Charge curve of Lithium-ion cell | CC-CV method charging
This charge curve of a Lithium-ion cell plots various parameters such as voltage, charging time, charging current and charged capacity. When the cells are assembled as a battery pack for an application, they must be charged using a constant current and constant voltage (CC-CV) method. Hence, a CC-CV charger is highly recommended for Lithium-ion batteries.
- The CC-CV method starts with constant charging while the battery pack’s voltage rises.
- When the battery reaches its full charge cut-off voltage, constant voltage mode takes over, and there is a drop in the charging current.
- The charging current keeps coming down until it reaches below 0.05C.
- The battery reaches full charge voltage some time after the CV mode starts (as soon as one of the cells reaches its full charge voltage). At this stage, estimating SoC (state of charge) based on the battery voltage would mean that the battery is fully charged.
- The battery reaching its full charge voltage at this stage does not mean that it is 100% charged. Trickle charge mode kicks in immediately after this stage, where a reducing charging current charges the remaining battery capacity while balancing the cells at the same time.
- When every cell has been balanced and has reached its full charge voltage, at this point, the battery pack is really 100% charged. One way to know this is when the charging current has reached close to 0.05C.
The above concept is why EV manufacturers do not promise a 100% charge at a DC fast charging station. The industry standard is to provide 80% fast charge, then the charging current comes down and eventually, trickle charge mode comes in place.
Discharge curve of Lithium-ion cell at various C-Rate of discharge
This discharge curve of a Lithium-ion cell plots voltage vs discharged capacity. A flat discharge curve is better because it means the voltage is constant throughout the course of battery discharge.
But a flat discharge curve also means the battery might not deliver close to 100% DoD (depth of discharge) because the battery cuts off if one of the cells reaches its lower cut- off voltage.
LFP cells have a flatter discharge curve when compared to NMC cells. Hence, LFP cells deliver lesser DoD then NMC cells and have more balancing issues when assembled into a battery pack.
C-Rating – C-Rating is associated with charging or discharging a battery. C-Rate of discharge is a measure of the rate at which the battery is being discharged when compared to its rated capacity. A C/2 or 0.5C rate means that this particular discharge current will discharge the battery in 2 hours. For example, a 50Ah battery will discharge at 25A for 2 hours. A similar analogy applies to the C-rate of charge.
The science of electrochemistry dictates that lower the C-Rate of charge, more energy can be stored in the battery. Similarly, the lower the C-Rate of discharge, the more energy can be delivered from the battery. Hence, charging and discharging the battery differently from the standard continuous charge current and standard continuous discharge current mentioned in the cell datasheet can yield different results for the energy stored and energy delivered by the cell.
Discharge curve of Lithium-ion cell at various temperatures
Lithium-ion cells can charge between 0°C and 60°C and can discharge between -20°C and 60°C.
A standard operating temperature of 25±2°C during charge and discharge allows for the performance of the cell as per its datasheet.
Cells discharging at a temperature lower than 25°C deliver lower voltage and lower capacity resulting in lower energy delivered. On the other hand, cells discharging at a temperature higher than 25°C deliver higher voltage and higher capacity. Higher operation temperature allows for better flow of electrons resulting in higher energy delivered. But there is a drawback, i.e. reduction in cell cycle life when operating at higher temperatures.
Another environmental aspect to keep in mind is the range of relative humidity. Lithium-ion cells are supposed to operate between 0% and 60% RH.
Cycle life of Lithium-ion cell
According to the industry standard, the cycle life of a Lithium-ion cell is defined as the number of charge-discharge cycles of the cell by the time it reaches 80% retention capacity of its original capacity. The recommended Depth of Discharge taken for the cycle life testing is 80%. The cycle life of a Lithium-ion cell increases as its DoD decreases.
For example, an NMC 18650 cylindrical cell having a cycle life of 500 cycles at 80% DoD would provide 750 cycles at 60% DoD, 1250 cycles at 40% DoD and 2500 cycles at 20% DoD. On the other hand, this same cell would provide 300 cycles at 100% DoD.
This curve of a Lithium-ion cell plots capacity retention vs cycle number. It is defined at 1C charge rate and 1C discharge rate.
NMC cells have a cycle life of 500-2000 cycles, while LFP cells have a cycle life of 2000- 5000 cycles depending on the cell form factor, charge-discharge C-Rating and operation temperature. LTO cells have the highest cycle life of up to 25,000 cycles at 100% DoD. LTO cells are the safest in the industry as they do not catch fire, can operate in water and can even operate when punctured. LTO cells support up to 10C fast charging and can charge up to 80% in 6 minutes.
The capacity retention has reached 85.36% after cycling for 2000 cycles. It can be predicted that the industry-standard definition of 80% retention capacity will be achieved at around 2600 cycles by plotting the curve downwards. Hence, the cycle life of this cell at 1C charge rate and 1C discharge rate at 25±2°C is 2600 cycles.
Standard operation temperature of 25±2°C is assumed in the cycle life graph if it does not mention the operation temperature. Higher operation temperature leads to a reduction in battery cycle life.
About the Author
Rahul Bollini is an independent R&D consultant in the field of Lithium-ion cells and batteries. The author can be reached at firstname.lastname@example.org.
This article was first published in EVreporter Jan 2022 magazine that can be accessed here.
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