Sizing Supercapacitors for Minimum Weight Across Twelve Applications: End-of-Life Design, Self-Discharge, Heat and Life, Sensitivity, Converters and Standards
The mass of a supercapacitor bank is set by how many cells it needs, so sizing is the step that decides how heavy a hybrid storage system is. This paper sizes the bank for each application the earlier Hybrid Power Pack papers identified, and finds the lightest one that still does its job on the last day of its service life.
Status. This is a preprint on Zenodo. A manuscript based on it is under peer review at Energies (MDPI). The same analysis appears as Chapter 10 of the consolidated monograph; this is its stand-alone version. Every result is computed from stated inputs, not measured.
The numbers in brief
Table 1. Selected banks from the paper, sized at end of life. Cell mass only. Computed, not measured.
| Application | Event | Cell mass | What sets the size |
|---|---|---|---|
| Engine cranking, 12 V (six cars) | Peak-power start | from 1.44 kg | ESR: 4.3–5.8× the capacitance energy alone requires |
| 10 kW fuel-cell step | Small event | 3.6 kg (60 in series) 14.4 kg (240 in series) | Series count sets a mass floor |
| Battery-electric car, WLTC | Drive cycle, 5–10 s low-pass split | 30.6 kg | Battery Joule heat 26 % lower |
| Battery-electric car, US06 | Drive cycle, 5–10 s low-pass split | 61.6 kg | Battery Joule heat 54 % lower |
| Metro wayside store | 353 kW for 12.3 s, repeated | 470 kg EDLC 233 kg lithium-ion capacitor | End-of-life allowance (26.5 % of mass) and event size |
| Photovoltaic smoothing | Cloud transient, low-pass split at τ = 20 s | 1.1 t per MWp | Battery peak power 9.2 % lower |
| Ship-to-shore crane | 9,152 kJ cycle | 1,426 kg | ESR: 2.1× the capacitance energy alone requires |
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.
Keywords: supercapacitor sizing; electric vehicle; photovoltaic smoothing; low-pass filter; electric double-layer capacitor; end of life; equivalent series resistance; self-discharge; thermal design; lithium-ion capacitor; hybrid energy storage; engine cranking; regenerative braking
Three regimes, three different things to design for
Peak-power events: the resistance decides
When the event is short and intense, as in engine cranking or a crane lift, the bank has to deliver its power without its terminal voltage collapsing across its own internal resistance. That resistance, not the stored energy, sets the cell count. The cranking banks end up with four to six times the capacitance the energy of the crank would suggest, which is why a low-ESR cell is worth more here than a high-capacitance one. The arithmetic behind the power ceiling is set out in how a supercapacitor buffer is sized.
Repeated energy events: plan for the last day, and for heat
When the bank cycles all day, as at a metro wayside station, two things dominate: the size of each event, and the allowance for ageing. Designing for a cell that has lost a fifth of its capacitance and doubled its resistance accounts for about a quarter of the bank's mass (26.5 % in the metro case), and it is what makes the bank still adequate in its final year. Repeated duty also heats the cells, and heat ages them. The paper puts a number on the trade: adding 10 % to the cell mass lowers the ageing factor from self-heating by 19 %.
Small events: the series count sets a floor
For a small event on a high-voltage bus, the bank cannot shrink below the number of cells needed to span the voltage. The fuel-cell example makes the point: the same 10 kW step needs four times the cell mass at 240 cells in series as at 60. Choosing the voltage window and converter arrangement is therefore part of sizing, not a separate decision.
Where the battery gains most
For a battery-electric car, a modest bank fed through a low-pass split of 5–10 s gives the most reduction in battery heating per kilogram added. For photovoltaic plants, a capacitor sized for the first 30 s of a cloud transient carries only 2.8 % of the transient energy, which supports giving the fast part of the duty to the capacitor and the slow part to the battery. The same division of labour is the subject of Two Stores, One Power Pack, and the reason a short, high current costs the battery more is explained in why a short high current costs more.
Where this paper sits in the series
| Work | What it covers | Record |
|---|---|---|
| Two Stores, One Power Pack | The physics of battery–supercapacitor hybridisation and the patented architecture | 10.5281/zenodo.22958076 |
| The Hybrid Power Pack as a Platform | Claim-by-claim engineering analysis of US 10,523,019 B2 | 10.5281/zenodo.22970638 |
| Sizing Supercapacitors for Minimum Weight (this paper) | Sizing the capacitor bank for each application, at end of life | 10.5281/zenodo.23301485 |
| Two Stores, One Power Pack — consolidated monograph | The whole series in one volume; this paper is its Chapter 10 | 10.5281/zenodo.23021957 |
Declarations
Competing interest. 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. A patent grant records what is claimed; the technical results here rest on the stated method and inputs.
Nature of the results. All results are computed from datasheet values, published event data and stated assumptions; each input in the paper is labelled as sourced, computed or assumed. Costing is deferred to a later paper.
Licence. CC BY 4.0.
How to cite
Rohera, H. K. (2026). Sizing Supercapacitors for Minimum Weight Across Twelve Applications: End-of-Life Design, Self-Discharge, Heat and Life, Sensitivity, Converters and Standards. Preprint, Zenodo. https://doi.org/10.5281/zenodo.23301485