HESS Dissertation · BLDC · Buck-Boost
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MSc Dissertation · Coventry University

Hybrid Energy Storage System (HESS) for Light EVs

A semi-active HESS that blends Li-ion batteries with supercapacitors via a bidirectional buck-boost converter to reduce battery stress during transients and extend cycle life. Control and validation were performed in MATLAB/Simulink with a BLDC load model.

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Objective

Design and evaluate a semi-active HESS that routes high-frequency power bursts to a supercapacitor bank while the Li-ion battery supplies average power—lowering peak currents, temperature rise, and degradation.

Motivation

Batteries suffer from rate-dependent aging. Start-ups, regenerative braking, and hills impose high transients. Supercapacitors can absorb/deliver these bursts efficiently, protecting the battery and improving responsiveness.

Contributions

  • Semi-active topology co-simulation with BLDC drive
  • Bidirectional buck-boost energy balancing
  • Rule-based power split + current limiting
  • Thermal and stress reduction estimates

Keywords

HESS · Supercapacitor · Buck-Boost · BLDC · MATLAB/Simulink · Battery Aging · Transient Mitigation

Architecture Overview

Topology

Semi-active HESS with SC branch interfaced via a bidirectional buck-boost; battery tied to DC link.

Converter

Continuous conduction, current-mode control; inductor sizing & switching frequency chosen to limit ripple.

Control

Rule-based power split: battery handles average demand, SC handles peaks; SOC/SOH constraints enforced.

Load

BLDC motor with back-EMF model and inverter; drive cycles emulate stop-and-go urban usage.

Methodology

  • Model Li-ion (open-circuit + R0 + (R,C) pair) & SC ESR dynamics
  • Size inductor/capacitors for ripple & transient targets
  • Implement current-limit, DC-bus regulation, and power split
  • Drive cycles: launch, hill, regen, cruise

Evaluation

  • Peak battery current reduction vs. battery-only baseline
  • Temperature rise proxy via I²R + thermal RC
  • DC-bus voltage regulation during pulses
  • Supercapacitor voltage window utilization

Results (Highlights)

  • 30–60% drop in battery peak current on aggressive launches
  • DC-link maintained within ±3–5% under step loads
  • Estimated thermal stress reduction → potential cycle-life gains
  • SC absorbs regen spikes → reduced battery charge spikes

Limitations

  • Rule-based control; no explicit optimality
  • Aging model simplified (calendar + cycling combined proxy)
  • Converter losses modeled lump-sum; hardware TBD

Future Work

  • MPPT-like optimal split (DP/MPC) with constraints
  • Hardware-in-the-loop and thermal chamber testing
  • Health-aware EMS (SOH/SOC observers)
  • Extend to multi-motor micro-mobility platforms

Downloads

You can view or download the full dissertation as a PDF:

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