cycle life. It would also run cooler, great for use in higher ambient temperatures and/or at lower risk.
So which is the true rating? They all are. It’s up to the manufacturer to decide what applications the cell is going to be used in and to set a rating that helps their huge commercial customers decide whether that cell is a good choice or not.
For example…
If a cell is being used very hard in a handheld electric vacuum cleaner, only for a couple minutes at a time, the cell might have been given a very high current rating. The priority is power, how much they can get from the cell for short periods.
The vacuum cleaner manufacturer doesn’t need the cell to last ten years. They need to have the smallest battery pack possible that can run the vacuum cleaner well. The manufacturer of the cell knows this and would give a cell made for this application a high current rating with a temperature limit to prevent excessive damage. The true continuous current rating would be a lot lower.
So how can we use a manufacturer’s current rating? And what about the ratings for all those rewrapped and non-manufacturer wrapped cells? That will be covered in Part 5B to be posted soon.
