This TechLetter will focus on the thermal management of batteries. Indeed, battery performance and life are greatly affected by the temperature of the cells. It is therefore important to control cell temperature to ensure a good productivity of an electric equipment and a good profitability by controlling battery life.
How important is thermal management of batteries in heavy duty application?
A poor thermal management of li-ion batteries leads to the following issues:
- Capacity loss in winter (20 to 60%)
- Overheating preventing battery charge, especially in summer
- Fast charge stopped prematurely due to battery overheating
- No fast charge possibility without affecting the battery life
- Big difference of cell temperatures inside a battery pack, leading to accelerated aging, and thus, early capacity loss.
- Overheating disabling the battery, especially during high power uses
- No possibility to charge at low temperatures
- Significantly reduced battery life.
If you have experienced such issues, read carefully the following lines.
How does a lithium-ion cell store energy?
The principle of a lithium-ion cells is that the lithium ions are moving from one electrode to the other one by diffusion in a liquid electrolyte through an electrically insulation separator, as illustrated in the picture here below. When charging the cell, ions are pushed into the active material of the negative electrode (anode) ; when discharging the cell, ions are pushed into the active material of the positive electrode (cathode).
The molecular structure of active materials at both electrodes enables li-ions to be inserted into free sites of the material. As you can see, the active materials (Graphite and LiM02 on the above picture) are coated on metallic electrodes (copper for the graphite and aluminum for the LiMO2). Increasing the thickness of the active materials results in a increased number of sites to store lithium ions for the same volume of other materials (electrodes, separator…). This leads to a higher energy density li-ion cell. Since the electrodes are thicker, there is more distance to cover through the material for the lithium ions and it increases the internal electrical resistance of the cell. Thinner electrodes on the contrary enable faster charge and discharge rates but limit the energy density, resulting in a high power cell.
Influence of low temperature on battery performance
Some of you may have noticed that their smartphone is quickly losing its energy under very cold conditions (skiing day, strong winter…). Indeed, high energy li-ion cells are using very thick electrodes, and lithium-ions have a harder access to the free insertion sites located deep inside the electrodes. The access to these sites become impossible at low temperatures due to the increased resistance in the more viscous electrolyte and harder diffusion into the less flexible active materials. The figure below shows the way the ions have to follow to reach deeper active materials insertion sites.
The lithium-ion cells thus exhibit a much lower capacity at low temperatures, which can be measured easily. This phenomenon is amplified for high energy cells, with the thickest electrodes. Typical numbers are given in the table below.
|Capacity at 25°C
|Capacity at -10°C
Due to the increased internal resistance of the negative electrode, charging at low temperature (approx. 5°C or less) can lead to lithium plating on the negative electrode. Lithium plating ages the cell by consuming lithium-ions which are not available anymore to store energy. But most importantly, lithium plating can lead to dendrite formation and cause an internal short-circuit of the cell (more information in this Elsevier whitepaper from Xianke Lin and all). The internal short-circuit can lead in the best case to an early end of life of the cell and in the worst case to a thermal runaway, resulting in a battery fire and/or explosion.
Influence of high temperature on battery performance
As explained in our previous TechLetter on Lithium-ion battery aging, numerous studies show that the aging of any type of li-ion battery is divided by 2 to 3 between a usage at 25°C and the same usage at 45°C (as example : Elsevier – Transportation Research Part B 103 (2017) 158–187 or the PhD thesis of Arnaud Devie http://www.theses.fr/2012LYO10231 ).
Furthermore and for safety reasons, li-ion batteries cannot be used above a defined maximum temperature to prevent thermal runaway (see TechLetter on battery safety for further information). This is why most li-ion cells cannot be charged above 45°C and cannot be discharged above 60°C.
The weakest cell limits the performance of the whole battery
A battery pack is composed of a lot of lithium-ion cells connected in series and in parallel to reach the requested power and energy (see TechLetter on battery system for further information). As illustrated in the figure below, every cells connected in series sees the same current; so if one cell has got less capacity than the others, it will reach its end of discharge before others and limit the performance of the whole battery.
It is therefore prominent to ensure that the cells have a very homogeneous aging to maximize battery pack life. The thermal management of the battery pack has to make sure that all the cells are working at the same temperature.
Due to the physical phenomenon involved in the lithium-ion cells, temperature greatly impacts their performance and aging. Controlling temperature over the life of the battery is then prominent for applications requiring constant performance and long life. This temperature control has to make sure that cells temperatures are very homogeneous in the whole battery pack so that the global performance is not limited by the most aged cell.
Battery pack air cooling is not efficient enough. Water cooling leads, by design, to a wide temperature spread between cells and increases fire risks in case of water leakage in the battery.
By using immersion cooling and a patented cooling architecture ensuring very homogeneous temperature in the whole battery pack, WATTALPS’ technology enables high performance and longer life, together with improved overall battery safety.
More information on WATTALPS’ cooling performance in a coming TechLetter to be published this summer.