title: New results for battery impedance at very low frequencies

authors: Scott, Jonathan B.

subjects: Science & Technology

abstract: In search of an equivalent circuit model for rechargeable batteries, many authors start with a measurement of battery impedance, spanning what is presumed to be the frequency range of interest. Various networks have been suggested in the literature to account for the measured impedance characteristic. Most incorporate two or more resistors, at least one capacitor, some include at least one Warburg element, and more recently “constant phase elements”(CPE), otherwise identified as fractional-derivative capacitors. Networks that are more successful at reproducing the measured impedance have from five up to tens of degrees of freedom. The frequency range upon which most models are based extends only to 1mHz. This is surprising since many batteries see a daily or longer usage cycle, corresponding to a frequency of ≈ 11.6 μHz or lower. We show in this manuscript that the most-cited impedance measurement instrument, and one of the few that can operate below 1mHz, can be unreliable at and below this boundary. We present a novel impedance measurement algorithm robust against the issues present while measuring the impedance of electrochemical systems to as low as 1 μHz. Next, we present reliable impedance data extending to a lower frequency limit of 10 μHz. A remarkable characteristic appears at the lower frequencies, suggesting a surprisingly simple and elegant equivalent circuit consisting of a single fractional capacitor. A new model is proposed, which requires only four parameters to predict the measured impedance as a function of frequency.

text: New results for battery impedance at very low frequencies In search of an equivalent circuit model for rechargeable batteries, many authors start with a measurement of battery impedance, spanning what is presumed to be the frequency range of interest. Various networks have been suggested in the literature to account for the measured impedance characteristic. Most incorporate two or more resistors, at least one capacitor, some include at least one Warburg element, and more recently “constant phase elements”(CPE), otherwise identified as fractional-derivative capacitors. Networks that are more successful at reproducing the measured impedance have from five up to tens of degrees of freedom. The frequency range upon which most models are based extends only to 1mHz. This is surprising since many batteries see a daily or longer usage cycle, corresponding to a frequency of ≈ 11.6 μHz or lower. We show in this manuscript that the most-cited impedance measurement instrument, and one of the few that can operate below 1mHz, can be unreliable at and below this boundary. We present a novel impedance measurement algorithm robust against the issues present while measuring the impedance of electrochemical systems to as low as 1 μHz. Next, we present reliable impedance data extending to a lower frequency limit of 10 μHz. A remarkable characteristic appears at the lower frequencies, suggesting a surprisingly simple and elegant equivalent circuit consisting of a single fractional capacitor. A new model is proposed, which requires only four parameters to predict the measured impedance as a function of frequency.

year: 2019

title: Structural and thermal performance of cold-formed steel studs with slits

authors: Ghosh, Kushal

abstract: Cold-formed steel (CFS) studs in lightweight construction increasingly incorporate web slits to limit thermal bridging, yet these openings can reduce axial strength. This thesis integrates structural and thermal assessments through validated non-linear elasto-plastic finite element analysis (FEA) and complementary heat-transfer modelling. Shell-element FEA models were developed and validated against tests, then used in a 960-case parametric study spanning section dimensions, slit geometry, thickness, and member length to establish the influence of slits on concentric axial capacity. The results were evaluated against the Direct Strength Method (DSM) in AS/NZS 4600, leading to strength-reduction recommendations and modified DSM expressions that achieve reliability indices β ≥ 2.5. The programme was extended to combined axial compression and minor-axis bending using 1,134 FEA models with six eccentricities, demonstrating a systematic discrepancy in current AS/NZS 4600 interaction checks: strengths are generally underestimated at eccentricities of 10–25 mm and overestimated at 50 mm. A new interaction equation incorporating element and web slenderness ratios is proposed and verified within the AISI S100 reliability framework. Subsequently, a parametric three-dimensional heat-transfer study quantified the dependence of heat flux on geometry and slit parameters, providing a quantitative basis for thermal performance gains and their balance with strength. The thesis proposes design recommendations and revised equations for slitted CFS studs that satisfy codified reliability targets and support informed selection of slit configurations to balance energy efficiency and structural capacity in practice.

text: Structural and thermal performance of cold-formed steel studs with slits Cold-formed steel (CFS) studs in lightweight construction increasingly incorporate web slits to limit thermal bridging, yet these openings can reduce axial strength. This thesis integrates structural and thermal assessments through validated non-linear elasto-plastic finite element analysis (FEA) and complementary heat-transfer modelling. Shell-element FEA models were developed and validated against tests, then used in a 960-case parametric study spanning section dimensions, slit geometry, thickness, and member length to establish the influence of slits on concentric axial capacity. The results were evaluated against the Direct Strength Method (DSM) in AS/NZS 4600, leading to strength-reduction recommendations and modified DSM expressions that achieve reliability indices β ≥ 2.5. The programme was extended to combined axial compression and minor-axis bending using 1,134 FEA models with six eccentricities, demonstrating a systematic discrepancy in current AS/NZS 4600 interaction checks: strengths are generally underestimated at eccentricities of 10–25 mm and overestimated at 50 mm. A new interaction equation incorporating element and web slenderness ratios is proposed and verified within the AISI S100 reliability framework. Subsequently, a parametric three-dimensional heat-transfer study quantified the dependence of heat flux on geometry and slit parameters, providing a quantitative basis for thermal performance gains and their balance with strength. The thesis proposes design recommendations and revised equations for slitted CFS studs that satisfy codified reliability targets and support informed selection of slit configurations to balance energy efficiency and structural capacity in practice.

year: 2025

title: Place attachment of Ngāi Te Ahi to Hairini Marae

authors: Teddy, Lani

subjects: Hairini

abstract: Twelve members of Ngāi Te Ahi, a hapū from Tauranga, were interviewed to explore how they talk about their place attachment to Hairini Marae.  This was organized around five key dimensions of place attachment taken from the literature—continuity, distinctiveness, symbolism, attachment and familiarity.  We found that in discussing all dimensions, place attachment was equally about social and cultural relationships, history, and socialization.  Place attachment for groups such as Māori is complex because it encompasses all social relationships past and present.  The implication for those working with Māori is to take seriously the wider connotations of place when talking to Māori about marae, traditional homelands, and their land.

text: Place attachment of Ngāi Te Ahi to Hairini Marae Twelve members of Ngāi Te Ahi, a hapū from Tauranga, were interviewed to explore how they talk about their place attachment to Hairini Marae.  This was organized around five key dimensions of place attachment taken from the literature—continuity, distinctiveness, symbolism, attachment and familiarity.  We found that in discussing all dimensions, place attachment was equally about social and cultural relationships, history, and socialization.  Place attachment for groups such as Māori is complex because it encompasses all social relationships past and present.  The implication for those working with Māori is to take seriously the wider connotations of place when talking to Māori about marae, traditional homelands, and their land.

year: 2008