Diode battery isolators allow simultaneous charging of two or more batteries from one alternator, without connecting the batteries together. Discharging the accessory battery for example will not result in also discharging the starter battery.
The Argo battery isolators feature a low voltage drop thanks to the use of Schottky diodes: at low current the voltage drop is approximately 0,3 V and at the rated output approximately 0,45 V.
All models are fitted with a compensation diode that can be used to slightly increase the output voltage of the alternator.

This compensates for the voltage drop over the diodes in the isolator.

Argo Diode Isolator Datasheet