Many square and cylindrical (18650) battery cells are connected in parallel to form a module, and many modules are then connected in parallel to form a pack, which serves as the power source for electric vehicles. Some people online have disassembled Tesla Model S batteries, showing an 85kWh battery pack with 7104 cylindrical cells manufactured by Panasonic, arranged in 16 groups of 444 cells each, connected in series between the groups to form the battery pack.
Musk's willingness to do this is not something everyone can justify. Firstly, Panasonic's small cylindrical battery technology is highly refined, balancing low cost and consistent performance. There's a joke that if you test a Panasonic battery with two different machines and the resulting curves are different, you should first suspect a problem with your testing. Battery manufacturing is complex, and many factors affect cell consistency. From start to finish, besides the consistency of the materials themselves, there are factors such as batching/mixing/coating/cold pressing/slitting/winding/liquid injection/formation. To improve battery consistency, manufacturers add a sorting process after formation to remove batteries with large resistance deviations or severe self-discharge. Poor cell consistency severely impacts battery lifespan and shortens the battery pack's lifespan. After many charge-discharge cycles, a single cell's reversible capacity will decrease due to factors such as SEI film growth, increased side reactions, and blockage of separator pores. When the capacity decreases to 80% of its initial capacity, it will severely affect the functional design of the electric vehicle. If the cells in a battery module have poor consistency, using the same charging and discharging current will cause some cells to be overcharged or over-discharged, which will significantly shorten the overall cycle life of the battery. Inconsistency will also cause a domino effect-like chain reaction in other cells within the battery pack. For example, a cell with a cycle life of around 1000 cycles might only actually complete about 200 cycles in the battery pack. However, if the cell consistency is inherently poor, can the entire battery pack still be used?
Traditional BMS systems would definitely reject this approach, but SDB (software-defined battery) systems might be an exception. A traditional BMS uses a single charging cable to charge/record/cut off all batteries before outputting the data. If cell consistency is poor, the problems described above will occur. Furthermore, if we think outside the box and install two types of batteries in a battery pack—one for low-rate discharge during low-speed cruising and the other for high-rate discharge during high-speed emergency stops—different cell systems have different characteristics, and commonly considered metrics such as energy density, power density, cost, lifespan, and flexibility often conflict. If different cell systems need to be used in the same module to create a product with outstanding overall performance, traditional BMS systems will struggle.
Microsoft believes this can be achieved by adding a smart switching circuit to each battery, collecting the charging/discharging voltage, current, and resistance data for each cell. This allows for intelligent current allocation, ensuring that the charging and discharging behavior of each cell closely resembles that of a single individual cell, maximizing the cell's usability. The main challenge of this cell management model lies in the analysis and current allocation of the collected data. Microsoft has developed a complex algorithm for this and defines battery packs using this management model as Software-defined batteries.
More importantly, this battery management mode allows for the mixed management of different chemical systems. There are countless energy storage methods, each with its own advantages and disadvantages. Can batteries with different voltages, resistances, and charge/discharge rates function properly when combined? Microsoft conducted an experiment using SDB (Battery Management for 2-in-1s) and obtained good results. Furthermore, in smartwatches, flexible solid-state batteries can be integrated into the watch band, paired with a polymer lithium battery under the watch face, to better extend standby time. This requires SDB to better regulate the interaction between solid-state and liquid batteries.
Regarding the introduction of so many charging and discharging micro-devices, the bigwigs say, "It's not a problem (We believe the BoM cost and space requirements of our SDB solution will not be significant)." In the future, this SDB system will also connect to the vehicle's Internet of Things (IoT) to understand your personal behavior and user schedule, serving your travel needs. Wow, our batteries are getting into the big data IoT era too! How sophisticated!
