Battery Switch and Isolator Buying Guide: Switches, ACRs and Dual Battery Setups
Quick answer: Every boat with a battery needs a battery switch as a master disconnect. If you have two or more batteries, add an automatic charging relay (ACR) so your engine charges both banks while your start battery stays protected from house loads. Choose a switch rated above your engine's cranking current, and pick the simplest setup that keeps a charged start battery every time.
The fastest way to ruin a day on the water is turning the key and hearing a click because the stereo, livewell and fishfinder drained the only battery. Battery switches and isolators solve that by splitting your power into a dedicated starting battery and a separate house bank, then managing how they charge. This guide explains switch types, isolators versus ACRs, and the most common dual-battery layouts.
Step 1: Understand the switch types
- ON/OFF (single circuit): a master disconnect for one battery or bank. Every boat needs at least this, and ABYC calls for a battery switch on most installations.
- Selector (1 / 2 / BOTH / OFF): the classic four-position switch. Lets you run from either battery or combine them. Simple, but it relies on you remembering to turn it to the right position.
- Dual Circuit: two separate ON/OFF switches in one unit, one for start and one for house. Keeps them separate at all times.
- Dual Circuit Plus: adds a third "combine" position so you can parallel the batteries for an emergency start if the start battery is dead.
- Remote (electronic) battery switches: controlled from a helm button or panel. Useful when batteries are far aft and hard to reach.
Step 2: Size the switch for cranking current
Switches are rated for continuous current and a higher intermittent (cranking) current. Outboards and small inboards are well within the range of compact switches. Big diesels, windlasses and inverters need heavy-duty switches.
| Switch class | Typical continuous rating | Good for |
|---|---|---|
| Compact (mini) series | about 300A | Outboards, center consoles, bass boats |
| Standard series | about 350A | Most runabouts, sterndrives, small inboards |
| Heavy duty series | about 600A | Diesels, large inverters, thrusters |
Ratings vary by model, so check the spec sheet against your engine's cranking amps and your biggest DC load.
Step 3: Alternator field disconnect (AFD)
Turning a battery switch off while the engine runs can spike voltage and destroy the alternator's diodes. Switches with AFD shut off the alternator field before the main contacts open. If your engine has an external field wire (common on inboards), choose an AFD switch. Most modern outboards are fine without it, but never switch to OFF with the engine running.
Step 4: Isolator or ACR?
- Diode isolator: splits alternator current to two or three batteries and prevents them from draining into each other. Reliable and solid state, but it drops roughly 0.6 to 0.7 volts, so batteries may not fully charge unless the alternator is adjusted. Also used on charger outputs.
- Automatic charging relay (ACR) or voltage sensitive relay (VSR): watches voltage. When the engine is running and the start battery is charged, it connects the house bank so both charge. When voltage drops (engine off, house loads running), it disconnects, protecting the start battery. No voltage loss. This is the better choice for most modern boats.
For lithium house banks, check that your ACR or charging device is lithium compatible, or use a DC-DC charger, since lithium can draw more current than an alternator should supply continuously.
Step 5: Pick a layout
Simple two-battery boat (most common)
A Dual Circuit Plus switch with an ACR. Start battery runs the engine, house battery runs electronics and pumps, and the ACR charges both. Combine only for an emergency start.
Bass boat with trolling motor bank
A master ON/OFF switch for the cranking battery, a separate ON/OFF for the trolling motor bank, and an onboard multi-bank charger to recharge everything at the dock.
Cruiser with house bank and inverter
Heavy duty ON/OFF switches for start and house, an ACR or DC-DC charger between them, and proper Class T fusing for the inverter.
Step 6: Install safely
- Mount the switch as close to the batteries as practical and where you can reach it quickly in an emergency.
- Use battery cable sized for the full cranking current and tinned lugs on every connection.
- Protect positive cables with fuses close to the battery, except the engine cranking circuit as permitted by ABYC.
- Label positions clearly so guests know which way is OFF.
- Bilge pumps that must always work are typically wired direct to the battery with their own fuse, bypassing the switch.
Our picks by boat
All in stock at Boat Supplies Direct.
Master ON/OFF for a single battery
Blue Sea 6006 m-Series Mini ON/OFF, compact and inexpensive, or the Blue Sea 9003e e-Series ON/OFF.
Classic selector switch
Blue Sea 9001e e-Series Selector or the 9002e with Alternator Field Disconnect for inboards.
Two batteries, kept separate
Blue Sea 6011 m-Series Dual Circuit Plus or the larger Blue Sea 5511e e-Series Dual Circuit Plus.
Automatic charging (ACR)
Blue Sea 7611 BatteryLink ACR 120A, or go all-in-one with the Blue Sea 7650 Add-A-Battery Kit (switch plus ACR). For three batteries, the Egis Triple Battery ACR.
Diode isolator
ProMariner ProIsoCharge 120A, 1 alternator, 2 batteries.
Heavy duty for diesels and inverters
Blue Sea 3000 HD-Series ON/OFF or the remote-controlled Blue Sea 7700 ML-Series Remote Battery Switch.
Shop all battery isolators and switches →
Frequently Asked Questions
What does a battery isolator do?
It lets one charging source, such as an alternator, charge two or more batteries while preventing them from discharging into each other, so house loads cannot drain the starting battery.
What is the difference between an isolator and an ACR?
A diode isolator uses diodes and causes a small voltage drop. An ACR is a voltage-controlled relay that connects batteries when charging and separates them when not, with no voltage loss.
Do I need a battery switch and an ACR?
Many two-battery boats use both. The switch provides a master disconnect and manual control, and the ACR automates charging and protects the start battery.
Should I run my boat on BOTH?
Not as a habit. Running on BOTH lets house loads drain the start battery too. Use separate batteries in normal use and combine only to help start the engine.
Can I turn the battery switch off with the engine running?
No. It can damage the alternator. Shut the engine down first, or use a switch with an alternator field disconnect.
Where should a battery switch be mounted?
As close to the batteries as practical, in a dry spot that is easy to reach in an emergency.
This guide is general information. Follow ABYC standards and the manufacturer's wiring diagrams, and consult a qualified marine electrician for complex systems.
