The cell is the smallest energy source of an electric vehicle battery. When these are connected to several equal cells, it forms a module. Hence, connecting many modules made from the same cells culminate in a battery pack. Actually, to become a battery pack it is also necessary to account an electronic control system, the hardware the battery pack. This is called, battery management system BMS. This article proposes a brief summary of BMS and main causes of EV battery deterioration.

Battery Management System BMS

In the chemical point of view, since there is no water inside the cell, in case of overcharged or overdischarged electrolysis, there is no cooling effect. Hence, controlling the thermal balance of the cell inside the series is necessary. It is important not only for performance but also for safety. This is defined as the Battery Management System, which is the electronic board. Renault Zoe, one of the first electric cars, was developed with a very simple air cooling system. Its BMS controlled overcharge and overdischarge well. This allowed a safe range and life cycle of 300,000 Km. The problem with this system is that the battery and BMS were overdimensioned. In summary, this car was heavier than similar models. Certainly this limit its performance.

Battery pack assembly

The assembly principle has four stages, the battery cell, module, system and vehicle integration. These require multidisciplinary and integrated team work. To ensure a system with many cells operating well, the development team should be capable to transit between chemistry and design design departments to perform a redesign when necessary. Therefore, it is evident that EV battery manufacturing is multidisciplinary learning design cycle that should have the proper time and investment. 

Battery pack degradation

The usual stable health of an automotive battery is about 80% of the original efficiency. The mileage at which the battery reaches this condition is very difficult to figure out. The reason is due to the many variables involved, it is a thorough statistical analysis. For instance, fully using an electric car’s battery range and recharging it quickly accelerates cell degradation. This damages the cells in the process. Another interesting aspect is the good practice to stop using the car when the battery charge is above 20%. Keeping the battery charge above 20% is important. This also accelerates the battery pack degradation. Moreover, the constant use of full-power mode also deteriorates the cells. All those factors complicate buying a second-hand electric car. There are no methods for battery evaluation. This difference between electric and internal combustion engine powered cars. The more than 100 years of development of the last ones provides a well established market and regulations. In comparison, the history of lithium-ion batteries in electric cars has less than 10 years regarding its co-development.

Battery pack main problems

There are three main concerns in EV batteries, flammable electrolytes, electrolyte breakdown (sealed cell) and thermal runaway. The first is important since in case of leakage, the electrolyte is organic and flammable. If the electrolyte contacts the graphite, gasses are produced. These gasses increase pressure and can destroy the battery. Usually, this results in the explosion of the battery pack when it is being discharged or recharged. The minor consequence is cell aging. Another problem is the thermal runaway during a car crash, since the electrodes inside must not be in contact.

Safety strategies

The main safety strategies include thermal management, which is intrinsically connected with the cooling system design. Another strategy is the battery management system, aimed at providing devices with trouble-sensing capabilities. Additionally, lithium-ion cell innovations like self-healing materials and improved electrolytes are important. Lastly, encapsulation techniques utilize silicones, epoxies, and lightweight foams.

References

• This article was based on the lecture notes written by the author during the Vehicle Components Lectures of the Advanced Automotive Engineering Master Degree course at Università di Modena e Reggio Emilia;