The temperature of the LFP cells
What is the operating temperature of the LFP cells?
The operating temperature of the LFP cells is based on the specification and the application. There are several points to be clarified in this matter. The first to clarify is the three different temperatures in relation to the batteries.
The ambient temperature
The ambient temperature is usually given as the air temperature of any environment where the batteries and related equipment are kept. The ambient temperature is given at the specification of the cells. It is also called the operation temperature.
The cells can be stored and operated within these temperature limits. However the performance of the cells is dependent on the temperature so the performance of the battery differs in various temperatures. In simple words, when the cell is cold (bellow zero) the performance is limited, when the cell is hot (above 40 °C) the drain of the cell must be limited as well in order to protect against overheating.
The cell temperature (surface)
The cell temperature is the temperature measured on the surface of the cell. As the heat has the tendency to go up, the temperature of the cell is usually measured at the top part of the cell.
When not in operation the cell temperature will be similar to the ambient temperature. Since the cell is mostly composed of metals, the cell temperature is usually slightly lower than the ambient temperature.
When in operation the cell emits (dissipates) heat and starts warming up. So during the operation the cell temperature is usually higher than the ambient temperature. When the cell is cold (bellow zero) the cell temperature will increase quickly to above zero and keep growing depending on the drain rating (the current in Amp). When the cell is in hot environment, the temperature of the cell will increase quickly as well. That is why it may be necessary to arrange for ventilation, cooling or air-conditioning to maintain the temperature within the limits.
The cell internal temperature (inside)
It is the estimated temperature inside the cell at the hottest point. During the operation the cell warms from inside. When continued operation under high currents, the temperature of the cell may increase quickly. Inside the cell is the warmest place. However it is not possible to measure this temperature, as it is inside the cell and there is no access. This temperature is only estimated.
Typically, the internal temperature maybe about +15°C higher than the cell temperature on the surface. The internal cell temperature is mainly important in hot operating environments. It is necessary to avoid the over heating of the cell. That is why the cell may needed to be cooled to keep the internal temperature within the construction limits.
Example for Winston cells
The specification for the ambient temperature is given at -45°C to 85°C. The typical operational temperature of the cells (at the surface) is at 0°C to 40°C (max 60°C). The internal temperature under load should not exceed the maximal limit (<85°C) that is why the surface temperature of the cell should be kept within a limit of 40°C (to 60°C).