Sizing Supercapacitors for Minimum Weight Across Twelve Applications: End-of-Life Design, Self-Discharge, Heat and Life, Sensitivity, Converters and Standards

Abstract

Background. The mass of a supercapacitor bank is set by its cell count, so sizing is the step that decides how heavy a hybrid storage system is. Companion papers analysed the physics and the patented architecture of a battery–supercapacitor Hybrid Power Pack. This paper sizes the supercapacitor itself for each application those papers identified and asks what sets the weight in each. Method. Each bank is sized by a constant-power discharge of a capacitance in series with its equivalent series resistance (ESR), at the end-of-life values that the cell datasheet defines (capacitance −20 %, ESR +100 %). Where a converter is used, its efficiency is taken as 95.3 %, the lowest mode peak measured on a published prototype. A search over seven datasheet-verified electric double-layer capacitor (EDLC) cells returns the lightest bank that meets the event energy, the power, the voltage window and, for repeated duty with a known power profile, a temperature-rise limit. Event data come from published measurements, simulations and standards, and every value is labelled as sourced, computed or assumed. Results. Across the banks summarised, cell masses range from 1.44 kg (12 V engine cranking, six cars) to 1426 kg (a 9152 kJ ship-to-shore crane cycle), with 470 kg for a 353 kW, 12.3 s metro wayside event. Three regimes appear. In peak-power events the ESR sets the size: cranking banks carry 4.3–5.8 times, and the crane bank 2.1 times, the capacitance that energy alone requires. In repeated energy events the end-of-life allowance (26.5 % of the metro bank mass) and the event size dominate, and a temperature-rise limit trades mass for life: 10 % more cell mass lowers the ageing factor from self-heating by 19 %. In small events the series count sets a mass floor: a 10 kW fuel-cell step needs 14.4 kg of cells at 240 in series and 3.6 kg at 60 in series. For a battery-electric car on standard drive cycles, the most battery-heat reduction per kilogram comes from a small bank with a 5–10 s low-pass split: 30.6 kg of cells cut battery Joule heat by 26 % on WLTC, and 61.6 kg cut it by 54 % on US06. For photovoltaic smoothing, a capacitor covering the first 30 s of a cloud transient handles 2.8 % of the energy, and with a low-pass split the best return, at τ = 20 s, is 1.1 t of cells per MWp for a 9.2 % lower battery peak power; both support a battery–capacitor split. A lithium-ion capacitor halves the metro bank to 233 kg. Self-discharge, self-heating and battery-ageing effects are quantified and the applicable standards listed. Conclusion. Sizing at end of life with the ESR and the heating in the model, and matching the cell, series count and voltage window to the event, gives the lightest bank that still meets its duty on its last day of service. Costing is deferred to a later paper. Relation to earlier work: the analysis also appears as Chapter 10 of the author's monograph (doi:10.5281/zenodo.23021958); this is its stand-alone version. All results are computed, not measured. Disclosure: the author is the named inventor of the Hybrid Power Pack patents (US 10,523,019 B2 and family) and founder of Rohera Emerging Energies Private Limited. Preprint, not peer reviewed.