Dual Battery Setups

Introduction

Whenever you add accessories such as a winch, off road lights, 12V refrigerator, or a high-power stereo, it puts a strain on the trucks charging system. Many people add a higher output alternator, but there are times when a dual battery setup is needed. This page discusses using a dual battery solenoid or a dual battery isolator to add a second battery to your truck.

Dual Battery Solenoid vs Dual Battery Isolators

The primary difference between a battery isolator and a battery solenoid is how they manage the flow of electricity. A battery isolator is a solid-state device that acts like a one-way check valve; it uses electronic components (like diodes or FETs) to allow an alternator to charge multiple batteries simultaneously while keeping them electrically separated. This prevents a discharged auxiliary battery from draining the starting battery. Because they have no moving parts, isolators are highly durable, though traditional diode models can cause a slight voltage drop that may slow down charging.

In contrast, a battery solenoid is a mechanical, electromagnetic switch that physically connects or “bridges” two batteries together. When the solenoid is triggered—usually by the vehicle’s ignition—it creates a direct metal-to-metal connection, allowing the batteries to charge with zero voltage loss. While solenoids are often more affordable and efficient at delivering full voltage, they contain moving parts that can wear out over time and typically require a power source to keep the switch closed.

NOTE: YOU CANNOT use a dual battery solenoid or dual battery isolator on a 2019 or newer Ford Ranger. You need to use a DC-DC Charger. For more information check out: Adding An Auxiliary Battery To A 2019+ Newer Ford Ranger

Dual Battery Solenoid

Painless Performance

I have used a Painless Performance Product dual battery current control system in my 1983 and 1996 Ford Ranger.  The system includes a solenoid, switch, pre-terminated wire/connectors, fuse holder, fuse and mounting hardware.

Painless Performance Dual Battery Control P/N 40102 – Click For More Details & Reviews

Painless Performance Dual Battery Current Control (Waterproof) P/N 40103 – Click For More Details & Reviews

Click HERE to view the instructions for this kit.

Dual Battery Isolators

Many vehicles have multiple batteries: one dedicated for starting the engine and the other to power accessories. Without a Battery Isolator, a battery with a lower charge will draw power from a battery with a higher charge, until the batteries equalize. This problem is known as multiple battery drain and could leave you with dead batteries.

A Battery Isolator will prevent current from flowing from one battery to the other and each battery will act as an independent power source. This will ensure that the battery you dedicated for starting never gets drained.

A Battery Isolator will also allow you to simultaneously charge both independent power sources at the same time. The amount of current which flows into each battery is then determined by the battery’s own state of charge (based on voltage regulator settings).

 

Victron Energy Argofet Battery Isolator

In contrast with Argodiode battery isolators, Argofet isolators have virtually no voltage loss. Voltage drop is less than 0,02 Volt at low current and averages 0,1 Volt at higher currents. When using Argofet Battery Isolators, there is no need to also increase the output voltage of the alternator. Care should be taken however to keep cable lengths short and of sufficient cross section.

Example: When a current of 100 A flows through a cable of 50 mm² cross section (AWG 0) and 10 m length (30 ft), the voltage drop over the cable will be 0,26 Volt. Similarly, a current of 50 A through a cable of 10 mm² cross section (AWG 7) and 5 m length (15 ft) will result in a voltage drop of 0,35 Volt!

12/24 Volt auto ranging: The Argofet will automatically adjust to a 12V or 24V system.

Alternator energize input: Some alternators need DC voltage on the B+ output to start charging. Obviously, DC will be present when the alternator is directly connected to a battery. Inserting a Diode or FET splitter will however prevent any return voltage/current from the batteries to the B+, and the alternator will
not start. The new Argofet isolators have a special current limited energize input that will power the B+ when the engine run/stop switch is closed.

Installation:

1. Always disconnect the battery minus cables before making alterations to the electrical system.

2. Connect the positive output of the supplying source (alternator) to the input of the battery isolator.

3. Connect the positive connection of the battery sets to output 1, 2 and (optional) 3 respectively.

4. Connect the ‘Energize’ blade terminal to the engine run/stop switch (optional). Minimum cable cross section: 2,5 mm².

5. Connect the ‘Ground’ blade terminal to the common negative bus bar. Minimum cable cross section: 2,5 mm².

6. Connect the negative poles of the battery sets to the common negative bus bar.

The blue LED will light up when voltage is present on the input of the Argofet.

Note: I chose to show the Argofet instead of the Argodiode because they have less voltage drop and don’t generate as much heat as the Argodiode which could become a fire hazzard.

Note: I like Victron products and use their DC-DC charger in my 2021 Ford Ranger FX4.

Victron Energy Argofet Battery Isolators 200-2AC (2 Batteries 200 amp)

Keyline Chargers Battery Isolator Kit

DUAL BATTERY ISOLATOR 12V KIT – Measuring only 2.6″ x 2.6″ x 2″, it’s the perfect dual batteries isolation kit for under the hood or Battery Relocation Kit. Say goodbye to power-robbing traditional battery isolator diodes, solenoids, or battery disconnect switch 12v with our Battery Isolator 12V that delivers optimal performance without sacrificing power.

DUAL BATTERY KIT by Keyline Charger is a Voltage Sensitive Relay that charges two battery systems. It ensures that the primary battery is always charged and ready to start the vehicle by cutting in at 13.3 volts and cutting out at 12.8 volts. It’s like having two power sources with only one alternator.

RUGGED DURABILITY: The ideal choice for those who require a reliable marine battery isolator or a dual battery smart isolator for their car audio battery charger needs. The KeyLine Automatic 140 Amp Dual Battery Isolator is a durable product that can withstand extreme weather conditions and vibrations. Its IP65 certification guarantees its safety for off-road use, even in dusty and wet environments.

WHAT YOU GET: (1) 140 Amp Voltage Sensitive Relay (VSR); (1) 20 ft red battery cable; (1) 2ft black ground battery cable; (2) positive brass marine type battery terminals; (1) negative brass marine type battery terminal; tinned copper lugs & heat shrink; cable ties; easy-to-follow instructions. Easy-to-follow directions for the most novice DIYer. Power wire in, power wire out. What could be easier?

BUY WITH CONFIDENCE: Every 140 Amp Dual Battery Isolator is backed by an industry leading 12-month warranty. If you are not 100% satisfied with your purchase, simply take advantage of the No Questions Asked, 30-Day Money Back Guarantee!!

Dual Battery Isolator Kit 12V – 140 Amp Voltage Sensitive Relay (VSR) for Dual Battery Setup

Recommended Charging Cable Gauge Size

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Last Updated: April 16, 2026