Traction Battery Pack Design

TRACTION BATTERY PACK DESIGN

Before commencing battery pack design for EV applications, it is important to understand the behavior of the battery chemistry. It is important to start with the requirements to ensure that the battery design falls within the boundaries of the technology. It is important to determine the maximum discharge rate of the battery in addition to the operating and storage temperature ranges.

Consider the cell configuration—Pb-acid, NiMH, and Li-ion batteries can be configured in the series or parallel configuration or a combination of the series-parallel combination. The selection of the configuration is based on the requirements for input voltage and battery pack discharge requirements. Battery cells can be configured in series to provide the necessary input voltages. Alternately, battery cells can be configured in parallel to provide the battery capacity to yield the required run time. Battery configuration can be determined based on the two methods.

Based on the lower battery cutoff voltage (LCV) and the minimum voltage from the battery (e.g., 12.5V), the number of cells connected in a series combination is expressed by the equation,

LCV/12.5V = # of cells in the series combination

Based on the average discharge current (Iavg), the number of batteries connected in parallel are expressed by the equation,

(Run Time [hr] ¥ Iavg)/Battery Ahr = # of batteries in a parallel combination

In the second method, the average discharge power (Pavg) is known and is determined by the equation,

(Run Time ¥ Pavg)/# of series batteries ¥ Battery W-hr = # of parallel batteries

Battery pack electronics includes pack control (and possibly battery charge control). In addition, a simple fuel gauge or smart-battery circuitry is integrated into the battery pack. Contacts for the battery terminals must be designed to eliminate shorts by coins or other metallic objects. In addition, the contacts must exhibit good corrosion resistance and provide low electrical resistance.

Multilevel battery pack-control circuitry ensures reliability and provides protection against overcharge, over-discharge, short circuits, and thermal abuse. Charge-control electronics is typically part of the battery pack and is located outside the battery pack.

Estimation of the battery pack size may be performed during the design phase based on the electrode size (15cm ¥ 30cm). In addition, battery pack weight may also be estimated for the EV. The battery electrodes are capable of yielding 30Ahr.

Submodule design dimensions: 15cm ¥ 30cm ¥ 11.5cm Electrode size: 14.7cm ¥ 29.7cm = 67.7in2 Electrode capacity: 500mAhr/67.7in2 = 30Ahr Volumetric energy density: 50V ¥ 30Ahr/15kg = 100Whr/kg Gravimetric energy density: 50V ¥ 30Ahr/(1.15dm ¥ 1.5dm ¥ 3.0dm) = 1500Whr/5.175dm3 = 290Whr/L

The weight components for the 1.2V unit cell battery pack are:

Cell Components Thickness (mm) Weight (g) Conductive Plastic 0.10 4 Negative Electrode 0.85 85 Separator 0.25 12 Positive Electrode 1.08 117 Conductive Plastic 0.10 4 Total 2.38 222

For a 27kWhr (300V ¥ 90Ahr), the submodules are rated at 50V, 30Ahr. Overall nominal battery dimensions are 15cm ¥ 120cm ¥ 69cm, which allows for a battery pack configuration with a low-profile battery allowing a low center of gravity.

The sub-module capacity is 30Ahr ¥ 50V = 1,500Whr Dimensions: 15cm ¥ 30cm ¥ 11.5cm Weight: 15kg The module capacity is 30Ahr ¥ 300V = 9kWhr Dimensions: 15cm ¥ 30cm ¥ 69cm Weight: 90kg

The battery module design: 90Ahr ¥ 300V = 27kWhr Dimensions: 15cm ¥ 120cm ¥ 69cm = 125 litres Weight: 360kg (792lbs) Battery volumetric energy density is 27kWhr/125ltr = 216Whr/ltr Battery gravimetric energy density is 27kWhr/360kg = 75Whr/kg