Why Does IR of Prismatic Cells Fluctuate?

Prismatic Lithium-Ion Cell Internal Resistance Fluctuations ExplainedPrismatic lithium-ion cells are used worldwide for applications in electric vehicles (EV’s), energy storage systems, and consumer electronics. One significant parameter affecting prismatic cell performance is internal resistance (IR) – understanding what causes fluctuations in IR is one way to improve performance.
What is Internal Resistance?
Internal Resistance is the measure of a battery’s resistance to current flow through it, and can be thought of as a measure of the cell’s efficiency being represented by the degree of loss of energy through resistance to flow causing heating of the battery when current is flowing. In this respect, higher internal resistance translates to a lower efficiency and a slower charging of the cell at higher temperature, potentially leading to a shorter lifespan.
In practical terms of what best practices need to be done to maintain the performance characteristics of prismatic lithium-ion cells, the measurement of internal cell resistance is an important gauge of a cell property that does not vary over time. Measurement of IR of prismatic cells is made typically by passing a small current and measuring voltage drop that occurs, either shorttime or long-term measurements being made. If easily published, a stable IR measurement is a sign of good cell, while significant broad changes over time in cell IR are an alarm that things are going south.
What Causes Internal Resistance Fluctuations and Fluctuation of Other Parameters with Performance, for Prismatic Lithium-Ion Cells?
Surrounding Temperature – Temperature has a direct effect on the IR, an increase in the temperature will decrease the IR since at higher temperature ions are freer in the electrolyte and hence the impediment to motion they experience is lower. The effect of this fluctuation is indicative of unaffected good cell – but high temperature may burn the internal resistance out of it in the long run.On the other hand, lower temperatures tend to slow down ion movement which influences IR negatively. In freezing environments for example, the IR of prismatic cells may appear to be a lot higher, which in turn impacts the overall performance and efficiency of the cell. This fluctuation can be used to explain the slight rapid decrease in IR that we termed IR during the summer, and the gradual increase in IR observed towards winter.
(b) State of Charge (SOC)
The state of charge (SOC) is yet another reason that leads to the fluctuation of IR in prismatic cells. When the battery is fully charged, the IR may be on the high side. A lower SOC value implies that the battery has discharged to a level where IR is lower, thus promoting the smooth flow of current.
However, as the SOC lowers to an extent, the IR of the cell may scale up as there are not a lot of available ions to get quite a decent conductivity through.
These fluctuations caused by the normal effects of SOC on IR are a common phenomenon, but they could still have a significant impact on battery performance in some applications where a constant good power output is required.
(c) Ageing and Cycle Life
As prismatic cells grow old under many charge-discharge cycles, fluctuations in IR are more common, as frequent cycling can lead to the gradual breakdown of the cells, ie the electrodes, electrolyte, etc. The growth of products of degradation such as a solid electrolyte interphase (SEI) plays a part in affecting how ably ions flow within the cell, hence the increase in the IR of cells that have gone through more cycles. Over the next few years in the life of the cell, cells of higher cycle life may have greater fluctuations in IR as being affected by the other factors mentioned above.a. Manufacturing Variability
Even if using great care to manufacture prismatic cells, small variations may remain which may lead to a cell with a lower or higher internal resistance. A cell may be slightly higher, or lower in IR simply because of minute differences in thickness of electrode material, electrolyte concentration, and so on in the assembly process.
Such fluctuations are generally tolerable but may have implications for the battery pack at large. Cells which tend to be on the high side may run hotter and degrade more quickly, and which run lower may have the advantage of slight extra efficiency, but perhaps at risk of damage in tough conditions.
b. Current and Load Conditions
The load, or current drawn, from the battery can also affect its internal resistance. High current draws will tend to cause a rise in this case due to the higher natural demand on the battery. This is most notable in applications producing and using high power, such as electric vehicles and heavy industrial machinery.
By applying a current, the battery’s temperature indeed rises, and this may slightly shorten the immediate IR, although again, applying the load, turning off and waiting a little for it to cool should see an approximate return to normal. Wear from repeated high currents will of course building up a cumulative increase.
c. Cell Imbalance
In many batteries such as that used for EVs or energy storage, there will be several cells used alongside of one another. If these tend to fall out of lockstep from one another, then they may well cause an overall IR which reflects that of the cells at the time of reading” the other cells, so IR will fluctuate at the pack level.
For example, suppose a cell has an IR of 50mΩ whereas the other three cells have IRs of approximately 5mΩ. The BMS then has to account for differences in IR (causing fluctuations), amongst other things. And naturally, the total IR would cover up for it. You might see something like the following when checking the IR through your BMS app: “90mΩ,” “80mΩ,” “100mΩ,” “200mΩ”. If all the cells are more or less even, then it won’t appear that way.
“So should I ditch the battery?” you ask. Not necessarily. The solution is to use a BMS and make sure it’s properly monitoring each cell, suspension and vibration that the cells and batteries see and managing the “health” of the cells. It should be able to balance each cell to make sure everything is status quo: no suspicious jitter in the IR. What you want is a system that is balanced and stays balanced.
While IR fluctuation can occur between charge cycles within a battery cell, it is as a function of many things, including battery construction, usage, and the adverse effects of self-discharge and ageing. You monitor it and share it with your battery monitor app, which syncs it with your mobile app.