Basic battery calculations and battery arrangements. - swapnilmankame1995/EV-course GitHub Wiki

Now that we have calculated our average energy consumption with respect to a WLTC Drive cycle, we can take that data and move on to selecting a battery to accommodate the energy consumption,

The battery parameters have a significant influence on other components and attributes of the vehicle, like:

Maximum traction motor torque
Maximum regeneration brake torque
Vehicle range
Vehicle total weight
Vehicle price

Pretty much all major aspects of a pure electric vehicle (EV) depend on the parameters of the battery.

Before we move on to selecting our battery, we need to calculate All the secondary appliances in a car that will require power as well,

Because on top of the energy needed for propulsion i.e Ep= 112.7 Wh/km calculated in the previous chapter, the battery must supply the energy for the vehicle’s auxiliary devices Eaux [Wh/km], like 12 V electrical system, heating, cooling, etc. Also, we have to consider the efficiency of the powertrain ηp [-] during the conversion from electrical energy to mechanical energy.

CodeCogsEqn (19).png (1)

The prolonged electrical loads (headlights, multimedia, etc.) and intermittent loads (heater, brake lights, wipers, etc.) use on average 430 W of electrical power.

The duration of the WLTC cycle is 1800 s (0.5 h), which gives energy of 215 Wh for the auxiliary loads.

If we divide it to the length of the WLTC driving cycle (23.266 km), we get an average energy consumption for the auxiliary loads E-aux of 9.241 Wh/km.

Even if Wh/km is not really energy but factorised energy, since it’s divided per unit of distance (km), for simplicity, we are going to refer to it as average energy.

The direct current (DC) supplied by the battery is converted into alternated current (AC) by the inverter. This conversion is taking place with an associated loss. Also, the electric motor and driveline have some losses which we need to consider. For this exercise, we are going to use an average efficiency ηp of 0.9 from the battery to the wheel.

Replacing the values with Ep = 112.7 and Eaux = 9.241 in (1) gives the average energy consumption

CodeCogsEqn (1).gif

The battery pack will be designed for average energy consumption of 134.13 Wh/km.

We have already learned the basics of the working of Li-ion Batteries in Target group 2.

Lets look at how batteries can be arranged to give us the needed outputs,

Individual li-ion battery cells may be grouped in parallel and/or series as modules. Further, battery modules can be connected in parallel and/or series to create a battery pack. Depending on the battery parameters, there may be several levels of modularity.

The total battery pack voltage is determined by the number of cells in series. For example, the total (string) voltage of 6 cells connected in series will be the sum of their individual voltage.

Image: Battery cells string

In order to increase the current capability the battery capacity, more strings have to be connected in parallel. For example, 3 strings connected in parallel will triple the capacity and current capability of the battery pack.

Image: Battery cell strings in parallel

Examples

1. Mitsubishi i-MiEV

The battery pack of Mitsubishi i-MiEV consists of 22 modules made up from 88 cells connected in series. Each module contains 4 prismatic cells. The voltage of each cell is 3.7 V and the total voltage of the battery pack 330 V.

Image: Battery pack (modules and cells) Credit: Mitsubishi