For boats with two or more batteries (which is very typical), diode isolator devices electrically isolate the batteries to prevent the battery with the higher charge from draining into that with the lower charge. Also, all connected batteries can be charged simultaneously from one source (alternator).
A battery isolator is a one-way electrical battery that allows an electrical current to flow in…
A battery isolator can eliminate the problem of battery drain altogether. By using diodes to only allow current to flow in one direction, a battery isolator prevents a fully charged main battery from passing current to a partially charged auxiliary battery.
With this setup, each battery is isolated and acts as an independent power source, allowing the main battery to conserve power for starting and essential functions.