Effects of VRLA Battery Formation on Electric Vehicle Performance

2 ELECTRIC VEHICLE BATTERY

EFFICIENCY

To develop a practical electric vehicle (EV), it is essential to understand the behavior of a set of batteries in a pack (when they are connected in a series or parallel configuration).

Figure 2–1 illustrates some of the factors that influence the battery efficiency of a traction battery (battery efficiency h is defined as the ratio of the energy output of a battery to the input energy required to restore the initial state of charge under specified conditions of temperature, current rate, and final voltage). Formation time, discharge rate, frequency of the charge/discharge of batteries, temperature of charging and discharging, etc., are some of the factors that affect the battery efficiency.

EFFECTS OF VRLA BATTERY FORMATION ON ELECTRIC VEHICLE PERFORMANCE

The formation process of batteries converts unformed paste masses to the charged state, which is stored as energy. In the case of lead-acid batteries, this involves the formation of lead dioxide (PbO2) with lead on the positive plate and sponge lead on the negative plate. This process of conversion of the unformed paste to an active mass is carried out in the presence of sulfuric acid. At the end of formation, the battery gains its final strength and the active mass exhibits a porous structure with a large internal surface.

However, several factors affect the formation of the batteries, including electrolyte temperature, concentration of the forming electrolyte, and current density during the formation cycles. These external factors singularly affect the battery efficiency h and performance of the EV. For example, a typical 85Ahr valve regulated lead acid battery requires 8 to 12 cycles to form up to 60% of its useful rated capacity (capable of providing 60 to 65Ahr), up to 17 to 20 cycles to form up to 65% of its useful rated capacity (capable of providing 75Ahr) and up to 36 to 40 cycles