Final answer:
The gel-cell battery's main role is to provide the necessary current to start the engine, with an alternator then powering the electrical components and charging the battery.
Step-by-step explanation:
The primary function of a gel-cell battery in a truck chassis battery bank is to supply a large current needed to start the engine. The operation of starting the vehicle requires this hefty current. Once the engine is running, the alternator takes over to power the vehicle's electrical components and to charge the battery. In circumstances where a truck uses a 24V starter system, batteries can be connected in a series-parallel configuration. Connecting two pairs of 12-V batteries in series provides 24 V needed for starting the engine, and connecting those pairs in parallel allows for additional capacity without increasing voltage, which is ideal for the 12-V electrical component operation.