## 4 Conclusions  
The following points have been stated based on extensive discussions:  
1. Regulatory constraints: The transition from diesel to electric propulsion in the context of the EURO 7 standards requires compliance with UNECE regulations, particularly R100. Since modifications to the certified bus body would trigger re-certification under R66, R80, R107, etc., battery units must be integrated without altering the frame. This presents a structural challenge, as the bus must now withstand longitudinal and transverse accelerations of 12 g and 10 g, respectively. * Feasibility of battery integration: The results confirm that battery systems can be safely integrated into existing diesel bus frames while meeting UNECE R100 acceleration thresholds, without requiring structural changes. This enables regulatory-compliant electrification of older buses using bolt-on designs. * Stress and plastic deformation: it is evident that keeping the battery units within the material elasticity (Hooke's law) is impossible and makes no sense (otherwise the material capacity and mass will be inadequate), so it is essential to control the amount of stress (must be within the limits of tensile strength--360 MPa) and \(\sigma_{max}\) locations (effective zones of plastic deformation). The value of \(\sigma_{max}\) varies between 364.89 MPa (\"3-10g-L\" mode) and 439.08 MPa (\"3-12g-B\" mode). * Deformation and Safety Factor: the maximum deformations are recorded in the models with the highest \(\sigma_{ave}\): \(\delta\) = 63.04 mm in the \"2-12g-B\" mode with \(\sigma_{ave}\) = 83.18 MPa. The minimal \(\delta\) of 6.95 and 7.95 mm are in the \"3-10g-L\" and \"3-10g-R\" modes with \(\sigma_{ave}\) of 40.90 and 43.02 MPa, respectively. Therefore, it is necessary to focus primarily on \(\sigma_{ave}\), but not on \(\sigma_{max}\), while evaluating the stress-deformed state of the battery unit. In addition, it is mandatory to comply with the manufacturer's requirements regarding the relative displacement (in this case, it should not exceed 45-50 mm). * CoG preservation: when switching from a diesel power unit (together with a gearbox) to an electric one, the position of the bus CoG must be preserved; otherwise, braking, handling, active, and passive safety will be affected. That is why mathematical modelling was proposed to determine CoG based on the linear mass density function. Consequently, it is an effective model that was proposed to determine CoG based on the linear mass density function. * Acceleration modelling: despite the regulatory requirements of R100 are the longitudinal 12 g and transverse 10 g accelerations, in reality, for heavy buses (M3 category), typical values during frontal crash tests range between 15 and 20 g and depend on the distance from impact point: the farther a component is, the lower the peak acceleration it will experience. At the same time, the indicated acceleration values do not correlate with the real ones, which may arise as a result of the problem of the mathematical attenuation model of acceleration propagation: an exponential decay model of acceleration based on battery CoG location and damping coefficient (like \(k\) = 0.05 m\({}^{-1}\)) was used. The proposed mathematical modelling is an effective way of predicting the acceleration that will act on a specific battery unit, like: \(a_{f}\) = 12.91 m/s\({}^{2}\) for the front battery unit and \(a_{r}\) = 9.56 m/s\({}^{2}\) for the back unit due to structural damping and wave dissipation. Conceptualisation, K.H. and E.K. ; methodology, K.H. ; software, K.H. ; validation, O.H. ; formal analysis, I.K., O.D., R.H., S.B. and Y.S. ; investigation, Y.R., V.R., R.H., S.B. and Y.S. ; resources, O.D. ; writing--original draft preparation, K.H. and O.D. ; writing--review and editing, K.H., T.W. and A.M.; visualisation, O.D., I.K. and Y.R. ; supervision, K.H., E.K., I.K., T.W. and A.M.; project administration, O.H. ; funding acquisition, E.K., T.W. and A.M. All authors have read and agreed to the published version of the manuscript.