Pack Gravimetric Energy Density

Pack Gravimetric Energy Density has the units Wh/kg and is a key pack metric. The optimum metric is a high Wh/kg. Hence:

  • High nominal voltage
  • High Ah capacity
  • Low mass

The best you can achieve will be less than or equal to the cell Wh/kg that the pack is made from.

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.

The only note of caution is the discharge rate at which the cell Ah capacity has been measured. For automotive C/5 to C/3 is quite representative for a range test. However, for a phone or laptop C/10 is perhaps closer. Most cell capacities are quoted by manufacturers at C/10.

Plotting this energy density versus total energy in the pack is as expected.

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Electrified Aircraft Propulsion Targets

In aircraft the gravimetric energy density is one of the key targets at pack level. As of 2022 the 400Wh/kg target from NASA is a very tough target when automotive packs are achieving ~170Wh/kg.

NASA electrified aircraft propulsion system targets

NASA state in their November 2021 presentation that they have the following requirements for the battery pack:

  • 400 Wh/kg required at the system level
  • 1000’s of cycles
  • Extremely high power requirements (C-rates) during takeoff and landing
  • Cruise power for long range flights
  • High reliability, limited maintenance
  • Improved safety for thermal runaway events

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