A tech company has launched a new line of high- performance laptops designed for business professionals. The company offers a warranty on the battery, promising free replacement if the battery capacity drops below 80% within the warranty period. The lifespan of the laptop batteries, until they drop below 80% capacity, follows a normal distribution with an average lifespan of 3.75 years and a standard deviation of 0.89 years. What length should the battery warranty period be set at by the company, such that only 2.5% of laptop batteries are eligible for replacement within this timeframe?