Requirements for rechargeable batteries for electric and hybrid electric cars

Requirements for rechargeable batteries for electric and hybrid electric cars

Research on the batteries of electric cars and hybrid electric cars has shown that the performance parameters of mass energy density and volumetric energy density are very important. In addition to these two electrical performance parameters, the cost, size, weight and life of rechargeable batteries ranked second. These two performance parameters of lead-acid batteries are very poor. Silver-zinc batteries (Ag-Zn) have the highest volume energy density and mass energy density, as listed in Table 1. After silver-zinc batteries, the electrical performance parameters of silver metal hydride (Ag-MH) batteries rank second. The electrical performance parameters of various commercial batteries, namely mass energy density and volume energy density, are summarized in Table 1.

Battery TypeVoltage range/VTemperature range/℃Cycle life/hMass energy density / (W·h/kg)Volume energy density / (W·h/L)Self-discharge (%/month)
Ni-MH1.4 ~1.2﹣30 ~65900 ~ 120065 ~ 85200 ~ 85015 ~20
Ni-H21.5 ~1.2﹣10 ~30>220045 ~ 6068 ~8440 ~60
Ni-Zn1.9 ~ 1.5﹣20 ~50326 ~ 65055 ~ 65100 ~14014 ~18
Zn-air1.2 ~1.00~4520 ~ 30150 ~ 220160 ~ 2405 ~10
Zn – AgO1.8 ~1.5﹣20 ~6050 ~8580 ~100175 ~1854~6
Table 1 Characteristics of different aqueous rechargeable batteries

The electrical parameter values ​​summarized in Table 1 are valid for rechargeable batteries designed, developed and tested during 1990-1995. In addition, the parameter error is within ±5%. In the past 15 years or so, the electrical parameters of these batteries have improved significantly.
Currently, various automakers are focusing on electric cars and hybrid electric cars. The battery requirements of all-electric cars are different from those of hybrid electric cars. To be more cost-effective, rechargeable batteries must generate more electricity per unit of mass or volume. The battery of a hybrid car is charged from an on-board power supply or from a charging device composed of an alternator and inverter, which converts alternating current (AC) into direct current (DC). Hybrid operation usually only accounts for a small part of the battery dry cycle.
In some hybrid cars, the capacity of the battery is only sufficient for one cycle of operation of the car, after which the battery needs to be recharged. On the other hand, the battery of an all-electric car is charged by the on-board power supply, and the time for full discharge varies from a few hours to a few days before it needs to be recharged. This is a fundamental difference between the charging requirements of all-electric cars and hybrid cars.

①Test requirements for rechargeable batteries required for electric and hybrid cars
Due to the special needs of electric cars, tests for other types of batteries cannot be used to predict the rechargeable batteries of electric cars. Tests are usually carried out on the basis of actual or virtual driving behavior; however, the test results of electric and hybrid car batteries have been accepted. In the United States, battery testing procedures have been developed and approved by the joint government and industry authorities. The American Society of Automotive Engineers (ASAE) in Warrendale, Pennsylvania has standardized testing as a “recommended practice.” Automakers in Japan, Europe and South Korea have developed similar test procedures, and the test results have been approved by their respective government and industry authorities. Recently, internationally standardized tests have been approved. If the performance requirements of electric or hybrid cars are clearly specified, standardized tests can reasonably predict the performance of the rechargeable batteries of that particular car. In addition, the requirements for batteries must be carefully determined to meet the performance requirements of compact, medium or full-size electric or hybrid cars . The battery test results can be incorporated into the rechargeable battery model, which is based on the specific technology used in the battery design to predict the performance of electric cars or hybrid electric cars.
②Battery life prediction for electric and hybrid cars
As discussed, it is difficult and expensive to predict the life of rechargeable batteries if possible. The degree of uncertainty stems from the configuration and technology used to design that particular battery. A battery pack for an electric car usually requires 100 or more electrochemical cells in series. It is extremely difficult to maintain balance with so many batteries due to manufacturing differences between batteries and battery-to-battery temperature changes when the car is running.
According to different types of rechargeable batteries, various control mechanisms and maintenance procedures are required to achieve a perfect balance. In some cases, thermal management systems, periodic overcharging, and active electronic systems are required to maintain the battery’s state of charge within its expected operating range. Under these complex requirements, battery life is highly variable and therefore unpredictable.