If you are in a battery design role there is always pressure on increasing usable SoC window. Nobody wants to pay for and carry around unused battery capacity. However, there are some really good reasons to restrict that window.
High SoC and hence high cell voltage stresses the cell and significantly reduces the lifetime. Going over the maximum cell voltage risks safety of the cell and pack. Although this top end SoC is controlled by cell voltage, any error in SoC estimation needs to be taken into account when setting the SoC limits.
At low SoC the Open Circuit Voltage (OCV) is decreasing and the internal resistance of the cell increasing. Hence with a discharge load the cell voltage will drop even further and more rapidly approach the minimum cell voltage. This minimum cell voltage will be set by the cell manufacturer to avoid damage and extend the cell lifetime.
In order to increase the usable SoC window we need to understand:
- thermal behaviour of the cell
- thermal behaviour of the pack
- electrochemical behaviour of the cell
- electrical behaviour of the pack
- thermal behaviour of the system
- control system
- ageing performance of the cell
- thermal, electrical, control and operation of the total system and it’s environment
Note: all of these bullets can be expanded and we will do that as a separate deep dive into the subject of Ageing vs SoC Window.
This data collection and analysis all takes time and experience. Hence the initial usable SoC window might be smaller than required in a new application or with a new chemistry.
Note: cell-to-cell variations and non-uniform degradation of the pack can lead to local issues where the anode can reach 0V with reference to Li and lead to dendrite formation. Especially at high charge rates (Sreekanth Pannala).
The following plot of the usable SoC window versus battery pack year of production is perhaps not unexpected.
Data over the air and software updates over the air are having a significant impact on this. Mixing real user data with laboratory test data increases the depth of knowledge around the application.
One example is if you know the users are happy to manually select a lower maximum SoC for their daily use of a vehicle and only set this to a maximum when they need it you can probably increase the maximum SoC. Thus giving the user a better experience.
A look at the estimation of State of Charge (SoC) using voltage profiling and coulomb counting. These two methods give a good overview of the difficulty and errors associated in estimating this critical battery parameter.