Visually inspecting the battery itself for corrosion or leaks and listening to the engine for delays or sluggishness are ways of monitoring battery health. Visible damage or fluid leaks are always signs that a new battery is needed, while an apparent lack of power may be caused by cable issues.
If a car engine cranks more slowly than usual, the battery may not be providing as much power. A battery case that has bloated or shows battery acid corrosion at the connection posts is ready for replacement. Some batteries have a translucent portion of the casing to let the owner check the fluid level, and if the fluid is under the lead plates that conduct energy, a battery test is necessary. If the battery is older than three years, it needs an annual inspection whether it works or not.