The energy a battery holds can be measured with a battery analyzer by applying a full discharge. The battery is first charged and then discharged at a controlled current while measuring the time to reach the end-of-discharge point. A capacity of 100 percent delivers the specified Ah; 50 percent is shown if the discharge time is cut in half.
Measuring capacity by discharge gives the most reliable assessment, a method that is most effective with portable batteries. A periodic analysis ensures that the batteries are kept within an acceptable performance range and prompts replacement if the capacity is below a required target threshold and can no longer be raised. Larger batteries are normally not cycled as this is time-consuming and stresses the battery. That’s where non-invasive test methods come in.
Progress is being made with rapid-testing but this only provides estimated state-of-health results and the accuracies vary with the technology used. While older rapid-test methods measured the internal battery resistance, advanced rapid-test methods look at diffusion, charge transfer and migration that are present in Li-ion and lead acid batteries.