En 
SvWhy not have an arbitration period?
An alternative to a divider relay is to use a divider diode.
Isolating diodes introduce a voltage drop of 0.7 V that causes you to lose half your charge.
From a safety point of view, the relay is also preferable because you only need cables between the generator and the starter battery, i.e. no components that can break if the worst comes to the worst.
The isolating relay is located after the starter battery, which means that there is no "manoeuvre" between the generator and the starter battery that can break.