DF225 battery isolator / alternator output

[waverunner]

we are attempting to add a battery isolator /charger to charge three batteries using the single wire from the engine alternator.
are there any tricks on how to wire it up.
we have tried to disconnect the wire that connects the alternator output to the main positive battery lead and connect the output of the alternator directly to the externat batery isolator / charger but the result is that we get no or very little power to the engine and the trim moves very little and slowly.

basically we are trying to avoid connecting the alternator output to the main battery lead and the isolator in parrallel, ( the isolator / charger will charge the main battery).

[courtenay7500]

batterys

Put in a heavy duty selinoid between no.1 and no.2 battery and throw away isolater.