The best battery isolators for a second or auxiliary battery are voltage-sensing automatic relays or DC-DC chargers — like the Blue Sea Systems ML-ACR or Redarc SBI12 — that combine batteries only while the alternator is charging and isolate them the moment the engine shuts off, so accessories never drain your starter battery.
Adding a second battery for a winch, fridge, inverter, or auxiliary electronics only works safely if something keeps that battery separated from your starter battery when the engine's off. A basic solenoid, a smart voltage-sensing relay (ACR), or a full DC-DC charger each solve this differently, and the right choice depends on your alternator type, battery chemistry, and how much load you're actually running.
Continuous amperage rating that comfortably exceeds your total accessory draw, not just your alternator's peak output
Voltage-sensing (ACR) versus manual solenoid versus DC-DC charger — DC-DC is required for lithium auxiliary batteries or smart/variable-voltage alternators
Waterproof or marine-grade housing rating if the unit will be mounted under the hood or in a wet engine bay
Compatibility with your specific alternator type — older fixed-voltage alternators work with simple relays, but modern smart alternators usually need a DC-DC charger
Manual override switch so you can combine batteries on demand for emergency jump-starting if the auxiliary battery is fully charged
Our picks
Best overall
Blue Sea Systems ML-ACR Automatic Charging Relay
Blue Sea built its reputation in marine electrical systems, and the ML-ACR brings that same reliability to dual-battery vehicle setups by automatically combining batteries when it senses charging voltage and isolating them otherwise. It's a straightforward, well-documented relay rather than a full charger, so it won't boost charging for lithium batteries the way a DC-DC unit does.
Automatic voltage-sensing combine/isolate logic
Marine-grade build quality and corrosion resistance
The SBI12 monitors both batteries' voltage independently and only connects them when conditions are right, which makes it a solid middle ground between a basic solenoid and a full DC-DC charger. It's a strong pick for overlanding rigs with standard alternators, but lithium auxiliary batteries are usually better served by a dedicated DC-DC unit.
Independent dual-battery voltage monitoring
Compact design suited to tight engine bays
Reliable isolation logic for off-road and touring builds
National Luna built its name in the overlanding and expedition-vehicle world, and its intelligent solenoid is a common pairing with dual-battery trays and fridge-freezer setups on long off-grid trips. It's purpose-built for that use case rather than daily-driver electronics, so it's overkill if you just want to run a stereo amp.
Proven in remote overlanding and expedition use
Pairs naturally with fridge/freezer and winch setups
Rugged construction for off-road vibration and dust
Unlike a passive relay, the D250SE actively charges the auxiliary battery at the correct profile for its chemistry, which is essential if you're running a lithium (LiFePO4) auxiliary bank or a vehicle with a smart/variable-voltage alternator that a basic ACR can't read properly. It's more expensive and more involved to install than a simple isolator, so it's not necessary for a basic lead-acid dual-battery setup.
Proper charging profile for lithium (LiFePO4) batteries
Works correctly with smart/variable-voltage alternators
A battery isolator connects your starter battery and an auxiliary battery while the engine is running and charging voltage is present, then separates them when the engine is off, so accessories powered by the auxiliary battery can't drain the battery you need to start the vehicle.
Do I need a battery isolator if I add a second battery?
Yes, unless you're comfortable manually switching between batteries every time — without an isolator, relay, or DC-DC charger, any load on the second battery can eventually pull down your starter battery too through the shared connection, leaving you stranded.
What's the difference between an ACR, a solenoid, and a DC-DC charger?
A basic solenoid mechanically joins the batteries when the engine starts, a voltage-sensing ACR does the same thing but automatically and more intelligently based on measured voltage, and a DC-DC charger actively charges the auxiliary battery at a profile matched to its chemistry, which is required for lithium batteries and vehicles with smart alternators.
Can a battery isolator help if my car won't start with a clicking noise?
A properly isolated dual-battery system prevents that scenario by keeping your starter battery protected from auxiliary loads, but if you're already hearing a clicking no-start, the isolator itself isn't the fix — you'll need to jump-start or charge the starter battery first, then verify the isolator is functioning correctly afterward.
Do smart alternators cause problems with basic battery isolators?
Yes. Many newer vehicles use smart or variable-voltage alternators that don't hold a steady charging voltage the way older alternators do, which can confuse a basic voltage-sensing relay; in that case a DC-DC charger is the more reliable solution since it manages charging independently of the alternator's variable output.
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