How To Get More Cycles Out Of Lithium Ion CellsPosted on: Tue Apr 05 2022
In a perfect world, a lithium ion battery pack would last forever. In the real world, however, battery packs are known to degrade over time. At some point, a battery pack will degrade so much that it should no longer be used. The number of times a battery pack can be cycled before it needs to be discarded or repaired is known as its cycle life.
Depending on the type of cells you have and how you treat them, lithium ion cells can have a cycle life ranging anywhere from 500 to 2000 or more cycles.
The most effective things you can do to get more cycles out of your 18650 cells are to lower the charge voltage and raise the cutoff voltage. You can also minimize stresses on your cells by using as many cells in parallel as possible. When a battery pack has more parallel cells, it will produce less heat when charging or discharging.
In this article, we will discuss tips and procedures that will ensure that you get the most cycles out of your lithium ion battery packs.
So, What Exactly Causes Lithium-ion Cells To Age?
Lithium-ion battery cells work based on the movement of ions between the positive and negative electrodes. In an ideal battery, this mechanism would work perfectly forever. Unfortunately, however, the lithium ion charging and discharging process causes a tiny amount of damage each time.
This manifests itself as the battery losing its ability to hold as much of a charge as it did when it was new.
A battery's State of Health is a function of its capacity, internal resistance, level of self-discharge, and other factors. When it comes to 18650 battery cells, there are several important parameters to keep up with.
These parameters are especially important if you are working with salvaged cells. So, if you would like to learn more about these key aspects, then check out our article on how to properly salvage 18650 cells.
Capacity is rated in mAh (milliamp Hours). A milliampere/hour is how long a battery can sustain a constant discharge of electric energy at a certain rate. A cell's actual mAh value is a function of its discharge current, temperature, and wear level. In fact, how a cell is charged can even affect its capacity.
CDR (Continuous Discharge Rating)
CDR is a measure of how much current a cell can deliver continuously. This figure will always be lower than a cell’s peak discharge rating. For example, if a cell can deliver 5 amps continuously, it may have a peak discharge current of 20 amps or more. The manufacturers spec sheet will state how long the cell can sustain the temporarily higher current draw.
Charge current is the amount of amps that a given cell can be charged at. Most of the time, there are two values listed for charge current.
These values are typical, minimum, and maximum. If you are wanting to extend the life of your cells, choose a charge current that leans more toward the cell’s typical or minimum values.
So, What Exactly Is A Lithium Ion Cell Cycle?
Believe it or not, there is really no clearly defined understanding of what a complete cycle is. Generally, when a 18650 battery is fully charged, fully discharged, and then fully charged again, that is considered to be one cycle.
The confusion comes from the fact that the charge voltage and cutoff voltages can be chosen by the user. So really, you choose what a cycle really is.
18650 lithium-ion battery cells have a maximum charge voltage of 4.2 volts. The low voltage cutoff, which is when a cell is considered to be totally depleted of usable energy, is somewhere between 2.5V and 2.8V.
Lithium ion cells also have a max discharge rating. The rating is usually only achievable for 10 to 30 seconds. If you ever go past this mark, it will accelerate the degradation of the cell.
This is not an exact figure for all lithium ion cells and depends on the cells you are using. So always make sure to refer to your cell's datasheet when possible.
What Is Discharging?
As the name implies, discharging is the opposite of charging. So, if charging is when power is going into the battery, then discharging is when power is coming out of the battery.
How Is Cycle Life Calculated?
The SoH or (State of Health) is a percentage that represents your batteries overall health. While SoH is not based on capacity alone, you can get a pretty decent estimation of SoH by calculating the ratio of a cell's measured capacity compared to the ratings provided in the manufacturer's spec sheet.
For example, if your cell is supposed to be able to store 2200mAh, but now it is only able to store 2000mAh in it, that cell has a 91% SoH. Generally speaking, an 18650 battery cell is no longer good if it has a SoH of 80% or less.
Also, as a lithium ion cell ages, its ISR (Internal Series Resistance) increases. Brand new cells have an ISR of around 20 to 50 mOhms. As a cell is cycled more and more, its internal resistance will increase up to and beyond 400 mOhms. Once a cell gets to this level, it should no longer be used.
How Many Cycles Do 18650 Cells Have?
The average 18650 cell will have a cycle life of 300 - 500 cycles when used at its rated limits. It is important to note, however, that when a cell is exposed to high temperatures and high-drain situations, this can lower cycle life to around 200 cycles.
