Batteries don't need to be plugged in 24/7. They just need to be kept charged, so they don't run down. Depending on the parasitic draw on the rig, this will determine how often to charge. A digital volt meter is your BEST friend regarding batteries.
A fully charged battery is at least 12.6 volts. When it gets down to around 12.2 volts, it should be charged, and not allowed to drop lower, so you don't risk damaging it.