Inputs to Pack Sizing

Lets look at the inputs to pack sizing, what do we need to know in order to size a pack?

  • Usable Energy [kWh]
    • Discharge C-rate or power [Crate or kW]
    • Ambient temperature [°C]
    • Minimum voltage [Vmin]
    • Maximum voltage [Vmax]
    • Cooling power [kW]

At the early stage you don’t need to think about total energy, concentrate on the usable requirement and conditions at which that needs to be achieved.

The minimum and maximum voltage is likely to be defined by the components drawing power from the HV system, eg: motor & inverter, 12V DC-DC, electric air con, cooling pumps.

  • Discharge Power [kW]
    • 10s Peak discharge power [kW10s]
      • Minimum voltage [V]
    • Continuous discharge power [kW]
      • Minimum voltage [V]
      • Continuous cooling power [kW]
      • Ambient temperature [°C]
    • End of Life
      • minimum peak 10s power
      • minimum continuous power

The discharge power will change depending on SoC, cell temperature, SoH and it will be a function of discharge time.

The End of Life power is sometimes better defined as a percentage of beginning of life requirements, eg 80%.

  • Regen Power [kW]
    • 10s Peak regen power [kW10s]
    • Continuous regen power [kW]
      • Continuous cooling power [kW]
      • Ambient temperature [°C]
    • End of Life
      • maximum peak 10s regen power
      • maximum continuous regen power

Regeneration is effectively the charge power from the system in use (not whilst being charged from an external source). It is important to understand what the system might want to input into the battery.

This requirement might be for a number of reason, eg braking using the motors and hence converting mechanical to electrical power.

  • Charging
    • Full charge time [minutes]
    • 0 to 80% charge time [minutes]
    • Charger types
    • End of life charge time

Will this battery pack need to be charged from an outside source? What type of charger and how fast does it need to be charged. This will allow you to start to select cells and think about the system sizing.

  • Temperature Range [°C]
    • Full operation
    • Reduced Operation
    • Full charge capability
    • Reduced charge capability
    • Ambient temperatures

Battery cells typically work between -20°C and 60°C, however, you cannot charge most below 0°C and they are optimum between 20°C and 45°C.

Therefore, map out the temperature requirements for full and reduced operation as these will be important in cell selection and system heating/cooling concepts.

  • Control
    • System network architecture and protocols
    • SoX data outputs from BMS
    • Required accuracy of outputs
    • Checks needed prior to discharge
    • Checks required prior to charge
    • Service data requirements
    • Updates over air
    • Data over air

What will the battery control system communicate with? What data does the system need from the battery and what system information does the battery need to know. Establish these requirements early as these will drive the design of the control system and unearth other requirements.

  • Dimensions
    • Maximum x, y, z [m]

Often there are a few critical dimensions. Also, establish whether a simpler rectangular shape is more important than other requirements as this could significantly impact the complexity and cost.

  • Mass [kg]

Just remember that there might be a significant volume of cooling fluid and hence mass.

  • Mechanics
    • Mounting system
    • IPx rating
    • Vibration exposure

The mechanical requirements of the pack are important, for initial sizing this can be kept to a simple overview as to the environment that the pack will be exposed to in use, eg inside or outside of vehicle, isolated or not isolated from shock and vibration.

  • Legislation
    • Application: automotive, aerospace etc
    • Country (market(s))
    • Timing (year of release)

This will be determined by the application, country and timing.

Some of these requirements will be iterative. Some will be unknown and will have to be developed in conjunction with the system and application. The Inputs to Pack Sizing has been written to prompt and encourage battery designers to carefully think about these requirements, to document them and to agree them.

Ducati V21L


A good way of understanding your battery design is to look at how others have approached the design.

Our battery benchmarking is gradually expanding in depth and number of packs.

Leave a Comment