A Relay-based Isolator have no apparent voltage drop while for Diode-based Isolator there will be minimally 0.7V – 1.5V drop across the diode depending on the amount of current pass through. The higher the current, the bigger the voltage drop and the lower the efficiency of charging the dual battery.
Relay-based Isolator is best seen as switch that can direct current, enabling primary and auxiliary bank of batteries to be charged from a single source, using a solenoid. When the engine starter is engaged, the Relay-based Isolator compares the voltage in both main and auxiliary batteries. If the Relay-based Isolator determines that there’s not enough voltage in the primary source to perform an essential function, such as starting an engine, it will open the solenoid and allow current to flow from the auxiliary battery to make up the difference. However, if the drain on the charging system by the auxiliary or main battery bank reduces the system voltage below a certain point, it will disconnect the battery banks from each other, protecting them from excessive drain.
Thus, in most of the scenarios, Relay-based Isolator is the preferred choice for Dual Battery Applications involving battery banks of at least one Main Battery and one Auxiliary Battery.