# Pack Mass

When looking at battery design we need to know the pack mass for a number of reasons, irrespective of the application:

• transportation
• case structural requirements
• vehicle or location structural requirements
• recycling

Early in the design we often don’t have enough detail to create a complete bill of materials based summation and then it is good to estimate the pack mass.

As the battery pack energy increases the mass of the pack increases. Pack mass in kg is roughly 6.7x total kWh.

However, this is a very rough approximation.

Data from Battery Pack Database.

#### Pack Mass Estimation

Battery pack mass estimation is a key parameter required early in the conceptual design. There are a number of key reasons for estimating the mass, one of the main ones being the significant percentage it is of the overall mass of the complete system.

This calculator uses benchmark data to estimate the mass of everything other than the cells.

We can see based on battery pack benchmarking data that the pack mass is 1.6x the cell mass as a first estimate.

This relationship is used in the Pack Mass from Cell Density calculator. All data available in the Battery Pack Database.

#### Pack Mass from Cell Density

The key relationship we have is between cell and pack gravimetric energy density.

This graph has been pulled together by scouring the internet for cell and battery data.

The ratio of cell density to pack density is 0.6235 and this is very close to the total cell to pack mass relationship of 1.6034

Reinforcing this is a plot of the pack gravimetric energy density versus the cell energy density. Some deviation here as the number of data points available varies. However, 1/1.6 = 0.625 and the gradient of this line is very close.

In order to improve this estimation we need to look at the battery pack design in more detail.

The ultimate estimation is to list all of the components and estimate the mass of each part and sum all of them up. The parts list for a battery is a good place to start in the creation of a complete BOM. We can also look at the key factors relating to mass of each component.

Contactor mass plotted versus continuous current rating.

Not very much data with just 23 parts, but it appears that we are seeing distinct masses based on frame size.

Hence a relationship to continuous current, but not a very clear relationship.