If you NEED to join 2 cables together, use the cable drum to keep the connectors off the ground, better still, you can buy a waterproof connector shroud to enclose the plug and socket to keep them dry.
You don't need 2.5mm2 if your just hooking up to a smart charger to charge the batteries. My self build uses Lithium batteries hooked up to a 2kw inverter to power the van. The 30A charger draws just over 2 amps (AC).
It's good practice to use 2.5mm2. 1.5mm2 is usually 15A rated, which means that it gives opportunity to cause voltage drop/heating issues. Maybe less significant on short runs, less than 5m or so. But always use the proper rated cable for certain connectors, especially when unfused.
