Page - 73 - in Hybrid Electric Vehicles

relatively high voltage DC, which is about 400 V. In addition, the server/controller monitors, calculates and controls the amount of electricity received from the electrical grid based on some data, including electricity price and grid condition. Furthermore, the server manages the electricity to and from the battery and the charging rate from a quick charger to the con- nected vehicles. In the battery unit, a bidirectional DC/DC converter and battery management unit (BMU) are introduced to facilitate controllable charging and discharging behaviours according to the control values from the server. In the quick charger, a DC/DC converter and a charging control unit (CCU) are introduced to facilitate active management during vehicle charging. The number of quick chargers can be more than one, depending on the conditions. The battery is adopted to store the electricity in case of the presence of remaining contracted power capacity and lower electricity price (during off-peak hours). In addition, the battery dis- charges its stored electricity in case of high electricity price due to high demand for charging or peak hours. The stationary battery having relatively large capacity is generally employed to sufficiently facilitate simultaneous charging of multiple vehicles. Therefore, the charging service can be maintained with high quality. According to the charging and discharging behaviours of the employed stationary battery and the source of electricity for charging, quick-charging modes of the BAC are classified as follows: a. Battery discharging mode Stationary battery releases its electricity to assist the charging. Therefore, vehicle charging is conducted using electricity received from the electrical grid and discharged from the stationary battery. This mode is introduced when a simultaneous quick charging of mul- tiple vehicles occurs, especially in case of high electricity price. Electricity in the battery discharging mode can be shown as follows: P grid + P bat = P QC1 + P QC2 + P loss (1) where Pgrid, Pbatt, Pqc and Ploss are electricity received from electrical grid, charged (negative value) or discharged (positive value) electricity from stationary battery, discharged elec- tricity for quick charging of vehicles and electricity loss, respectively. b. Battery charging mode When there is remaining electricity (margin between the contracted power capacity and the used electricity) or the electricity prices is getting down (because of surplus electricity in the grid, night time, etc.), the stationary battery is charged to store electricity. The flow in this mode is expressed as Eq. (2). P grid − P bat = P QC1 + P QC2 + P loss . (2) c. Battery idling mode Stationary battery might be in the idling (stand-by) mode in case of several conditions: (a) contracted power capacity can sufficiently cover the electricity demand for simultane- ous charging of vehicles (low charging demand), (b) stationary battery is empty or under Advanced Charging System for Plug-in Hybrid Electric Vehicles and Battery Electric Vehicles http://dx.doi.org/10.5772/intechopen.68287 73