Cell

This is the smallest building block of a functional battery.

power versus energy cell

Power versus Energy Cells

In simple terms the energy cell has thicker layers of active material, thinner current collectors and less of them.

This means the energy cell will have a higher electrical internal resistance meaning it will generate more heat based on I2R heating.

The energy cell will have poorer thermal conductivity in-plane and through-plane. Thus, it will need a higher temperature gradient to reject the heat.

Cell Sample Maturity

Cell Sample Maturity is normally defined by the A, B, C, D sample definitions. These stages of the cell design, production line development and material supply are key to the relationship between the cell manufacturer and cell customer. The customer needs to have confidence in the cell design, robustness and quality and this is only possible if they know the process that the manufacturer is working to.

cell sample maturity
cell enclosure functions

Function of the Cell Can or Enclosure

The function of the cell can or enclosure is to contain the chemistry over the lifetime of the battery cell and to allow the electrical, mechanical and thermal connections. It must also work in the extremes and have a controlled failure mode.

Manufacturing Process

cell manufacturing schematic

In order to engineer a battery pack it is important to understand the fundamental building blocks, including the battery cell manufacturing process. This will allow you to understand some of the limitations of the cells and differences between batches of cells. Or at least understand where these may arise.


Cell Definitions and Glossary

3D Electrodes – another way to increase energy density is with 3D electrodes. Increasing the surface area and connection to the active materials can improve a number of features of the cell.

3D Electrodes - Addionics

Ah – Ampere-hour is the unit of cell capacity.

Capacity – battery capacity is expressed in ampere-hours.

C-rate – a measure of the rate at which a battery is charged or discharged relative to its capacity. It is the charge or discharge current in Amps divided by the cell capacity in Ampere-hours.

Instrumenting Cells – if you are going to instrument a cell you need to be able to do this reliably and robustly. The process flow diagram illustrates the experimental stages employed for cell instrumentation and includes: sensor fabrication, cell modification and sensor insertion. The diagram highlights the different verification stages for assessing LIB performance, operation and ageing.

WMG Smart Cell instrumentation
typical OCV vs SoC

Open Circuit Voltage (OCV) – is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.

Specific Heat Capacity – for the main lithium ion chemistries the following generic heat capacities for a cell are:

  • Lithium Nickel Cobalt Aluminium Oxide (NCA) = 830 J/kg.K
  • Lithium Nickel Manganese Cobalt (NMC) = 1040 J/kg.K
  • Lithium Iron Phosphate (LFP) = 1130 J/kg.K
SoC definition for Samsung SDI 94Ah cell

State of Charge (SoC) – abbreviated as SoC and defined as the amount of charge in the cell as a percentage compared to the nominal capacity of the cell in Ah.

cell temperature limits

Temperature Limits – these temperatures will change with chemistry and by cell manufacturer, therefore, it is really important to use the limits as advised by the manufacturer. In addition you will need to test the cell to gain the detailed understanding of how the cell behaves in your application versus temperature.

The limits will also be blurred by the design of the battery and control system. One example is the maximum operating temperature for the cell.

Thermal Conductivity – If we look at the active layers of a cell the thermal conductivity in the plane of the layers is approximately 10x to 100x that through the planes.

This should not be unexpected as the electrodes are made from sheets of aluminium and copper. Two of the best materials for thermal conductivity.

These values though have a large range [Ref 1]:

  • 15 to 160 W/mK In-Plane
  • 0.2 to 8 W/mK Through-Plane
Thermal conductivity of a cell
Thermal Runaway Energy vs Electrical Energy

Thermal Runaway vs Electrical Energy – the energy released during Thermal Runaway versus the electrical energy stored in a battery.

The energy released during Thermal Runaway (TR) versus the stored electrical energy. A number has been bandied around for a long time that the energy released in a TR event was 2 to 6 times the electrical energy stored in the cell.

Usable SoC Window – If we want a battery cell to last a lot of cycles, extend the life in a power application or to ensure the available power is consistent then we need to set a usable SoC window that is smaller than 100%.

usable SoC window