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.
The calculations are quite simple as the energy content of the cell [Wh] = Vnom x Ahnom. 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.
- 325 Wh/kg Lithium Sulphur (ALISE 2018)
- 271 Wh/kg Panasonic NCR2170‑M
- 263 Wh/kg LG Chem M50 21700
- 260 Wh/kg Panasonic NCA 21700 (Tesla Model 3 2019)
- 241 Wh/kg Murata 18650VTC6 (Formula E 2019-21)
- 240 Wh/kg Panasonic NCA 1
- 169 Wh/kg XALT 53Ah HE NMC (Formula E 2014-18)
- 160 Wh/kg Lithium Iron Phosphate battery
- 100-150 Wh/kg Sodium Ion battery
- 70–100 Wh/kg Nickel Metal Hydride (NiMH) battery
- 90 Wh/kg Sodium Nickel Chloride (Zebra) battery
- 80 Wh/kg Sony first ever production lithium ion cell (1991)
- 50-75 Wh/kg Nickel Cadmium (NiCd) battery
- 35-45 Wh/kg Lead Acid battery
Perhaps the simplest of the battery metrics as the capacity of the cell is fairly easy to measure and the mass is just a set of scales.
This list of values gives a snapshot of chemistry and the development roadmap.
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?
- 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?
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.
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?
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.