A cordless vacuum battery may need replacement when it begins showing signs of deterioration, such as a noticeable decrease in runtime, where the vacuum runs for significantly less time even after a full charge. Another sign is longer charging times, where the battery takes much longer to reach full capacity than before. Overheating during use or charging is also a red flag, as it indicates internal inefficiency. Inconsistent performance, such as the vacuum intermittently stopping or losing suction power, can point to a failing battery. Finally, if the vacuum won’t turn on at all, even after a full charge, the battery may have reached the end of its life and likely needs replacement.