Explanation: Measuring a battery's voltage under a heavy load is crucial for determining its true condition of charge because it reveals the battery's ability to supply current. All batteries have internal resistance. When a heavy load is applied, current flows, causing a voltage drop across this internal resistance. A healthy, well-charged battery will maintain a higher terminal voltage under load compared to one that is partially discharged or nearing the end of its life, which will show a significant voltage drop. An open-circuit voltage (no load) can be misleading, as a dying battery might still show a high voltage but collapse under actual use. Therefore, measuring under load gives a more accurate assessment of its state of charge and capacity. Polarity is determined by meter deflection or sign, not load. While age affects performance, a voltage measurement under load indicates current condition, not the battery's age directly.