The cell temperature is a critical parameter that you need to know before charging or discharging a cell. A cell is a 3 dimensional structure that is also inhomogeneous and hence you will observe temperature gradients within the cell. The temperature limits, gradients and heat rejection rate will define the overall power capability of the cell.
Cell temperature is related to:
- Degradation rate
- Instaneous charge/discharge power
Heat generation in a cell can be defined quite simply for the case where the cell is operating within it’s normal limits. The first expression gives the heat flow [W].
The first part of this equation is the irreversible Joule heating term, the I2R term.
The second part is the reversible entropy term or Reaction heat terms. The charge and discharge reaction can be exothermic or endothermic under certain conditions.
Also, the cell can gain or lose heat to the local system or environment.
For each cell the manufacturer will define temperature limits for normal and safe operation.
Some of these temperatures are hard limits for the continued safe operation of the cell.
For most cells they will operate best between 15°C and 35°C.
Note that these temperature limits apply not just to the bulk average cell temperature, but to any discrete region. A cell can fail because one area gets very hot due to non-uniform discharge.
The easiest place to apply a temperature sensor to a cell is to the outside case. This assumes the temperature is uniform. This is perhaps ok for a small cell that is being discharged very slowly where the heat exchange with the environment is very small.
In reality there will be temperature gradients across the cell, induced by very non-uniform discharge or cooling applied to one surface. This means you need to understand the heat generation in different scenarios and use this knowledge to select the temperature sensor location(s).
As temperature is a critical parameter it is worth considering redundancy in the sensors and methods to remove calibration errors or failures of transducers.
The maximum temperature differential in a cell is normally specified as ~2°C to minimise the degradation in capacity of the cell. This requirement will drive the cell selection versus application along with the cooling system design.
A hot area of a cell will have a lower resistance, this means it will provide more current. Heat generation is a function of the current squared and so the hot area will heat up even more, reducing the resistance further and contributing at times to a positive feedback mechanism .
Temperature Estimation Algorithms
There are a number of ways of estimating the temperature of a cell, even without the use of an actual temperature sensor.
- one cell, one sensor direct measurement
- average of sensor array
- thermal model of the battery
- thermal model of battery and coolant system
- cell DCIR as an estimation of cell average temperature
For all cells there is an optimal temperature window in which to store the cells to reduce leakage currents and to reduce degradation. The temperature window for storage is typically 5°C to 15°C. However, it is best to consult the cell data sheet or manufacturer for the specific cell you are dealing with.
- Fleckenstein, M., Bohlen, O., Bäker, B., Aging Effect of Temperature Gradients in Li-ion Cells Experimental and Simulative Investigations and the Consequences on Thermal Battery Management, World Electr. Veh. J. 2012, 5, 322-333
- Ian A. Hunt, Yan Zhao, Yatish Patel and G. J. Offer, Surface Cooling Causes Accelerated Degradation Compared to Tab Cooling for Lithium-Ion Pouch Cells, Journal of The Electrochemical Society, Volume 163, Number 9
There are a number of fundamental functions that the BMS needs to control and report on.
Most of these algorithms are estimations and often inter-dependent on each other.
- State of Charge (SoC)
- State of Health (SoH)
- State of Function (SoF)
- State of Power (SoP)
- Cell Temperature
- HV Isolation
- Control contactors
- Cell Balancing