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Redarc Smart Start Battery Isolator - Dual Sensing - 12 Volt - 200 Amp

Redarc Smart Start Battery Isolator - Dual Sensing - 12 Volt - 200 Amp

Item # 331-SBI212D
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Redarc Battery Charger - 331-SBI212D
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Connect this isolator in-line with 2 batteries - 1 that is charged by your alternator and another that connects to an outside source - and it will transfer power in either direction to keep both batteries charged. Shuts off if battery drops to 12.7V. Call 800-298-8924 to order Redarc battery charger part number 331-SBI212D or order online at etrailer.com. Free expert support on all Redarc products. Guaranteed Lowest Price and Fastest Shipping for Redarc Smart Start Battery Isolator - Dual Sensing - 12 Volt - 200 Amp. Battery Charger reviews from real customers.
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  • Reviews (4)
  • Q & A (12)
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Redarc Battery Charger - 331-SBI212D

  • Battery Isolators
  • Redarc
  • 12V
  • Vehicle Battery to Auxiliary Battery

Connect this isolator in-line with 2 batteries - 1 that is charged by your alternator and another that connects to an outside source - and it will transfer power in either direction to keep both batteries charged. Shuts off if battery drops to 12.7V.


  • Dual-sensing battery isolator connects in-line with 2 batteries and transfers power in either direction
    • Transfers charge from start battery to auxiliary battery once start battery gains charge from alternator
    • Pulls power from auxiliary battery that is connected to outside power source to charge start battery
  • Isolator function stops power transfer from the charging battery if its voltage drops too low
    • Prevents excessive discharge and dead battery
  • Override wire allows for emergency starts and simple fault detection
    • Attach to a switch (sold separately) to manually reverse charge and jump-start a dead start battery
    • Connect to an LED (not included) that can be mounted in your cab for easy viewing of diagnostics
  • Silver contact points ensure long life and corrosion resistance
  • Durable housing protects sensitive electronics
  • Compact design allows for versatile installation
    • Mount the isolator in a convenient location near the start battery
    • 4-Gauge cables, fuses, and connectors sold separately
  • Made in Australia


  • Application: vehicles with standard or temperature-compensating alternators
  • Maximum continuous current: 200 amps
    • Standby current: less than 5 mA
  • Output connection: 12V DC
  • Overall dimensions: 3-15/16" long x 3-3/4" wide x 3-1/2" tall
  • 2-Year warranty

This dual-sensing battery isolator can transfer charge from either your start battery or your auxiliary battery to keep both batteries in your setup charged. Turn on your vehicle, and once the alternator begins to supply power to the start battery, the Smart Start will transfer some of that power to your auxiliary battery. If your start battery is low, make sure that your auxiliary battery is connected to an outside power source, like a solar panel, and it can transfer some of its charge to the start battery. The isolator function of this system will stop the charge transfer to and from either battery if the charging battery's voltage drops too low. This ensures that both batteries are protected so that you can venture out without the fear of running into a dead battery.

Parallel Charging for Optimal Functioning

REDARC Smart Start Dual Sensing Battery Isolator Parallel Charging Setup Diagram

The Smart Start connects in-line between your start battery and the auxiliary battery. This way it can pull the charge directly from the battery instead of pulling from an outside source, like your vehicle's alternator. This method of charging is called parallel charging.

REDARC Smart Start Dual Sensing Battery Isolator Priority Charging Setup Diagram

Most other battery isolators on the market use priority charging. In a typical priority charging setup, the alternator provides charge to either the start battery or the auxiliary battery, but not to both simultaneously. This can leave both batteries underpowered. With parallel charging, the power is drawn directly from the start battery, which removes stress on the alternator because it doesn't have to manage both batteries separately. This streamline connection of the Smart Start to the start battery also prevents potentially damaging voltage surges that can occur when a priority charger switches over between batteries.

2-Way Charging

Dual-sensing technology allows this Smart Start isolator to monitor the voltage of both the start battery and the auxiliary battery. If the auxiliary battery is connected to an external power source, then the Smart Start will recognize when the auxiliary battery reaches a full charge (13.2V) and it will redirect the current to provide a charge to the start battery. This allows you to use a solar panel (sold separately) or an additional battery charger (sold separately) to charge not only the auxiliary battery, but also the start battery.

Additional Features

The blue override wire attached to the Smart Start allows for additional features, like the Start Assist function. This function allows you to jump-start a dead start battery using the auxiliary battery. Just connect the blue wire to a push-button switch (sold separately) to create a bridge of power from the auxiliary battery to your dead battery. Great for those mornings when you leave the radio on too long or forget to close the vehicle door.

