There are large number of lithium cells out there. Many of them look similar, but their specifications and ratings are what set them apart. There’s a very long list of lithium-ion battery specifications. Here we will look at the most important specifications of the lithium-ion battery.
The capacity of a cell is probably the most critical factor, as it determines how much energy is available in the cell. The capacity of lithium battery cells is measured in amp-hours (Ah) or sometimes milliamp-hours (mAh) where 1 Ah = 1,000 mAh. Lithium battery cells can have anywhere from a few mAh to 100 Ah.
Occasionally the unit watt-hour (Wh) will be listed on a cell instead of the amp-hour. Watt-hour is another unit of energy, but also consider voltage. To determine the amp-hours in this case, simply divide the watt-hours by the nominal voltage of the cell.
Capacity ratings only tell you how much energy the cell can store and provide. They don’t give you any information about the power of the cell or its longevity. The highest capacity batteries usually have only moderate power levels. There is often a tradeoff between power rating and capacity. Therefore, the only thing you can use the capacity rating for is to determine how much energy is in a cell.
Also, don’t expect your cells to achieve their entire rated capacity. Manufacturers usually test the capacity of their cells using two “tricks” to seek out as much capacity as possible. They test at an incredibly low discharge rate. They also discharge the cells down to their absolute minimum rated voltage, often 2.5 V for most li-ion cells.
Discharging that low is possible, but it will decrease the lifetime of the cell if it is done too often. Most commercial products using Li-ion cells discharge down to around 3.0 V, if not higher, to get a longer life out of the cells.
Maximum Discharge Rate
The maximum discharge rating tells you the maximum load, which is to say the maximum current, that can be drawn from the cell.
There are two common discharge ratings, the “maximum continuous discharge current” and the “maximum peak discharge current”.
The maximum continuous discharge current is the better figure to use when making comparisons between cells. This is the maximum current that the cell can supply continuously without overheating or damaging itself. If the maximum continuous current rating is 5 A, then the cell can provide 5 A of current continuously from its full charge state until its empty state.
The maximum peak discharge current is the amount of current that a cell can provide for a short burst. Every manufacturer rates this differently, which is why it is hard to use this number for comparison. Some manufacturers consider this to be a 2-3 second burst, while others consider a 10 second or longer period for the maximum peak discharge.
Regardless, you should never exceed the maximum peak discharge rating for more than a couple of seconds because manufacturers usually rate their cells at the extreme end of what they are capable of, it is never a good idea to push them to the limit of these ratings.
Furthermore, cells that are operated near their rated limits tend to get very hot and operate inefficiently, robbing them of up to 10% of their designed capacity. So if you want to use the entire capacity of a cell, don’t push it to its maximum discharge current limit.
The C rate of a battery cell is a measurement of the rate that the battery cell can be discharged or charged in relation to the cell’s capacity. The C rate does not change based on the capacity of the battery cell; rather, it is an intrinsic property of the battery cell itself. That means that two cells that are identical in every way except for their rated capacities will also have identical C rates. The C rate is calculated as a multiple of the capacity rating of the battery.
For example, a battery cell with a rated capacity of 2 Ah and a maximum continuous discharge current of 4 A has a C rate of 2. This would be known as a 2 C battery.
In this example, we found the C rate by dividing the maximum discharge current rating by the capacity. This gives us a C rate of 2.
If that same 2 Ah cell had a maximum discharge rating of 6 A instead of 4 A, it would be a 3 C battery. If it had a maximum discharge rating of 10 A, it would be a 5 C battery.
The C rate is important because it is used to compare the relative power of cells, even when cells have different ratings.
A big 10 Ah cell might be rated for 10 A maximum discharge while a smaller 2.5 Ah cell is only rated for 5 A maximum discharge. At first, it might seem like the big cell is more powerful, as it can provide twice the current than the small cell can provide (10 A instead of 5 A).
However, it is the smaller cell that is more powerful relatively, as it has a higher C rating. The smaller cell has a C rating of 2 while the larger cell has a C rating of 1.
Occasionally lithium battery cells are marketed with just a C rating and not a maximum current rating. This can make it easier to compare the power level of battery cells of different capacities. As long as you know the capacity of the cell, you can use the C rate to quickly calculate the maximum current rating of the cell.
Maximum Charge Rate
The maximum charge rating is similar to the maximum discharge rating and is also fairly self-explanatory – it’s the maximum rate that you can charge the cell. Most cells will have a charge rating of not more than about 0.5 C.
Charging a cell at a rate near its maximum charge rating will shorten the life of the cell. It is recommended not to charge most lithium cells at more than 0.5 C, and charging closer to 0.2 C is much better for the cell’s health.
Remember how we calculated the C rating above for the discharge rate? It works the same for the charge rate. A 5 Ah cell charged at 0.5 C would be charged at 2.5 A. However, if you charged a 2.5 Ah cell at 2.5 A, that would be 1 C charging (and very fast charging as well, by lithium battery standards).
Maximum Number of Cycles
Depending on the type of lithium battery, the number of cycles could be anywhere from 200 to 3,000 or more. Cycle ratings can be difficult to compare from one cell to the next though, as manufacturers don’t always use a standard rating system.
The most common rating system is the number of cycles before a cell reaches 80% of its original rated capacity. The capacity of lithium cells slowly degrades over time with increasing charge cycles.
At 80% of the original rated capacity, many manufacturers consider the cell to have reached the end of its useful life. Some manufacturers give a rating of charge cycles until 70% capacity remains. Some don’t even specify, and just state a number of cycles until “end of life”, leaving it unclear exactly what they consider the end of life to be.
Regardless, when a cell reaches 80% or even 70% of its original rated capacity, it isn’t necessarily dead, it just isn’t going to perform as well. Not only it will not have as much capacity, but it will also have a larger drop in voltage under load. This will result in less power and even shorter working life for the battery.
There are many other ratings and specifications for most cells, but the ratings listed above are generally the most important parameters for choosing battery cells for most applications and also help with the comparison between multiple cells.
Thanks for reading about “lithium-ion battery specifications”. You may like my another article Types of Lithium-ion Cells & Working.