Your hybrid vehicle's battery is one of its most critical components — and one of the most expensive to replace. Knowing the early warning signs can save you thousands of dollars and prevent you from getting stranded. 5 Warning Signs Your Hybrid Battery Is Failing 1. Reduced Fuel Economy — If you notice your mpg dropping significantly, a degraded battery may be the cause. The internal combustion engine has to work harder when the hybrid battery can't hold a charge properly. 2