Cylindrical and Prismatic Cell Format Selection Criteria
Format selection dictates tooling capital, cooling architecture, and mechanical containment: cylindrical cells minimize stack stress, while prismatic cells maximize spatial fill.
Serpentine Cooling Ribbon
Meaning
Flexible heat exchanger made of thin, curved metal or plastic tubing that weaves between cylindrical battery cells to provide direct surface cooling. It governs the temperature distribution in high-density cylindrical cell packs, ensuring that heat is removed efficiently from each individual cell. This cooling element runs along the rows of cells and operates continuously during periods of high power demand or rapid charging.
Design Path
The cooling channel is shaped to follow the contours of the cylindrical cells, maximizing the contact area for heat transfer. A serpentine cooling ribbon is typically made of aluminum for its high thermal conductivity and lightweight properties. It sits between the cells, with a thermal interface material applied to fill any gaps and ensure good thermal contact.
This layout allows for a compact pack design while still providing effective cooling. It is particularly useful for packs with tightly packed cylindrical cells where space is at a premium. The flexible design can adapt to different cell layouts and spacing, giving pack designers more options to optimize the cooling system.
Thermal Uniformity
One of the main challenges with this design is maintaining a uniform temperature along the entire length of the cooling ribbon. As the coolant moves through the serpentine cooling ribbon, it absorbs heat, which means the cells at the end of the run receive warmer coolant than those at the start. Engineers optimize the flow rate and channel design to minimize this temperature difference, preventing uneven cell aging.
Keeping the temperature uniform across the pack is critical for maximizing overall lifespan. It helps prevent some cells from degrading faster than others, which can reduce the usable capacity of the entire pack. This uniformity is key to maintaining consistent performance and safety over the life of the battery system.
Structural Function
In addition to cooling, these ribbons are often integrated into the structural design of the pack to help hold the cells in place. This dual-use reduces the weight and complexity of the assembly, which helps increase the overall energy density of the pack. However, the ribbons must be designed to withstand the physical vibrations and impacts of mobile applications without leaking.
A leak would be catastrophic for the electrical systems inside the battery pack. This makes robust construction and rigorous pressure testing a key part of the manufacturing process. These testing procedures ensure that the cooling system remains reliable over many years of operation in demanding conditions.