Final answer:
High voltage drop across the main starter cable while cranking is termed as a 'voltage drop', it can lead to significant issues when starting an engine due to diminished voltage reaching the starter motor.
Step-by-step explanation:
High voltage drop across the main starter cable while cranking is called voltage drop. This occurs due to the resistance in the starter cable, which can cause a large current to create a significant voltage drop, thereby diminishing the voltage that reaches the starter motor from the battery. Using multiple batteries in series, rather than a single 12-V battery, can exacerbate this issue by increasing the resistance and the potential for a larger voltage drop, leading to difficulties in starting the engine.