High voltage battery design for large electric vehicles – Part 1

Large electric vehicles, such as buses and trucks, use standardized battery packs, such as the C pack and the G pack. This article will discuss these packs in more detail.

Did you know that the actual nominal voltage of a LFP cell is 3.22V? This is the nominal voltage for its standard C rate of charge and discharge. This is more evident in large cells where the internal resistance is lower. This number is taken more seriously in high-voltage EV battery packs.

C Pack

Standard C pack dimensions are 1060*630*240mm with liquid cooling type modules. It is popularly made using 206Ah/230Ah EV cells with dimensions 54*173*207mm and arranged in a 48S1P configuration. The battery rating is 154.56V, 206Ah (31.839kWh)/154.56V, 230Ah (35.548kWh). Three 16S modules would be connected inside one C Pack.

It can also be made using 280Ah/302Ah EV cells with 72*173*207mm dimensions in 36S1P configurations. The battery rating is 115.92V, 280Ah (32.457kWh)/115.92V, 302Ah (35.007kWh). Three modules of 12S would be connected inside one C Pack.

G Pack

Standard G pack dimensions are 950*630*240mm with liquid cooling type modules. It can be made using 206Ah/230Ah EV cells in 39S1P. The battery rating is 125.58V, 206Ah (25.869kWh)/125.58V, 230Ah (28.883kWh). Three modules of 13S would be connected inside one G Pack.

It can also be made using 280Ah/302Ah EV cells in 30S1P configuration. The battery rating is 96.6V, 280Ah (27.48kWh)/96.6V, 302Ah (29.173kWh). Three 10S modules would be connected inside one G Pack.

These EV cells have the capability to provide 3C power for 30 seconds and have the ability to cycle at 1C charge and 1C discharge with a minimum of 4000 cycles at cell level until 80% SoH. There are some companies promising up to 8000 cycles at cell level until 80% SoH with semi-solid state battery technology. These high-cycle life EV cells are also suitable for 1C operation BESS projects for managing the stability of the power grid for utility-scale applications.

C pack and G pack have IP68 protection, which makes them very usable for large EVs in day-to-day activities without much worry.

Both the C pack and the G pack look the same but vary only by their lengths. Adjacent is an image of a typical liquid cooled C Pack/G Pack.

High Voltage System

Example – To achieve a battery nominal voltage of close to 650V using 230Ah cells, one would connect one C pack with four G packs.

System Voltage = C Pack + 4*G Packs = 48S + 4*39S = 204S = 204*3.22V = 656.88V.

Similarly, different combinations of C pack and G pack can be connected in series to achieve the desired voltage as per the cell used. Some of the popular combinations are listed below:

Based on the above table, a suitable voltage platform and battery capacity can be achieved using C packs and G packs in combinations in one string. Multiple strings can be connected in parallel to double or triple the capacity. Multiple strings can work with a single liquid cooling unit.

One might have seen a large unit of battery cabinet placed behind the truck cabin; inside that cabinet, the batteries are placed like this image.

For electric buses, depending on low-floor (intra-city) or high-floor (inter-city) buses, the batteries are placed in various places for maximum space utilization.

Because of the 48S1P’s C pack architecture, earlier designs of BESS solutions would use 48S1P modules (but 800mm width instead of 640mm) and connect 8 such modules in series to make a 1228.8Vrated cluster for grid-connected projects. This design would have 10 such clusters in parallel, use 280Ah cells, and reach 3.44MWh BESS DC side capacity in 20ft HQ container dimensions.

48S1P module for BESS application

The new design uses 52S1P modules, and 8 such clusters reach a system voltage of 1331.2V. It connects 12 such clusters in parallel, uses 314Ah cells, and reaches 5016kWh BESS DC side capacity in 20-ft HQ container dimensions.

In part 2 of my article on high-voltage battery design for large electric vehicles, I will discuss the internal architecture of the battery system, its components, and its overall schematic design.

This article was first published in EVreporter Aug 2024 magazine.

About the author

Rahul Bollini is an R&D expert in Lithium-ion cells with 9 years of experience. He founded Bollini Energy to assist in deep understanding of the characteristics of Lithium-ion cells to EV, BESS, BMS and battery data analytics companies across the globe. Rahul can be reached at +91-7204957389 and bollinienergy@gmail.com.

Also read: Understanding battery energy storage system (BESS) | Part 6