Maximising Pack Energy Density

How can we go about maximising pack energy density? What can we look at to reduce the mass and volume? Where are the opportunities given a defined cell?

A great example of shrinking the size of a battery pack is the Mercedes Vision EQXX research car.

The advantage they had was a much more energy dense cell. However, they have also made some fundamental changes to the design. Some of these have been enabled by also looking at the car design.

Let’s think about what needs to be considered.

This post has been built based on the support and sponsorship from: Eatron Technologies, About:EnergyAVANT Future MobilityQuarto Technical ServicesTAE Power Solutions and The Limiting Factor. 

Size of the Opportunity

We are looking to optimise everything other than the cells, hence it is interesting to look at the pack mass minus the cell mass. This mass includes enclosure, cooling system, HV control, BMS, busbars and connectors.

Taking a battery pack that has a total energy of 100kWh we can see that the mass of everything other than cells has a spread from ~180kg to 310kg.

This shows that there is an opportunity of up to ~130kg.

However, there is also a spread in requirements here and levels of integration.

The Battery Pack Database is available as an excel download, this way you can see who can achieve what.

It is always very difficult to remove weight from a design where a number of parameters have been fixed. Normally we would expect to at most optimise and save 10%. That would equate to a weight save of around 25kg. Still worth having and don’t underestimate the level of challenge that it will take.

Mechanical Design

The simpler the pack layout the simpler the pack enclosure. This will reduce the amount of material required to enclose the volume and the amount of different types of support brackets.

The easiest way to illustrate this is to compare the Peugeot e208 battery design to the simple rectangular VW ID design. Both use a similar module shape and both cool the base of the module.

The e208 battery pack has a total energy content of 54kWh and an energy density of 152Wh/kg. The VW ID3 battery has a total energy content of 61.5kWh and a density of 168Wh/kg.

The complex pack shape of the Peugeot battery pack is down to it being an existing ICE platform. The VW ID3 is a ground up EV design. See Modular vs Dedicated BEV Platform for a more complete comparison of the pro’s and con’s of these two approaches.

Structural Integration

Integrating the battery structure with the body structure is a good way of optimising the overall system.

The Ford Mach-e body structure has a significant amount of extra stiffeners added to the siderails and floor. The battery pack for this vehicle also has substantial amounts of structure in the same areas/orientation.

Ford Mach E teardown by Munro

Overall this looks far from optimal, especially if you compare this to a Tesla Model 3 where the battery pack and body structure work together.

Cooling

The cooling system can add a considerable amount to the battery pack mass and volume. This will be sized for the continuous charge/discharge condition. For most automotive packs the most demanding condition is fast charge. This will drive flow rates and hence coolant channel sizing.

One thing we can do is look at the cooling pathway from the cell core to the coolant.

Edge cooling pouch cells schematic

Reducing this path will often take the form of reducing the number of interfaces and materials being used. The image above shows the optimisation of pouch cell edge cooling that has happened over the last decade.

This optimisation will allow the coolant system design to be revisited and minimised.

Fixings

The Hyundai Ionic 5 modules share fixings, this doesn’t quite halve the total number, but gets close.

This type of approach and thinking is required to drive the right mentality into the overall design.

Care about the grams and the kilograms will look after themselves.

Material Selection

The Ford Mach-e battery upper lid is a composite part. Munro Live show that this weighs 14.5kg, almost double the mass of the metal VW ID battery lid.

You can also see that this lid has required a significant number of fixings to ensure the seal works (this may also be required for EMC).

Sometimes the material you select is not the best at a system level.

This is not an exhaustive list of ideas, but hopefully will get designers thinking about their approach to reducing the weight. Please do share with us ideas and examples.

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