Peak vs continuous power is a recurring question across the electrification space. We need to deliver a repeatable amount of power for the user to have confidence in the machine and we need high power numbers to deliver the brochure wow factor. The transient peak power works well for a number of vehicle applications. However, a lot of commercial applications are all about the continuous power capability.
Spirit of Innovation is the Rolls Royce electric aircraft designed to set air speed records.
An application where reliable continuous power capability is critical.
Depending on the product, performance and drive cycle requirements there is a need to look at the peak and continuous ratings.
Peak
- limited by the cell ion charge transfer kinetics
- considered as up to 10s in duration
- use the thermal mass of the battery cell and components
- peak delivery assumes recovery periods
Continuous
- limited by the cell ion charge transfer kinetics
- limited thermally
- maximum cell temperature
- continuous heat rejection capability of the pack
- maximum cell temperature gradient
- maximum cell to cell temperature difference
- busbar continuous rating
This post has been built based on the support and sponsorship of: AVANT Future Mobility, Quarto Technical Services, TAE Power Solutions, h.e.l group and The Limiting Factor.
Cell Capability
Also, this requires a battery cell design with the right power and energy balance.
Power versus Energy Cells
In simple terms the energy cell has thicker layers of active material, thinner current collectors and less of them.
This means the energy cell will have a higher electrical internal resistance meaning it will generate more heat based on I2R heating.
The energy cell will have poorer thermal conductivity in-plane and through-plane. Thus, it will need a higher temperature gradient to reject the heat.
Lain and Kendrick [1] tested commercial cell electrodes at high C-rates to understand the limiting factors, pulse power tests at high rates typically showed three limiting processes within a 10s pulse:
- instantaneous resistance increase
- solid state diffusion limited stage
- electrolyte depletion/saturation
- on anodes, the third process can also be lithium plating.
Jow et al [3] look at the Factors Limiting Li+ Charge Transfer Kinetics “The Li+ charge transfer process starts from the solvated Li+ in the electrolyte to the reception of an electron (e−) from the electrode and becomes Li (i.e., LixC in graphitic anodes). This involves the de-solvation step of Li+ before entering into a layer of solid electrolyte interphases, or SEI, that is often referred to that on the anode such as carbonaceous materials, and the diffusion step of Li+ through the SEI, which is pre-formed on the electrode at the electrode and electrolyte interfaces, before receiving an e− from the electrode at the electrode and SEI interface.”
As we increase the discharge rate of a cell then less capacity will be available, the usable energy of a cell will be dependent on design, temperature, discharge rate and age.
Discharge rate capability of a new SAFT MP 176065 xtd battery [2].
As you can see, at a C/8 discharge rate (purple line), the cell offers a 5.8 Ah capacity, at 1.5 C, the cell capacity goes down to 5.5 Ah (green line).
The Ragone Plot is a good way of looking at cell capability.
This plot shows cell gravimetric power versus gravimetric energy density.
Plotted with 10s Peak and Continuous power data.
Cooling System
The power capability of the cell is determined by and limited by the cell temperature. Hence the cooling system design needs to be in line with the power requirements of the battery pack and the cell requirements.
Increasing the cell temperature will reduce the DC internal resistance, resulting in a smaller voltage drop and less I2R heating for a given current demand. This would require the cells to be heated uniformly and for the cells to be managed within a narrower temperature window. This is a one approach to cooling system design for very high performance battery packs such as: McLaren Speedtail or Mercedes 2023 C63 AMG.
If peak performance is only required very occasionally then the heat could be absorbed within the thermal mass of the cell. The heat can then be rejected to the cooling system over a longer time frame.
All of the cooling systems have pros and cons that need to be balanced in the overall design.
HV Components
The sizing of all of the other components needs to be considered carefully.
Thermal mass means heat generated in the cells might take time to conduct away from the core to the cooling system. Meaning you need to consider the number and position of temperature sensors as well as the algorithms that the BMS uses to estimate temperature versus limits.
The busbars are normally sized based on continuous current requirements. In electric cars this is often determined based on the fast charge cycle. Moving towards a more repeatable peak power means the sizing needs to be reconsidered. Cooling of the contactors, fuses and busbars should be considered in order to maintain lifetime or to offset mass increases.
References
- Michael.J. Lain, Emma Kendrick, Understanding the limitations of lithium ion batteries at high rates, Journal of Power Sources, Volume 493, 2021
- Lithium-ion batteries in use: 5 more tips for a longer lifespan, SAFT Batteries
- T. Richard Jow, Samuel A. Delp, Jan L. Allen, John-Paul Jones, and Marshall C. Smart, Factors Limiting Li+ Charge Transfer Kinetics in Li-Ion Batteries, Journal of The Electrochemical Society, 165 (2) A361-A367 (2018)
Continuous Current Rating
At some point in the development of a battery pack design you need to consider the continuous current rating. Do this for charge and discharge as this then gives you one for the fundamental requirements to determine:
- cell to cell busbars
- HV joint requirements
- HV distribution busbar cross-sectional areas
- contactor sizing
- fuse sizing
- connector sizing