Marine Battery Isolators

A battery isolator lets one alternator or charger charge two or more battery banks while keeping them separated, so the house bank can never drain the starting battery. This collection covers Powerline diode isolators from 70 to 400 amps (one or two alternators, one to four batteries, positive-ground and Delco CS130 versions), Victron Argo FET and Argo diode isolators, Victron Cyrix combiners and Hypertech battery combiners for lead-acid and lithium banks.

How to choose a marine battery isolator

• Amp rating: match or exceed the maximum output of your alternator.
• Alternators and banks: count the charging sources and the battery banks you need to charge.
• Alternator type: some models need an excitation or regulator terminal, or a version made for Delco CS130 alternators or positive-ground systems.
• Voltage drop: diode isolators drop some voltage; FET isolators and combiners reduce or avoid that loss.

What is a battery isolator?

It is a one-way device placed between the alternator and the batteries. Charging current flows to every bank, but the banks cannot discharge into each other.

Isolator or battery combiner on a boat?

A diode or FET boat battery isolator is fully passive. A combiner (voltage-sensitive relay) connects the banks only while charging voltage is present and separates them when it drops. See our battery isolator guide for typical wiring layouts.

Can I use an isolator with lithium batteries?

Yes, but check compatibility. The Victron Cyrix-Li-Charge and the Hypertech lithium combiners are designed for LiFePO4 banks. For more charge control between banks, also look at our battery chargers and DC-DC chargers.

If you are upgrading the charging source too, read our marine alternator and regulator guide.