Sometimes one is not just enough. A dual battery system in your car is essential for those who have an Audio and Video set up, those who take their vehicle camping and want to run their gear without discharging their main batteries. Some cars will have space in front of a dual battery set up but most of the time it will have to be situated at the back or the trunk.
Usually, dual battery set up vehicles are those that go camping or fishing. They need to power a small fridge or lights while camping. However it isn’t just for camping, having two batteries is surefire insurance that when one goes dead for some reason or another, you will have a backup handy. Camping, using 4WD, or winching can be hard on the batteries and when you are in the remotest part of the outback, it is always a good idea to have a spare so you can get back to civilization. For those who want to go camping with their vehicle, a second battery ensures that you can run appliances for your convenience while preserving the power of your main batteries for transport.
Another use for a dual battery system is for those who run high end and powerful audio set up in the vehicles. In the same manner as using it for camping. The second battery can be used to power the audio equipment without harming the main battery by using a dual battery isolator, this will enable the alternator to charge them both while running.
In order to maximize the dual battery set up, make sure to use the same battery size and capacity. Use identical batteries if you can. If you do not, the battery isolator will charge the smaller battery with the same amount of juice of the larger battery attached which may cause problems. Never just hook two batteries in parallel. The use of an isolator will prevent backfeed of one battery to the other. It is preferable to use maintenance free or gel cell batteries for this kind of set up.
SO there you have it, a dual battery set up is way better IF you are a camper and if you run some equipment from your car. Audio equipment especially the louder ones can take a toll on your regular car batteries. High power amplifiers and speaker will drain your battery flat in minutes.