Although there has been a significant advancement in increasing the range of electric vehicles, the charging speed is still a matter of concern. For instance, DC fast chargers can charge the battery to 80 percent in about 30 to 45 minutes. In contrast, it is possible to fill the gas tank in only a few minutes. Fast charging has its limitations, as the process generates a significant amount of heat. The high current and the internal resistance of the cable and the battery typically generate a significant rise in temperature.
EV batteries are typically rated at 400 V, and several factors limit their charging rate. This includes the cross-sectional area of the charging cable and the temperature of the battery cells. The temperature rise can be high enough for some fast-charging stations that necessitate liquid-cooling of their cables. Therefore, it would seem reasonable to expect that an increase in the battery’s voltage will boost the power it delivers.
Porsche, in their Taycan EV, has done just that. Their first production vehicle has a system voltage of 800 V rather than the usual 400. This would allow a 350 KW level 3 ultra-fast DC charging station to potentially charge the vehicle to 80% in as low as 15 minutes. But then, an EV design with an 800 V system requires new considerations for all its electrical systems, especially those related to managing the battery.
Switching the vehicle on and off requires the main contactors to electrically connect and disconnect the battery from the traction inverter. On the other side, there are independent contacts for connecting and disconnecting the battery to and from the charger buses and the DC link. For DC fast charging, additional DC charge contacts are necessary that can establish a connection from the battery to the DC charging station. Additionally, auxiliary contactors connect and disconnect the battery to electrical heaters for optimizing the passenger compartment temperature in cold weather conditions.
Moving to a higher battery voltage increases the potential for the formation of electrical arcs, and these can be damaging. Vehicle architectures operating at 800 V therefore, require stricter isolation parameters than those necessary for 400 V architecture. This can increase the cost of the vehicle.
For instance, higher voltage levels require the connector pins to have greater creepage and clearance between them to reduce the risk of arcing. Although connector manufacturers have managed to overcome these issues, the connectors are more expensive than those they offer for 400 V systems, thereby jacking up the total costs.
The maximum battery voltage decides the ratings of components that the traction inverter module uses. For battery voltages at 400 V, there is a wide range of selection of suitably rated components. But this range reduces drastically when the battery voltage is at 800 V. Most components for higher voltages come with a premium price tag attached. This raises the price of the traction inverter module.
A solution to the above problem is to use two 400 V batteries. To reduce the charging time, the batteries may connect in series. They can connect in parallel when driving.