TheUS Advanced Battery Consortium(USABC) states that the battery used by a plug-in hybrid (PHEV) is expected to last for over 15 years and 300,000 charging cycles.
However, its lifetime hinges on different stress factors that strongly affect degradation rate, with temperature being a huge factor. Further, by being the most expensive element in an electric vehicle, the automotive industry is focused on developing better battery thermal management systems to avoid premature and costly battery replacement.
What are the latest developments in battery thermal management systems?
The relationship between degradation and temperature can be formulated by an Arrhenius-type behavior where degradation rate increases exponentially with temperature:
“The exact relation depends on the specific electrochemistry and design of the battery. Therefore, there is no single life model that models all different chemistries.”
Although the capacity increases as the operating temperature is raised, the degree of capacity fade also increases. On the other hand, poor performance is observed at low operating temperatures. In addition, excessive or uneven temperature rises in a system or pack reduces its lifecycle significantly.
As an industry reference, EV batteries reach their last days once they reach a 20 percent capacity loss or 30 percent internal resistance growth, and both active and passive battery thermal management systems (BTMS) are the main cards that engineers play to tackle battery overheating and poor performance.
Further, there are various types of BTMS techniques depending on the purpose, source and cooling medium. By purpose, there are systems based on cooling only rather than both heating and cooling. Source includes passive, if cabin air is used without extra pre-conditioning, or active, if a specific heating or cooling component is installed in the system to change the air before entering into the pack. Lastly, by cooling medium could mean air versus liquid.
“Efficient temperature management systems contribute significantly to battery health and extend the overall lifespan,” says Lyu et al in their 2019 paper Electric vehicle battery thermal management system with thermoelectric cooling. “Moreover, as the capacity and charge and discharge rate increase, battery security issues need more attention. Subsequently, various BTMS have been developed to meet the demand for higher power, faster charge rates, and improved driving performance.”
Regarding passive BTMS, phase change materials, heat pipes and hydrogels are utilized, with the benefit of no additional power consumption. On the downside, the cooling process is difficult to manage. In opposition, the paper continues, traditional active methods generally lead to forced circulation and circulation of specific cooling materials and substances such as water and air: “The main issue is that the cooling effect can be very limited under certain circumstances.”
TEC combined with other BTMS techniques
The first method for effective heat mitigation in Li-ion batteries is the choice of the electrode materials, which is inherent to the battery cell technology. One BTMS that is drawing attention in the EV industry is thermoelectric cooling, or TEC. This is due to some benefits relating to their solid cooling capabilities and dependable working potential, besides being quiet, stable, and allowing an easier control of temperature by just adjusting the voltage supply.
Thermoelectric coolers are based on the conversion of voltage to the temperature difference. This Peltier–Seebeck effect, together with the Thompson effect, belongs to the thermoelectric effect. “The thermoelectric effect refers to all of the transformation processes from heat to electricity, and vice versa." Several noted studies have been carried out in recent years.
In one, the cold sides of the thermoelectric coolers were connected to the heat sink in these designs and maximum temperature was kept below 55°C. “The cold air was blown into the battery pack and cabin for cooling. Later, a heatsink-fan set for both the cold side cooling and the hot side heat dissipation was incorporated.”
Besides, thermoelectric cooling can be combined with other battery thermal management systems, such as forced air cooling and liquid cooling.
In the air cooling system, the battery is cooled by an airflow sweeping the battery pack. The air typically comes from outside, but also from the cabin and additional AC units in systems that are more complex. The advantage of this method is its simple nature, where no insulation between the air and the battery is needed or which allows less maintenance and lighter components.
There are also drawbacks to this approach. The limited specific heat capacity of air requires larger space to be used for components constituting the BTMS. Moreover, special geometry for the coolant channel needs to be used and only a few cells can be cooled at once. Due to the above considerations, the airflow velocity needs to be increased, which leads to lower energy efficiency.
As for liquid cooled systems, components which contain a coolant are located in between cells or modules. The heat is then transported to a heat sink, placed away from the battery pack. The heat sink can be a simple radiator or a more complex system dissipating the heat into a refrigerant circuit. Usually, both are used in combination, where switching is done depending on various parameters.
The disadvantages of such a system are a higher weight resulting from the extra components and the proximity of the liquid coolant to the high voltage components. Hence, during operation and maintenance, various safety measures need to be in place.
Both of the above approaches are called active, because they use external components such as pumps and fans which use additional battery power. An additional disadvantage is the creation of noise and vibration in a generally silent setting, the need for extra maintenance, and a higher cost of components over time.
A combination of TEC and these BTMS is possible
Here, the liquid coolant acts as the medium to remove the heat generated from the battery during operation. Such BTMS design is a combination of TEC with forced air cooling and liquid cooling in which the liquid coolant works as the medium to remove heat from batteries:
“Forced air assisted heat removal is performed from the condenser side of the thermoelectric liquid casing. Detailed experiments are carried out on a simulated electric vehicle battery system.”
The battery is placed vertically in the center of the coolant container. Flowing liquid takes away a considerable amount of heat generated by the battery during operation. A water pump is used to drive liquid circulation. The TEC is used to manage the temperature of the coolant afterward. Lastly, the hot end of the TEC will be cooled by the heat sink and fan attached to it.
From the same paper, experimental results reveal a promising cooling effect with a reasonable amount of power dissipation: “Moreover, the experimental test shows that the battery surface temperature drops around 43ºC – from 55ºC to 12ºC – using a TEC-based water cooling system for a single cell with copper holder when 40V is supplied to the heater and 12V to the TEC module.”
However, the system performance is to be constantly enhanced to meet the demand of progressively higher heat generation. In another study, it was reported that COP of the BTMS decreases gradually with increasing power supply to the TEC and the maximum temperature of the battery was less than 36.2°C.
The EV revolution is nothing if the cars themselves can't deliver. And that means not just after 15 days, or 15 months, but 15 years. As a result, it's no surprise that so much time and effort has gone into refining battery thermal management systems, and will continue to do so to ensure EVs function time and again.
Interested in Battery Thermal Management Systems and all they entail? Automotive IQ'sThermal Management for EVs and HEVs conferencetakes place in Berlin, Germany, from 10-13 February 2020.All the details are here.
Written by Cristian Tangemann 12/09/2019