Exploring
For a system you depend on, the Victron Argofet 200-2AC is the best battery isolator — no voltage drop and nothing to wear out. The KeyLine 140A VSR kit is the best value and by far the most owner-tested, and the VEVOR split charge kit ships with the cable you need to finish the job.
How We Picked
We compared battery isolators on how they connect the two batteries (a voltage-sensitive relay versus a solid-state FET), continuous amp rating, voltage drop, and whether the package includes the heavy cable and fusing the install actually requires. Voltage drop matters more than people expect — an old diode isolator can cost you enough charging voltage that the second battery never reaches full. For the batteries themselves see our guides to LiFePO4 batteries and deep cycle batteries, and DC-to-DC chargers for the lithium alternative.
1. Best Overall: Victron Energy Argofet 200-2AC Battery Isolator (2 Batteries, 200A)
Victron Energy Argofet 200-2AC Battery Isolator (2 Batteries, 200A)
Victron Energy
- FET-based — essentially no voltage drop
- 200 amp rating across two batteries
- No moving contacts to wear or weld shut
- Victron build quality and documentation
- 4.4★ from 77 owners
Why we picked it: The Argofet is the isolator you fit when you would rather not think about it again. Unlike old diode isolators it loses almost no voltage, so the second battery actually reaches full charge instead of stalling half a volt short — the single most common complaint about cheap isolators. Unlike a relay it has no contacts to pit, weld, or chatter. Two hundred amps is more than most vehicle alternators can deliver, and Victron's documentation is the best in the business if you are wiring this yourself.
2. Best Value: KeyLine Chargers Dual Battery Isolator Kit 12V 140 Amp VSR
KeyLine Chargers Dual Battery Isolator Kit 12V 140 Amp VSR
KeyLine Chargers
- 140 amp voltage-sensitive relay
- Connects only once the alternator is charging
- Automatic — no switch to remember
- Simple three-wire installation
- 4.6★ from 1,133 owners
Why we picked it: A voltage-sensitive relay is the pragmatic answer for most dual battery setups, and this is the one that has been through the most hands by a wide margin. It watches system voltage and closes only when the alternator has the starting battery above about 13.3 volts, then drops out when you shut down — so the house load is physically disconnected from the battery that starts your engine. Nothing to switch, nothing to remember. At 140 amps it suits typical van and truck alternators.
3. Best Complete Kit: VEVOR Split Charge Relay Kit 12V with 6m Cable
VEVOR Split Charge Relay Kit 12V with 6m Cable
VEVOR
- Includes six metres of heavy cable
- Automatic split charge relay
- Lugs and fuse holders in the box
- Enough length for a van or box truck run
- 4.4★ from 529 owners
Why we picked it: The part people underestimate on an isolator install is the cable. Running heavy gauge from an engine bay to a house battery at the back of a van is most of the cost and all of the hassle, and buying it separately usually costs more than this whole kit. Getting the relay, six metres of cable, lugs, and fusing in one box makes this the fastest path from ordering to a working dual battery system. The relay itself is conventional VSR design — the value is in the completeness.
Comparison Table
| Pick | Isolator | Rating | Type | Best for |
|---|---|---|---|---|
| Best Overall | Victron Argofet 200-2AC | 200A, 2 batteries | Solid-state FET, no voltage drop | Best for permanent installs |
| Best Value | KeyLine 140A VSR | 140A VSR | Automatic relay | Best value for most vehicles |
| Best Complete Kit | VEVOR Split Charge Kit | Relay + 6m cable kit | Automatic split charge | Best for a first install |
What to Look For
VSR relay, FET, or DC-to-DC. A voltage-sensitive relay simply connects the batteries once charging voltage appears — cheap and effective for lead-acid. A FET isolator does the same with no voltage drop and no moving parts. A DC-to-DC charger is different again: it actively regulates the charge, which is what lithium house batteries generally need.
Lithium changes the answer. A LiFePO4 house battery can pull more current than a stock alternator is built to supply, and it wants a charge profile a relay cannot provide. If your second battery is lithium, budget for a DC-to-DC charger rather than a plain isolator.
Amp rating and cable size. Match the isolator to your alternator's output, then size the cable to the run. Long runs of undersized cable throw away charging voltage and get hot — this is where most disappointing installs go wrong, not in the isolator itself.
Fuse both ends. Any cable that connects two batteries must be fused close to each battery. A chafed unfused cable between two batteries is one of the few genuinely dangerous mistakes in a 12 volt system.
Bottom Line: Which Should You Buy?
For most people: the Victron Argofet 200-2AC. Best for permanent installs.
Best value: the KeyLine 140A VSR. Best value for most vehicles.
Also consider: the VEVOR Split Charge Kit. Best for a first install.
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Frequently Asked Questions
What does a battery isolator do?
It lets your alternator charge a second battery while the engine runs, then separates the two batteries when it stops. That way house loads such as a fridge or lights draw only from the second battery and can never leave the starting battery too flat to crank the engine.
What is the difference between a battery isolator and a VSR?
A VSR (voltage-sensitive relay) is one type of isolator — it physically connects the batteries when it senses charging voltage. Solid-state FET and diode isolators keep the batteries permanently separated and route charge through electronics instead. FET types avoid the voltage drop diode isolators suffer.
Do I need a battery isolator for a lithium battery?
Usually you need a DC-to-DC charger instead. A LiFePO4 battery can draw more current than a stock alternator is rated for and needs a specific charge profile, so a simple relay is the wrong tool and can overwork the alternator.
Where do I mount a battery isolator?
As close to the starting battery as practical, in a place that stays dry and gets some airflow. Keep the heavy cable runs short, and fit a fuse near each battery rather than only at one end.
Will a battery isolator drain my starting battery?
No — that is the point of it. A correctly wired isolator disconnects the two batteries whenever the engine is off, so house loads cannot reach the starting battery at all.
Related Guides
How We Research
Picks are based on manufacturer specifications, current Amazon availability, and patterns across verified owner reviews. Specs, capacities, and included accessories vary by listing — confirm the current details at the retailer before buying.
Note: Fuse the cable close to both batteries, disconnect the negative terminal before wiring anything, and size the cable for the full alternator output over the whole run — an undersized cable between two batteries gets hot before it ever blows a fuse.