Final answer:
Batteries in series increase voltage but affect the whole circuit if one fails; parallel connections increase capacity but can lead to charge imbalance. Semi-tractor trucks use series connections for higher voltage to start heavy loads and parallel for normal operations.
Step-by-step explanation:
The advantages and disadvantages of connecting batteries in series and in parallel configurations differ significantly. When batteries are connected in series, the resulting advantage is increased voltage; however, this comes with the disadvantage that if one battery fails, it affects the entire circuit. On the other hand, connecting batteries in parallel has the benefit of increased capacity, which means the batteries can deliver the same voltage for a longer period or to support a higher current demand. Yet the drawback is the potential for an imbalance in charging among the batteries.
For semi-tractor trucks, which use four large 12-V batteries, the starter system requires 24 V to turn over the engine effectively given the heavy load, while normal operation needs 12 V. To produce the required 24 V, two sets of batteries can be connected in series and then those sets connected in parallel. To provide 12 V, all four batteries can be connected in parallel. The reasoning behind using 24 V for starting engines, especially for heavy loads, involves providing a higher voltage for a more reliable and potent turnover of the engine, overcoming the higher start-up resistance.