Final answer:
A battery sensor is utilized to monitor temperature for optimal charging, while battery testers measure internal resistance and terminal voltage to gauge a battery's health.
Step-by-step explanation:
A battery sensor is used to measure battery temperature so the charging system can alter charging output as needed. Battery testers, such as those pictured in various figures, often use small load resistors to draw current and measure if the terminal voltage drops below an acceptable level, thereby testing the battery's internal resistance. High internal resistance suggests that the battery is weak, shown through its low terminal voltage. Additionally, varying methods like using a hydrometer can measure the state of charge by checking the specific gravity of each cell within a battery.