Best Dual Battery Kits of 2026

Most kits here are a relay and a length of cable. The Add-A-Battery is a different animal: a Dual Circuit Plus battery switch and an SI-ACR charging relay in one box, wired so a single ON/OFF motion looks after both banks. On a boat that is the appeal — one thing to turn at the dock instead of a sequence to remember.

What you are paying for is the combine position. If the starting battery is flat, the switch links the banks so the house battery cranks the engine. No plain relay here does that, and it is the difference between a delayed start and a ruined morning. The rest of the time the ACR does the automatic work, combining when it senses charge and isolating at rest, and the tin-plated copper studs are made for a damp compartment rather than a dry cab.

Wiring is a bigger job than a two-cable relay — switch, ACR, grounds — though the diagram is one of the clearer ones in this set. With an IP54 housing it wants a locker rather than an open console where spray sits. Both are fair asks on the boat this was designed for.

Fixed thresholds are the quiet problem with voltage sensitive relays: the unit decides when your alternator counts as charging, and you live with the number it picked. This one lets you move the cut-in and cut-out, and after a few kits that do not, we think adjustability is worth more than the amps on the label.

The display is the other reason we keep coming back to it. It reads both batteries at once across a 4.5 to 20 volt range, so you can watch a draw overnight or catch the moment the relay joins. There is an adjustable delay of 3 to 60 seconds as well, which stops the relay chattering when voltage sits near the threshold — exactly what makes cheaper units click in and out at idle.

At 300 amps it handles work the 140-amp kits cannot: an inverter, a big winch, a lithium bank. It is rated for lithium and lead-acid both, which matters because a lead-acid-only relay will leave a LiFePO4 bank short of a full charge. Two cautions. No IP number is published, so mount it out of direct spray. And a relay that gives up under load can fail hot, so fuse the positive cable at the battery and keep the unit where you can see it.

The relay is a 2.7-inch cube, which sounds like a spec-sheet detail until you are standing in an engine bay working out where a box can live. It slots into the space beside a battery tray that the bigger units cannot use, and the IP67 sealing is the best in this set — dust-tight, and able to take a dunking.

Thirteen pieces come in the box, including long leads that reach a battery at the back of a wagon without a splice, plus the terminals and lugs to finish the job. Nothing in this kit left us hunting for a part, which is more than we can say for the relay-only option here.

What you give up is information. There is no display, no manual override and no adjustable threshold; it cuts in at a fixed point and does its job without telling you about it. If you want to watch voltages from the cab, the VEVOR or the KEMIMOTO will show you more. What this one does instead is disappear — you find out it worked when the fridge is still cold on Sunday morning.

Pioneer owners are not shopping for a universal relay. They want a second battery on the factory accessory circuit without a harness full of splices, and this is the one kit here shaped around a single machine.

You get the isolator and a digital double voltmeter, so both batteries read from the dash instead of from under the seat. It replaces part number UTV-SBI-CM, which means it takes the spot an OEM-style kit would and works with the machine's existing wiring rather than asking you to run new circuits. What we did not expect to care about is that display: a permanent readout turns is-something-wrong into a glance.

The catch is the obvious one. On a Pioneer it is the neatest option in this guide; on anything else it is not an option at all. The current rating is not published either, so if you are feeding a winch or a plow, confirm the relay can carry it before you commit. For lights, a stereo and a sprayer, it is a tidy fit.

This is the only kit in the guide with a battery in the box: an 18-amp-hour, 270-cold-cranking-amp sealed AGM, along with the bracket and hardware to hang it.

For a Maverick X3 running a stereo, a light bar and a winch, that changes the job. The kit plugs into the factory accessory plug instead of asking you to build a harness, the isolator charges the second battery while the engine runs, and the battery ships fully charged, so the install ends with a machine you can drive.

It is not cheap and it is not universal — one machine, premium money. For a loaded X3, though, we think the bundle is the sensible call: a separate battery, a bracket and an isolator add up to a similar bill and a fabrication job at the end of it. AGM is sealed and maintenance-free, but it is still lead-acid, so put it on a maintainer if the machine sits for months.

Same maker as our top pick, twice the relay. Where the 140-amp kit covers one auxiliary battery and a handful of accessories, this one is built for installs that outgrow it: more than 150 amp-hours of auxiliary capacity, a big inverter, a winch that pulls hard.

The specification that matters most is not the 300 amps — it is that KeyLine rates this for lithium and LiFePO4 as well as lead-acid. Plenty of cheap relays switch at lead-acid voltages and leave a lithium bank short of a full charge. It is IP65 and carries a three-year warranty, the longest here, which you appreciate when the relay is buried behind a panel.

Going bigger costs you space. The package measures 15.4 by 12.9 inches and weighs 8.6 pounds, so it is a box of cable and hardware as much as a relay, and it needs somewhere to live. Installation is more of a project than the 140-amp kit — the trade for the capacity. What it does not cost you is charging output: a diode-based isolator drops voltage, and KeyLine quotes almost 2 amps that a diode unit gives away.

Open the box and you will find the relay, mounting hardware and a ring terminal — and no cable. That is the whole proposition rather than a complaint: this is the least expensive way into a dual-battery setup here, and it assumes you already own a reel of heavy cable and a way to crimp it.

What you get is neat. A 140-amp VSR with IP66 sealing, a live LED readout of system voltage, and a two-year warranty that runs longer than the top pick's. At two inches square, it fits behind a panel where the bigger units will not go.

The trade is that you are doing the engineering — gauge, fuse, lugs and the run from the front battery to the back. That suits someone who has already wired a winch or a trailer. Everyone else is buying a Saturday back by choosing a complete kit.

The extra here is a digital dual voltmeter: two readings, one per battery, mounted where you can see them. On a UTV that is more useful than it sounds — you find out the auxiliary battery is low while you are still parked, not when the winch stalls.

Around it you get a 140-amp isolator in an IP65 housing sealed against mud and vibration, and fitment that covers the usual machines: Polaris, Can-Am, Cfmoto, Talon, Pioneer, Kawasaki. The relay charges the starting battery before it tops the auxiliary, which is the right order on something that has to start.

The one thing we flag on this kit is the crimps on the included positive leads. Lugs that pull off turn a twenty-minute job into an afternoon, and a two-second tug on each one before you mount anything tells you whether yours are sound. Past that the install is straightforward, and the two-reading display is the reason to pick this over a bare relay.