In the current context, the consistency of lithium-ion batteries refers to the convergence of key characteristic parameters of a group of lithium-ion batteries. It is a relative concept; there is no absolute "most consistent," only "more consistent." Ideally, the parameters of each cell within a series of cells in the same battery pack should all fall within a relatively small range for good consistency.
Incorporating the time dimension, consistency refers to the consistency of all characteristic parameters of all cells within the battery pack throughout their entire lifecycle. This includes considering inconsistencies such as capacity degradation, internal resistance increase, and aging rate. The lifespan of the entire battery pack is the ultimate focus of our attention to consistency.
Some scholars have plotted the interactions between parameters on a graph based on the passage of time. Time is the horizontal axis, and the parameters are the vertical axis. Several parameters change with time and are placed in a table, intersecting to form a network, which serves as our starting point for considering consistency.
The goal of pursuing consistency is not only to maximize the battery pack's capabilities (maximum power, maximum current, and maximum usable capacity) under the current conditions, but also to maintain these capabilities for as long as possible.
Within what range should the consistency of evaluation for lithium-ion battery sorting machines be assessed?
My understanding is that consistency refers to the consistency of all the battery cells that power an electric vehicle, regardless of whether they are connected in series or parallel. The following content does not provide a comprehensive commentary, but only provides examples.
Parallel connection
To illustrate, consider a parallel module (D) consisting of a low-discharge-capability cell (code b) connected in parallel with other normal cells. For example, this might be a module with 10 cells in parallel. During system discharge, each parallel module must supply the same current, say 100A. Other normal parallel modules discharge 10A per cell; since cell b can only discharge a maximum of 1A, the other nine cells must each discharge 11A. Under normal circumstances, with prolonged overload, these cells age faster than other parallel modules. Eventually, the overall maximum discharge capacity of this parallel module will fail to reach its design maximum. This parallel battery pack will then become the bottleneck for the entire battery pack's discharge capacity.
Series connection
Following the typical scenario in electric vehicles, series connection primarily occurs between modules. Continuing from the parallel connection scenario, a battery pack D exhibits a higher degree of aging than the others. D has a smaller capacity and higher internal resistance. Reflected in the SOC/open-circuit voltage curve, for the same SOC, the open-circuit voltage at D is higher. During charging of the entire battery pack, D reaches the charging cutoff voltage first, and the battery pack stops charging. The other modules haven't even had enough power, yet D is already overloaded because it's older and has a smaller capacity.
Therefore, the consistency of individual cells is not just a matter within a single welded module, but a requirement for all power lithium-ion batteries in the same vehicle.
