Battery dimensioning

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By Enrico Melotti * Initial parameters**

The first step to design a battery pack is to fix some variables:

  • Voltage:

We decide to use 144V battery pack, created with common LiFePO4 cells type, usually used in EV conversions.

  • Resistent (Drag) Power:

The main source of Resistent Power is the aereodynamic resistance (drag).

After few evaluations we consider about 8 kW at 80 km/h (see Note 1 in the footer).

Number of cells: The number of cells is really simple to obtain:

144V (our battery voltage) / 3,2v (nominal voltage of each cell) = 45 cells

Battery capacity:

The battery capacity is a bit more difficult to determine, because it depends on more than one variable.

Some of these are:

  • Max Discharge rate:

    The discharge rate, or “C” rate, required by the motor is obatained by this formula:

    Peak Power of the motor / Nominal capacity of the battery pack

    The Peak Power considered is 66KW

    The datasheet specifes 2C optimal discharge rate - to 5C max rate.

  • Driving style:

    Depends on the driver. We decided a normal driving style with good acceleration and not excessive speed.

  • Range autonomy:

    The range depends on driving style and capacity. We focus our attention in the 100-150 Km range.



After this introduction, our choice is among two battery cells: A=100Ah and B=160Ah.

Differences:

Weight: A = 140 Kg | B = 260 Kg

Volume: A = 89 liters | B = 160 liters

Price estimate (1,2$ per Ah): A = 5500<math>** | B = **8600</math>

Max C rate: C = 3

Max Continuous amperage = Cell capacity x C: A(100Ah) = 300 A | B(160Ah) = 480 A



Note 1: You may compare this result against the Powertrain dimensioning sheet we are using for the conversion and currently [tracked in the repo] (https://github.com/iaiaGi/iaiaGi_ZEV_Kit/tree/master/Projects/Kevin/Mechanical_Dimensioning)