Hybrid Lead-acid/Lithium-ion Energy Storage System with Power-mix Control for Light Electric Vehicles

The performance versus cost tradeoffs of a fully electric, hybrid energy storage system (HESS), using lithium-ion (LI) and lead-acid (PbA) batteries, are explored in this work for a light electric vehicle (LEV). While LI batteries typically have higher energy density, lower internal resistance and longer lifetime than PbA batteries, the module cost of LI batteries are typically three to five times the cost of PbA batteries. The objective is to design a HESS that 1) is cost-competitive with a PbA single energy storage system (SESS) and 2) maintains most of the performance benefits of a lithium SESS. A modular HESS architecture with a bi-directional dc-dc converter and controller is proposed, and a power-mix algorithm with active inter-chemistry battery state-of-charge (SOC) balancing is presented, simulated, and verified experimentally. The batteries, converter and control algorithm are modeled in MATLAB, and the effects of total ESS energy, vehicle loading, depth-of-discharge (DOD), and speed are explored. The cost and performance of the HESS are assessed side-by-side with PbA and LI single energy storage system (SESS) configurations of comparable total energy, using expected vehicle range as the performance metric. The experimental HESS has a total projected cost midway between the SESS PbA cost and the SESS Li cost, while providing 17% range and 22.5% efficiency increase over the SESS PbA vehicle.