En 
SvWhy not use a battery isolator diode?
An alternative to a divider relay is to use a divider diode.
Battery isolator diodes introduce a voltage drop of about 0.7 V, which means you effectively lose half of the charging performance.
From a safety perspective, a relay is also preferable, since only cables are required between the alternator and the starter battery—meaning there are no additional components that could fail if something goes wrong.
The battery isolator relay is installed after the starter battery, which means there is no device between the alternator and the starter battery that could fail.