Second batteries are fairly easy installations.

The basic installation requires a battery isolator, a cable to the primary
battery, and a nice high-gauge line to ground.

The next level up steps up to a dual-output alternator and other goodies.

Having done interesting things with power in the car audio arena I can say
that it's fairly trivial, but you have to be careful about long runs without
fuses.  A suitably high gauge lead (2 gauge or 1 gauge), a fuse or a circuit
breaker (not tremendously cheap at that gauge), and a good line to the
chassis ground are requirements.

--JRP
'89 Westy, '90 Fox