Cell Energy Density

When we say cell energy density we need to consider if this is gravimetric (Wh/kg) or volumetric (Wh/litre).

The energy content of the cell will be determined by the discharge rate, temperature and other parameters.

Discharge rate capability of a new SAFT MP 176065 xtd battery.

As you can see, at a C/8 discharge rate (purple line), the cell offers a 5.8 Ah capacity, at 1.5 C, the cell capacity goes down to 5.5 Ah (green line).

Hence, it is really important to establish the discharge rate at which a cell nominal capacity and nominal voltage are declared at.

Battery suppliers want to declare the highest possible values, these are probably at a much lower discharge rate than you will be using in your application.

Calculations

The calculations are quite simple as the energy content of the cell [Wh] = V_nom x Ah_nom. This value is then just divided by the volume of the cell to calculate volumetric energy density or divided by the mass of the cell to calculated the gravimetric energy density.

Typical Values

Caution

When looking at cell specifications there are a number of points that you need to consider:

• Actual dimensions
  • Cylindrical cell: might be declared as 18650 format, but it might not be 18mm in diameter and 65mm high
  • Pouch cell: do the dimensions given include the sealing swages and tabs?
  • Prismatic cell: are the electrical connections included in the cell height?
• Mass
  • Is this an average value?
  • Has it been estimated?
• Energy Content
  • At what C-rate are the nominal voltage and capacity declared?
  • What temperature were these measurements made at?
  • What lower discharge cutoff voltage was used?
  • What upper charge rate and cutoff voltage was used during the charge cycle?

Roadmaps

Once we have values for the energy density we will want to explore how those values are likely to improve over time. For that we need to look at the roadmaps.

Cell Energy Density Roadmaps

Wh/kg is a key metric that we look at when comparing cells. Looking at production values and adding roadmaps gives you an interesting view as to the future. Is 900Wh/kg credible when production cells have taken 30 years to move from 80Wh/kg to 300Wh/kg?

Usable Energy

In the simplest terms the usable energy of a battery is the Total Energy multiplied by the Usable SoC Window. The total energy is the nominal voltage multiplied by the nominal rated capacity.

However, if you have been through the Battery Basics you will have realised that the battery cell and pack do not have a linear performance and this is true for the usable energy.