Basics of Big Cell Vs Small Cell Battery Architecture

In Li-Ion battery industry, majorly 2 architectures are being considered on gross level.?

1. Big cell architecture, where you have 1,2 or 3 large capacity cells in parallel
2. Small cell architecture, where you have large number or small capacity cell in parallel to meet overall pack level Current requirement and energy capacity.

Manufacturing and Operations:

In big cell architecture, due to cell size, handling of the same need to be carefully managed and automations costs become higher. Whereas, small cell handling automation solutions available are lower costs and can be deployed by small scale battery manufacturers at their early stage of developments.?

On function level, big cell packs are easier to design and can prove to be much safer as each pack has lesser number of connections which are less likely to go wrong due to assembly space availability and design freedom provided. On other side, small cell architecture needs very precise tolerances and accurately designed assembly methods, where chances or inconsistency or width of tolerances become very narrow.

Basic math:

Let’s consider a conventional PPM method for a 5kWh with Nominal 72V designed in 2 architectures. I am not specifying any particular cell or form factor here and keeping the things simple.

Big Cell Architecture:

Cell: 3.7V, 69~70Ah, 19 in series and 1 in parallel

Here, in each pack we have 19 cells in series and 38 connection. At failure rate of 1 ppm,? I am likely to get 0.004% i.e. 1 in nearly 26315 packs will be problematic.

Small cell Architecture:

Cell: 3.7V, 7Ah, 19 in series and 10 in parallel

Here, in each pack we have 190 cell and 380 connections. At same failure rate of 1ppm, I am likely to get 0.038% i.e. 1 in nearly 2631 packs will be problematic.

Mechanical Design:

While designing for big cell architecture, a major change being faced is form factor and size standardization. Majority of bigger cells being used are Prismatic, with sizes being decided by individual manufacturers. Whereas, in case of small cells majority of them being cylindrical, standardized sizes like 18650, 21700, 32140 are available across cell manufacturers allowing battery pack manufacturers to have secure supply chains.

Now considering form factors, Pouch cell is something available across the widest variety of sizes and capacities. However, need careful design of enclosures and connections to ensure best of life and charging discharging performance. Factors like keeping cell under pressure, providing heat conductive path with reliable insulation are some of very important key challenges. On other hand, cylindrical cells, generally being most structurally stable, forgo some installation requirements and are simplest to automate for mass production of pack. However, have impact on energy density. Prismatics, basically being midway to cylindrical and pouch, enjoy benefits of both worlds in terms of mechanical design needs. It’s easier to handle than pouch cells while being more stable. But mechanical assemblies and automations are not as easy as cylindrical but provide easier power connection with wider options of process to be used.

Thermal Management:

A bigger cell will have higher temperature gradients across the cell body and due to which aging will be faster. This limits the C rates which can be considered while developing packs. Having said that, temperature sensing needs to be more precise and faster for Big cell architecture. Sensing temperatures on casing may not help and if at all to be sensed on casing, safety limits need to have higher buffer to ensure in time thermal runaway triggers.

Now with smaller cells, as thermal gradient is narrower and allows designer to have larger degree of freedom while placing temperature sensors. On flip side, due to size and number of cells being more, extracting heat out of small cell to casing reliably and uniformly across battery pack is challenge and needs con conventional approach for manufacturing as well. This results in benefits provided by cell being nullified on pack level.

Functionality across life

Assuming same packs as considered above, what issues are getting faces in function.

When big cell joint fails, and we have 1 in parallel, complete pack goes down and can’t be used at all and needs major rework.

With small cell pack, one joint failure results in <10% or so (Assuming 10 or more cells in parallel) power loss, but pack can keep supplying energy within limitations. Even though imbalance and thermal issues arise, it will still working in what we can say as Limp mode.

Serviceability?

Big cell architectures having large joints and wider tolerances are very easy to service and may need lesser hands-on skills and equipment to repair and bring them back to life. This may make really good sense for 2nd life usage, off site, stationary storage applications or so.

Small cell architectures, having inherent capability to operate in limp mode can be super useful for applications like surgery robots, mining back-ups, etc where must work always even if slow is more important than anything else.

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