I want to wire up two batteries, do I need a battery Isolator. Or should I wire them negative to negative and positive to positive.
In most cases you do not want to connect the batteries using a battery isolator. Battery isolators are hard on alternators because they have a voltage drop within the isolator and they are restrictive to current flow, so your alternator has to power that voltage drop and resistance. The only time you use a battery isolator is in motor homes or boats. When you sit for long times with you motor off and you want to be sure your vehicle starts in the morning, this is when you would use a battery isolator. If you are not concerned about sitting to long and not getting your vehicle started then you should connect multiple batteries in parallel like they are one single battery. Connecting them in parallel is positive connecting to positive and negative to negative.