EV Battery parameters

Battery parameters

As seen in the previous lectures, the battery is the most important part of the EV. It determines the cost of the EV to a large extent and its range. Hence, understanding the battery performance parameters is very important. It forms the basis of understanding battery charging. Let us now look at a few main performance parameters we should know:

1. 1. 1. Nominal voltage (V_nom, in V): It is rated voltage of the battery when it is fully charged. When a battery is discharged or is loaded, the voltage reduces gradually to a lower value, V_batt
      2. Nominal current (I_nom, in A): It is rated current of the battery for charging or discharging. Typically, the actual charging/discharging current I_batt <= I_nom. Further, electric cars normally have much higher peak discharging current than the peak charging current.
      3. Ampere-hour (Q_nom, in Ah): An ampere hour is a unit of electric charge that corresponds to the charge transferred by a steady current of one ampere flowing for one hour, or 3600 coulombs. The commonly seen milliampere hour (mAh) is one-thousandth of an ampere hour (3.6 coulombs). The total amount of energy that can be stored in the battery can be calculated by the product of battery ampere hours and battery nominal voltage. The effective ampere hour of a battery is determined by the charging/discharging current as described by Peukert’s law.  This results in the following formula: 
         
                  E_{nom =} Q_nomV_nom
         
         
      4. Charge/discharge Efficiency (in %): When charging or discharging a reversible battery, neither can all the energy sent to a battery be effectively stored nor can all the available electric charge inside a battery be retrieved successfully. The efficiency is used to represent the ability of a battery to store/retrieve electric charge or energy. The efficiency of a battery is not always constant even in one cycle, and it depends on the SOC, cell temperature and current. When we talk about efficiency, generally it refers to the overall cycle efficiency, which means the efficiency of a whole cycle.