The simplest and cheapest dual battery system is to use a 30amp lighting relay to connect the starting and auxilliary batter and activate the relay from your accessories circuit. Whenever your ignition is on batteries will be charging and when ignition is off, batteries will be isolated - thereby avoiding being caught out with a flat starting battery.
I ran this system in a D1 for years without a problem.
Systems such a Traxide are much better and offer numerous other advantages but are also a lot more expensive.
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