The 10 Best 12V Battery Isolators

Wiring a second battery is where most installs stall, and the KeyLine Iso-Pro140 kit arrives with the heavy red cable, a ground lead and battery terminals in the box. The sealed relay joins the two batteries once the engine is charging and splits them when the voltage sags, so the starting battery stays ready for the morning. It suits flooded and AGM batteries. A few units have arrived dead, and coverage lasts a year.

Anyone who has needed a jump in a parking lot will appreciate the button on the WirthCo Battery Doctor 20092. Press it and the second battery teams up with the weak one to crank the engine, as long as the flat battery still holds some charge. The rest of the time it tops up the starting battery first, then the second. The manual rules out lithium and calls for a fuse on the second battery's lead.

Big alternators and winch rigs are where the VEVOR VSR005 fits, with a 300-amp rating and settings you can change. You choose the voltages where it connects and disconnects and how long it waits, and a screen shows both batteries at a glance. VEVOR says the settings work for lithium as well as lead-acid. It's a recent model with little long-term record, and the seller doesn't state a warranty.

Car audio installers have relied on the Stinger SGP32 for years to tie a second battery to a big stereo system while the key is on. Rated for 200 amps, it's a plain coil and a set of heavy contacts, with no circuit board to fail, and Stinger backs it for one year. It needs its own wire from an ignition-switched circuit, and it can run hot under a heavy load.

Through the clear cover of the JayCorp True VSR12 you can see a light that lets you know when the batteries are joined, so checking on it takes a glance. It needs only the battery cables and a ground wire, with no ignition lead, and it carries 140 amps. A separate terminal lets you link the batteries by hand. Cables are sold separately, and the seller doesn't name a warranty.

For a small add-on battery that runs lights or a cooler, the Stinger SGP38 keeps costs down. It switches on with the ignition through a trigger wire and handles 80 amps, which covers light accessory loads but not a big alternator. The contacts are silver, and Stinger gives it a one-year warranty. Some units quit within months, so mount it somewhere you can reach to swap it.

A small screen on the Nilight 90333A shows battery voltage, so you can tell whether the second battery is taking a charge without pulling out a meter. It joins the batteries when the engine is charging and splits them as the voltage drops, and Nilight's warranty covers two years. Screws and ring terminals come in the box, but no cable. Nilight says it isn't for lithium batteries.

Trucks and boats running on 24 volts can use the NEWISHTOOL C60, which also works on 12-volt systems and picks the right mode by itself. Two red readouts show each battery's voltage, and an emergency button joins the batteries when you need a jump. The finned metal case helps shed heat. Its wiring instructions are vague, so sketch your own diagram before you start or hand the job to an installer.

Boat owners who want marine-grade switching can turn to the Blue Sea ML-ACR 7622, which joins batteries once charging voltage holds steady and splits them as it falls. It latches in place, so it draws almost nothing while connected, and both a knob on the unit and a dash switch can force it on or off. It's rated for 500 amps, and the very thick cable that rating calls for adds real cost.

The POWO BG006 covers the basics at a low price. It links the batteries when the engine charges and splits them when voltage falls, while a red readout shows where things stand. A button flips it to manual, which helps when you want to borrow from the second battery for a jump, and it cuts out if voltage runs very low or high. Coverage is only a 30-day replacement.