Final answer:
A voltage reading can identify defective cells in a battery. This is determined using a battery tester that measures terminal voltage under load, revealing any high internal resistance which indicates a weak battery.
Step-by-step explanation:
A voltage reading can identify defective cells in a battery. Battery testers apply a load using small load resistors, allowing for a current to flow and reveal the terminal voltage under stress.
If the internal resistance is high, indicating a weak battery, the terminal voltage will be low, which can signify defective cells or other issues like voltage leaks or reasons for battery failure.
Devices like the ones shown in Figure 10.9 and Figure 21.15 use this strategy to assess a battery's health. The large devices might be used for bigger batteries such as those in the military, while smaller digital devices can be employed for consumer electronics. Regardless of size, the principle of measuring terminal voltage under load remains consistent for diagnosing battery condition.