If you're fairly competent then the the job should take about 2 hours to complete.
If you're interested in Mazda Bongos check out the U.K. owners club here Bongo Fury
Step 1: What exactly are we doing here
I am not going to describe how to wire up your goodies or how to rewire the internal lights etc to run from the leisure battery (that's another howto). I am going to explain how to wire up a split charging circuit so that your leisure battery gets charged up as you drive along.
Your starter battery is connected to the alternator of the van so when you drive along it gets charged. If we connect our leisure battery in parallel to the starter battery it will also get charged as we drive along.
Unfortunately it's not quite that simple. There are a couple instances when we don't want the two batteries connected.
1) Since the starter battery is the most important battery we need to insure that it gets fully charged before we even think about charging the leisure battery.
2) If we are stationary and using the onboard electrics we don't want to risk a flat starter battery so we must ensure it's disconnected from the leisure battery.
To do this we use a special relay that connects / disconnects the two batteries according to the above criteria.
The special relay is a variously called a voltage sensing relay (VSR), smart relay, battery isolator and several other names I can't remember.
It works by only making its switched connection when the input volatge reaches a preset level and unmaking its switched connection below that preset level.
To make sure we only charge the leisure battery when the engine is running and the starter battery is fully charged we need the VSR to make at about 14 volts. When the engine is not running or the starter battery is not fully charged the VSR will see less than 14v and so won't make. This matches the criteria set out earlier.
So our simple parallel circuit now has a smart switch added.