When we look at the battery versus system voltage we have to remember that these are working together. In fact we have to look at the complete system and all components to ensure they can work together over the maximum and minimum voltage range. This will normally be the maximum charge voltage and the minimum voltage will be the under load transient condition.
This plot of maximum and minimum pack voltage versus the nominal voltage was used to show the increase voltage range as you move to Higher Voltage Packs.
What you also see within this is the clustering just below 400V, just over 600V and around 800V. OK, this is benchmarking data and so you do need to squint, but there is a reason for this.
What we are seeing is the operating voltage of the battery packs being positioned to work within the operating range of the power electronic devices.
Silicon Voltage Rating
Silicon and silicon carbide power electronic devices have a maximum blocking voltage. Blocking voltages of 650V, 900V or 1200V for devices are normal for electric vehicles (other ratings higher and lower are available). This rating drives the design and cost. Typically 650V devices are used in 400V nominal system designs. 1200V devices are used in 800V nominal systems.
The 900V devices are reasonably new to the market and offer a lower cost and a nominal system voltage around 600V.
Safety and Lifetime
The battery cell will have an upper and lower voltage limit. These are often time dependent, but have to be enforced as the result can be cell failure or even thermal runaway.
Motor Power Delivery
For any motor, the basic principle is:
- rotational speed is proportional to voltage applied
- torque is proportional to current pulled
The motor will be designed for a maximum continuous and peak current, defined by the windings and ability to remove the heat generated in them. Thus, as the voltage decreases the maximum rotational speed decreases and the ability to deliver the power will reduce as it is current limited.
The inverter will also have a maximum continuous and peak transient current rating.
As Power = Voltage x Current this means that as voltage drops the maximum power will scale with these current limits.
This means we need to model the battery, motor and inverter together in order that you can fully map the power and torque delivery.
Don’t forget the charger, it isn’t very useful designing a battery pack that cannot be charged fully as the maximum voltage is above normal charging unit capability.
In any electric vehicle there are numerous devices attached to the high voltage (HV) bus and the low voltage (LV) bus. These two buses are normally connected by a DC-DC that allows the HV bus to power the LV bus. However, the operating limits of this DC-DC need to be set such that it can deliver the required power even when the battery is under load.