Perhaps the most important item needed for an auxiliary battery system is a battery isolator. Isolators allow auxiliary batteries to receive a charge from the vehicles electrical system but as the name suggest, isolates it and prevents a back-flow of current and depletion of the auxiliary battery. Advanced battery isolators also give operators the ability to manually override isolation and to assist with emergency jump starting.
When adding an auxiliary battery system, a new a circuit is created and in some cases such as a surveillance van, many sub-circuits are created as well. Without compromise, anytime a new circuit is added, proper fusing needs to be installed. For auxiliary battery systems, in general I recommend using a master or a.k.a. limiter fuse in between the auxiliary battery and the vehicles electrical system. Additionally, any sub-circuit originating from the auxiliary battery should have its own dedicated fuse. Fuse sizes are dependent upon total load or in other words amperage demand that will be placed up the new circuit(s) and potential use. For example, an auxiliary system designed to support 40 amps worth of electronics and the potential emergency jump start, should probably use a 150 amp limiter fuse and a separate 50 amp fuse for the sub-circuits. In essence, the fuse size must be larger than the demand by a small margin.
Like fuses, the size of wiring will be dependent upon the total load and potential future use. Also like fuses, if the wiring is too small… it will simply burn up, but in this case pose a serious safety hazard. On that note, the bigger wire size (gauge, AWG) the better.
A perfect battery type to cover various mission needs is an SLA battery (sealed lead acid). SLA batteries are resistant to vibration can be mounted in any orientation, endure temperature extremes and provides a long shelf life for energy. When it comes to battery size, there are a lot to choose from and here again; it will be dependent upon use. Like wiring, get the largest size possible or that the vehicle storage space will afford. The larger the size, the longer it will be able to support operation with engine off and the better it will be able to provide emergency starting power. For reference, SLA batteries are rated in amp hours (aH) which indicates how much amperage it can deliver in a given hour before depletion. A great battery for initial consideration that can serve many application needs is a 60aH battery.