This is the first of two parts.
First things first, what is a cycle?
It’s the full discharge of a cell down to its low voltage cutoff, typically 2.5V for a standard li-ion cell, and then a full charge back up to 4.20V. It is the use of the cell at its full capacity rating.
If you remove 3000mAh of charge from a 3000mAh cell, and then recharge it fully, that would be one cycle. If instead you discharge it down to 50%, removing 1500mAh, and then fully charged the cell that would be 1/2 cycle. You would have to do that twice to count as a full cycle.
Two half cycles don’t exactly equal one full cycle since they age the cell differently but it’s close enough for our purposes for now. Theoretically though, two half cycles should age a cell a tiny bit less than one full cycle.
The cycle life rating is the minimum number of these full cycles a cell should deliver, at a certain discharge and charge rate, before the cell’s capacity drops to a certain level. This level is typically between 60%-80% of the original capacity.
For example, a 3000mAh cell with a 300 cycle life rating set at 70% would mean that the cell should still have at least 3000mAh x 70% = 2100mAh of capacity left after being cycled 300 times at the discharge/charge rates mentioned in the datasheet. It might have a little bit more or a lot more capacity left after 300 cycles but it must have at least 70% left if that is the rating.
This point can be used as the end of the cell’s life or the cell can still continue to be used. After dropping to 60%-80% of the cell’s original capacity though many people would be frustrated at having to recharge that cell (or battery pack) more often and would replace the cell/battery pack.
But for some applications, like powerwalls or other low-power energy storage uses, the cell might be cycled many more times to extract as much as possible from the cell to reduce the overall costs of the system.
This all has to be done with an eye on safety though. Continuing to use a cell at well beyond its rated cycle life can mean the cell’s internal resistance has gone up, which makes it hotter during use, and any damage inside the cell is building up too.
Eventually this can lead to cell failure, which can be the cell just self-discharging slowly down to zero volts (ruining the cell) or it can mean the cell short-circuits internally and catches fire (very rare though). A very good battery management system (BMS) is important whenever li-ion cells are used but is critical when cells are used at beyond their rated cycle life.
This BMS can be a few protections built into a vaping device or flashlight, it can be a simple circuit board added to an e-skateboard or e-bike battery pack, or it can be an incredibly sophisticated monitoring, reporting, and protection system for a large energy storage facility.
Let’s look at the cycle life specs of two theoretical cells and compare them…
Cell A is rated to have at least 70% of its capacity left after 250 cycles at 5A discharge and 1A charge at room temperature.
Cell B is rated to have at least 60% of its capacity left after 200 cycles at 5A discharge and 1A charge at room temperature.
Which cell will last longer?
Cell A will. It will only drop to 70% (or higher) after 250 cycles. Cell B could drop to as low as 60% after only 200 cycles. Cell A is the better choice if cycle life is your priority (assuming it has a high enough current rating).
What’s critical when looking at the cycle life rating of a cell is to note the rate at which the cells are discharged and charged. You cannot directly compare the cycle life ratings of two cells unless that rating was set the same way for both cells!
Cell A will deliver at least 300 cycles before dropping to 70% for 10A discharges/4A charges.
Cell B will deliver at least 500 cycles before dropping to 70% for 3A discharges/1A charges.
If we didn’t read how the cycle life rating was set then we might think that Cell B is the better choice since it would deliver at least 500 cycles before dropping to 70% of its original capacity, compared to Cell A only lasting 300 cycles before dropping to 70%.
But Cell A was cycled at a much higher discharge/charge rate than a Cell B. This means that Cell A was being used much harder and that ages the cell faster. We can not directly compare the cycle life ratings for these two cells.
If Cell A was discharged/charged as slowly as Cell B then Cell A might actually last longer. We can’t know though unless the testing was done under the same conditions. This often makes directly comparing the cycle life specs of two cells very difficult.
Part two will be posted soon.