Hyundai announced it is adding a lifetime battery replacement guarantee to its Sonata Hybrid coverage to expand the benefits of the Hyundai Assurance warranty program.

The coverage applies to all 2012 model-year Sonata Hybrid models over the life expectancy of the vehicle and thousands of miles beyond the average duration of new-car ownership, Hyundai said. The protection ensures that if the Sonata Hybrid lithium polymer battery technology fails, Hyundai will replace the battery and cover recycling costs for the old powerplant free of charge to the owner.

Hyundai said it tested Sonata Hybrid and its Blue Drive hybrid drivetrain for more than 300,000 miles with minimal degradation of its output or duration of operation.

“Expanding Hyundai Assurance to include Lifetime Battery Replacement Guarantee was another opportunity for us to demonstrate our confidence in the durability of our product, and pass that peace of mind on to our owners,” said Michael O’Brien, vice president of corporate and product planning.

The heart of Hyundai’s Hybrid Blue Drive technology is its lithium polymer battery pack. The lithium polymer cells, developed with partner LG Chem, use a manganese spinel chemistry that provides balance between power delivery, energy density and thermal stability.

This thermal stability is critical to ensuring durability, eliminating the need to replace the battery pack during the normal lifespan of the vehicle, Hyundai said. The electrodes in older lithium-ion chemistries expand and contract with the heating and cooling that occurs during charging and discharging.

This thermal expansion causes cracks in the electrodes, which ultimately reduces the cell's ability to hold a charge. Manganese spinel lithium polymer cells have much lower expansion rates and can go through tens of thousands of charge cycles, even without having to use a heavier, liquid cooling system, according to the automaker.