If you go over the maximum discharge current limit, it's even worse. This will decrease the cycle life down to as little as 40 or 50 cycles.
In ideal conditions, 18650 cells can achieve much more than 500 cycles. In fact, if you do things right, you can get several thousand cycles out of your cells.
It's Important To Optimize Temperature
Cell temperatures are often overlooked. Always check your spec sheet, but a typical 18650 cell is rated to run between 20 and 25 degrees C. So, when running a battery outside of those ranges, you will see an efficiency loss. In fact, for every 10-degree deviation, you will lose a significant amount of capacity.
While not as dangerous, when it comes to the cycle life, running 18650 cells too cold can be just as bad as running them too hot. And while standard lithium ion chemistry is not as sensitive to low temperature conditions as lithium iron variants, make sure to never charge 18650 cells when ambient temperatures are below freezing
Lower Your Charge Voltage
You do not have to charge cells to 4.2 volts per cell. In fact, as you can see in the chart below, if you want the most cycles from your cells, then you should charge them at no more than 4 volts per cell. Charging to 4 volts per cell gives you a little less capacity, but a lot more cycles.
NOTE: The chart above is for *reference* only and was sourced here. You should *never* charge a lithium ion cell over 4.2 volts.
Raise Your Cut-off Voltage
To achieve the longest run time, lithium ion battery packs generally have a default cut off voltage of 2.5 to 3.0 volts per cell.
If you want the most cycles out of your battery, you should increase the cut off voltage to something higher than the LVC (Low Voltage Cutoff) stated in the spec sheet. This partial cycle setup will greatly reduce the stresses on your battery pack, and will, therefore, provide a much longer cycle life.
If the battery pack is optimally designed and the best charge and discharge methods are used, it's not uncommon for an 18650 cell to reach 1000 or even 2000 cycles.
Lower Your Charge Current
Not all chargers let you change the charge current. So, if possible, get a charger that has that feature. If you don’t want to have to adjust the settings, then you could always buy an undersized charger. For example, if your battery pack calls for a 4 amp charger, you could always get a 3 amp charger.
The slower charger will be compatible with your battery, and it will stress your cells less. The ability to lower charge current is crucial for extending the cycle life of your 18650 battery pack.
If you cannot lower the charge current, you can effectively lower the amount of charge current each cell receives by increasing the number of parallel cells in your battery pack.
While a fast charge of 1 amp per cell in parallel or more will charge a battery pack quickly, it will reduce the battery packs overall life.
Lower Your Discharge Current
If you want to make your battery pack last as long as possible, it's important to take into consideration your load when designing your battery pack.
If possible, design your system so that an individual cell is exposed to no more than 0.5c of discharge current. This can be achieved by building your battery pack with as many cells in parallel as possible.
The amount of current each cell has to provide can be calculated like this:
Single Cell Amps = (Load Watts / Battery Volts)/Parallel Cells
For example, if you have a 7S4P battery that is a 500 watt load, that comes out to about 19.3 amps. Each cell will have to provide 4.8 amps of current. If you doubled your parallel cells by building a 7S8P battery, then each cell would only experience 2.4 amps of current while running the same 20 amp load.
NOTE: As battery voltage drops, it will require more current from the battery pack to sustain the same load current. So, when planning your build, it's always a good idea to plan around how many amps your battery will have to provide at the lowest voltage it can run at.
Do 18650 Cells Have A Memory?
No. Lithium-ion batteries don't have a memory effect. So, that means that the first charge does not have to be an 8 to 10 hour charge.
Also, because lithium cells have no memory effect, they can be charged and discharged at any time without affecting their capacity. This is in contrast to older battery technologies such as nickel cadmium, which needed to be fully charged and fully discharged to get the most capacity out of them.
What Is The Shelf Life Of 18650 Batteries?
18650 cells have a very long service life when properly stored. They can last up to 10 years on the shelf if kept at typical room temperatures and at their nominal voltage per manufacturer's spec sheet. This will typically be 3.7 volts.
Battery packs don’t last forever. Due to the unfortunate lack of perfection in the underlying processes that make lithium ion batteries work, they wear out over time. There are, however, some things you can do to get more cycle life out of 18650 cells.
Lowering the charge voltage and raising the cutoff voltage are the most effective ways that you can get the most out of your cells. If you want to take things a bit further, you can lower the charge and discharge current by using as many cells in parallel as possible, which will make your battery pack run cooler.
We hope this article helped you learn how to get the most cycle life out of your 18650 cells, thanks for reading!