Battery packs are highly sensitive to operating environment and their interactions with other systems in electric vehicles (EVs). Control of the operating environment and understanding of the influences of these interactions on battery performances are required to maximise their energy capacity and cycle life in EVs.This paper presents a modified parameter diagram (P-diagram) which is a part of systematic effort to design a robust battery pack. In the modified P-diagram, the physical inputs that affect the performance of a battery pack are identified and categorised into noise factors and control factors, where the former limits the performance and the latter can be used to improve it. Different noise factors are conceptually analysed in conjunction with various control factors and graded according to their relative influence on the performance of a battery pack. The performance is measured in terms of ideal function output and potential error states. The error states are subsequently broken down into inherent losses and undesired response(s). With such systematic understanding, a robust battery back can be designed for EVs.