For what reason do you wish for the second battery Mick?
If as a parallel starter battery for extra ‘ooomph’ on startup, does it need it? Or for a leisure battery setup? Two batteries in parallel is never the perfect scenario it is expected to or should be, due to the cells being in a bank and doing their own thing within each battery and not getting an even charge across the battery.
If you are having starting issues in cold temperatures and need lots of cranking power, the second battery was added by Toyota to assist with this. If you’re not, you don’t need it. Just put the best quality highest capacity battery in the single battery tray.
If on the other hand you want to add a battery for leisure purposes, that loom will not be ideal. If running the same chemistry battery then a simple intelligent split charge relay would suffice. If utilising an AGM or sealed gel type battery or even lithium, you need a separate charger to manage the charging of that battery to give it the best possible life expectancy. Even a standard battery of the same chemistry as the starter battery would benefit from this, but the benefit is greater when dissimilar chemistry batteries are used. A DCDC charger is an ideal solution to look after a second battery.
