When looking at the key parameters in fast charging a battery pack it is worth looking at the complete system. Also, it is good to look from the cell at atomic scale through the thermal system design to the charging algorithms and handshakes [1].
The drive for a 10 min fast charge to reach 80% state of charge is tough against the other pressures of reducing cost and shrinking the pack. In most cases this fast charge is the worst case in terms of power requirements for the battery pack.
Also, fast charging in 10 minutes encourages the drive to reduce the battery pack size. This is the case whether it is a 2500mAh cell in your phone or a 100kWh pack in your car. A fast charge that allows you to continue is way around the low battery warning anxiety.
Atomic Scale
- Electric potential
- Diffusion
- Charge transfer
Nano/Micro Scale
- SEI growth
- Li plating
- Mechanical degradation
- particle cracking
Cell
- Capacity fade
- Impedance rise
- Temperature and current heterogeneities
- Current collector sizing
- anode and cathode homogeneity
- anode and cathode thickness
- Cell level modelling and control
Module
- Cell to cell variations
- temperature
- voltage
- current
- pressure
- Voltage measurement accuracy
Pack
- Cell-cell or module-module variations
- temperature
- voltage
- Cell balancing
- Busbar and joint electrical and thermal rating
- Fuse electrical and thermal rating
- Contactor electrical and thermal rating
- Current sensor accuracy
- Connector electrical and thermal rating
- BMS
- State of charge estimation accuracy
- State of health estimation accuracy
- Cell, pack voltage measurement and algorithm estimation accuracy
- Current measurement and algorithm estimation accuracy
- Cell temperature measurement and algorithm estimation accuracy
- Pack thermal plant model accuracy versus boundary conditions
- Algorithm/approach:
- Constant Current – Constant Voltage (CC-CV)
- Constant Power – Constant Voltage (CP-CV)
- Multistage Constant Current – Constant Voltage (MCC-CV)
- Pulse charging
- Boost charging with a CC-CV-CC-CV scheme
- Variable Current Profile (VCP)
- Stepped Constant Current – Constant Voltage (SCC-CV)
System
- Thermal management
- sizing vs ambient
- conflicts eg cabin conditioning
- Cable electrical and thermal rating
- Connector electrical and thermal rating
- Electrical rating of complete system from charge inlet to cell
- Ambient conditions
- Safety
- User drive cycles / habits
Charger
- Charging algorithms
- Thermal rating and duty cycles
- Durability
- Charging handshake with battery pack
- Ambient conditions
Grid
- Rating of local and national grid
- Ambient conditions
- Time of day
References
- Anna Tomaszewska, Zhengyu Chu, Xuning Feng, Simon O’Kane, Xinhua Liu, Jingyi Chen, Chenzhen Ji, Elizabeth Endler, Ruihe Li, Lishuo Liu, Yalun Li, Siqi Zheng, Sebastian Vetterlein, Ming Gao, Jiuyu Du, Michael Parkes, Minggao Ouyang, Monica Marinescu, Gregory Offer, Billy Wu, Lithium-ion battery fast charging: A review, eTransportation, Volume 1, 2019, 100011, ISSN 2590-1168
- Hong Zhao, Li Wang, Zonghai Chen, Xiangming He, “Challenges of Fast Charging for Electric Vehicles and the Role of Red Phosphorous as Anode Material: Review“, energies
- Michael Nicholas and Dale Hall, “LESSONS LEARNED ON EARLY ELECTRIC VEHICLE FAST-CHARGING DEPLOYMENTS“, the icct, July 2018
Stepped Fast Charge Limits
We often see stepped fast charge limits as shown in this BMW iX3 graph.
Often the result of a limited test regime applied by the cell supplier (in this case CATL) to establish the maximum charge current for the cell.
These limits are them applied by the BMS team and without more data they have to drop back to safe charge current limits between these points.