Mounting the battery is very important and must be done using good quality parts and brackets. Mounting the battery in a tool box or caravan boot is fairly easy but make sure you allow for ventilation, use a sturdy hold down clamp and the battery must be placed in a marine battery box or kept separate from tools, canvas and other loose items with a divider to prevent acid damage, short circuiting and possible battery rupture.
12 volt power pack units have a battery compartment and the power pack basically just needs to be secured in a safe upright position. When fitting the battery as a dual battery under a bonnet, there are a few things to keep in mind if you can't find a custom-made battery tray or carrier to suit your vehicle. Check your bonnet clearance before making a carrier or buying the battery - some systems require a low mount battery with recessed terminals similar to the BMW style batteries to obtain the bonnet clearance and where possible ensure the positive is the further away from the outside edge of the vehicle in case of off-road body damage. When making the carrier, use at least 25x25 angle and support the battery in at least 4 solid mounting points bearing in mind that there are corrugations and potholes on the roads you will likely travel.
With the battery (or power pack) mounted in a secure safe position, you can now connect them up to the charging system. Power Packs are supplied with cables and instructions and have built-in isolators to prevent draw from the starting battery when the engine is not running and with these it is just a matter of connecting the supplied cable to the starting battery and you are done. These cables are normally fitted with Anderson plugs for easy removal of the power pack when not in use, or when power is needed away from the vehicle for use at a camp site or in a boat.
With permanent mounted batteries, or dual battery systems, you will need to do the wiring to the charging system. To prevent voltage drop over long runs of cable it is necessary to use fairly heavy cable between the starting battery and auxiliary battery. The minimum cable thickness that should be used is 100 amp 8 B&S cable. In extreme situations it may be needed to use 140 amp 6 B&S cable. These cables must be run safely away from stone damage and sharp edges. When wiring up to a caravan or camper trailer use Anderson plugs at the hitch and use a fresh clean earth point - never use the hitch as an earth source. Use good quality lugs and brass battery terminals for all the connections between the two batteries.