There are a number of temperature limits of a battery cell, some harder limits than others. It is worth understanding these in general before looking at a specific cell.
These temperatures will change with chemistry and by cell manufacturer, therefore, it is really important to use the limits as advised by the manufacturer. In addition you will need to test the cell to gain the detailed understanding of how the cell behaves in your application versus temperature.
The limits will also be blurred by the design of the battery and control system. One example is the maximum operating temperature for the cell. This needs to take into account:
- temperature sensor measurement error
- linearity between sensor measurement and hottest point in cell
- estimation error, the temperature of every cell will not be measured
- depending on how fast the cooling system can react will also determine how close the cells can operate to the maximum allowed temperature
A number of the fundamental parameters of the cell will change even within the normal operational temperature window of the cell.
Capacity
The capacity of the cell drops with temperature. This is very dependent on the chemistry and manufacturer.
You can heat the cell or manage the load such that it heats up during the early stages of discharge.
Note that you need to be careful with measurements of cell performance versus temperature as the cell can heat up under the load of the test and as such the data might not be what you think it is.
Internal Resistance
The internal resistance of a cell decreases with temperature. For a given power demand the voltage will drop further and the current will increase. The increasing resistance and decreasing OCV at low states of charge create a very significant falloff in the discharge power capability at low SoC and low temperature.
This is the measured DCIR for a Panasonic 18650 3.2Ah cell.
State of Health
We could just let the cell get hot, but as you can see the lifetime of the cell decreases as the temperature of the cell increases [1].
If we let the cell get too hot it could lead to a breakdown of the cell or in a worst case thermal runaway.
There is also an issue with this cell at lower temperatures. Hence we need to both cool and heat the cell to increase the lifetime, especially if we want to use the cell in more extreme environments.
If you want to remove heat from the cell you will create a temperature gradient. If you want to cool fast, eg for a fast charging event, you will need to drop the temperature of the cooling medium, drop this too far and you risk lithium plating the cell. That permanently removes lithium ions from the active material and hence reduces the cell capacity.
References
- Xiao-Guang Yang and Chao-Yang Wang, “Understanding the Trilemma of Fast Charging, Energy Density and Cycle Life of Lithium-ion Batteries“, The Pennsylvania State University
- Seon Jin Kim, Gino Lim, Jaeyoung Cho, “A Robust Optimization Approach for Scheduling Drones Considering Uncertainty of Battery Duration“, Proceedings of the 2017 Industrial and Systems Engineering Conference
Battery Cooling Options
When you see this narrow “Normal Operation” window you can see that you need to cool and heat the cell if you want the most out of it. Designing the right system is a challenge and so it’s worth looking at the options.