You can also connect the blue wire to an LED (not included) to create an in-cab fault indicator. There is an internal LED on the unit itself that provides fault detection. This system communicates by causing the LED to flash a certain number of times to indicate errors. For example, 2 flashes signals over-voltage, and 3 flashes signals a voltage drop or excessive current draw fault. By connecting an additional LED to the blue wire, and running that up to your cab, you can more quickly assess these faults than if you had to peer into your engine compartment to see the light on the unit itself.

For assistance in wiring the Smart Start to include these optional features, please refer to the diagrams below:

Redarc Smart Start Battery Isolator Optional Wiring

Compact Design for Versatile Mounting

Mount the battery isolator in a location near your vehicle's start battery. The compact design of the unit allows you to have a good deal of flexibility when selecting a mounting location. A couple of places to consider would be in front of the radiator or behind the headlight assembly. You will need to make sure that the location you choose is not exposed directly to engine heat.

Once the unit is securely mounted, attach each battery to one of the terminals on the Smart Start. It is important to use the proper gauge wire for your particular application. If the cable length that you need is under 10', make sure that the wire gauge is 4 AWG. If you will be using between 10' and 20' of cable, you will want to select a thicker 3-gauge wire. To prevent damage from overload and short circuits, you will want to use properly rated fuses or circuit breakers (sold separately) to ensure that current is efficiently transferred.

For assistance in properly wiring the Smart Start, please refer to the diagram below:

REDARC Smart Start Dual Sensing Battery Isolator Wirisng Diagram

SBI212D REDARC Smart Start Battery Isolator - Dual Sensing - 12 Volt - 200 Amp

Installation Details 331-SBI212D Installation instructions

California residents: click here

Customer Reviews

Redarc Smart Start Battery Isolator - Dual Sensing - 12 Volt - 200 Amp - 331-SBI212D

Average Customer Rating:  5.0 out of 5 stars   (4 Customer Reviews)

Connect this isolator in-line with 2 batteries - 1 that is charged by your alternator and another that connects to an outside source - and it will transfer power in either direction to keep both batteries charged. Shuts off if battery drops to 12.7V.

- 331-SBI212D

With a flip of a switch from the drivers set, I'm able to re-charge the coach batteries from the vehicle battery via this Redarc Battery Charger while driving to the next camping adventure. 811409

- 331-SBI212D

I purchased the Redarc 331-SB1212D battery charger during the 2018 Christmas holiday season, it shipped with good efforts from etrailer. It is installed on a 2009 Chevy 5500 diesel powered cab and chassis truck , 2 on-board factory batteries in combination with add on 2 battery auxiliary bank, chassis mounted. I have extra heavy auxiliary power leads wire loom insulated routed with 350 amp Anderson connectors located at the ends , 1 lead to the front bumper , 2 leads to the rear bumper and a small 2/0 lead routed to the cab with 175amp Anderson connector for a 650 watt inverter. This application provides jumpstart service and remote power for dump trailer and 3000watt inverter power . This system has an additional provision to combine all batteries for heavy winching or sub zeros self jumpstart . My installer provided the additional switching to Redarc design specification. 734539

- 331-SBI212D

Product works as expected and I would recommend it to anybody needing a product like this. 671485

- 331-SBI212D

This bad boy had me at dual sensing, and 200 amps is exactly what I need for my beefy house battery bank. One knock might be a short ground wire, but otherwise great product. 460888


Ask the Experts about this Redarc Battery Charger

  • Recommended In-Vehicle Battery Charger for House Battery on Ford Transit
    The Redarc Smart Start Battery Isolator # 331-SBI212D referenced in your question will install between your Ford Transit's start battery and house battery. This will essentially pull power from either battery in an effort to keep both charged. It can potentially charge the house battery to 100%, but this is dependent on the start battery being completely charged as well, which likely won't be the case for the most part. This particular product isn't so much a standard battery charger, but...
    view full answer...
  • Charging Battery Bank Options from Alternator, Solar, and Shore Power
    The # 331-SBI212D is what you'd want for charging your battery bank from the solar panel and alternator power source, but when you want to charge it from shore power you'll need a different charger setup as the Smartpass can only accept DC input power. Then for a shore power charger the CTEK part # CTEK56674 would be a really great option. It has a max amperage rating of 25 amps and was intended for batteries with amp ratings between 50 and 500. It also has reconditioning modes as well...
    view full answer...
  • Redarc Auxiliary Battery Isolator for 1992 Jeep Comanche with 90 Amp Alternator
    You basically just need to go with the Redarc Smart Start Battery isolator that has at least the amount of amperage required that your alternator puts out so for a 90 amp alternator you'd want the part # 331-SBI12D.
    view full answer...
  • Can Redarc Smart Start Battery Isolator # 331-SBI212D pass 200 Amps to Deep Cycle Battery
    The Redarc Smart Start Battery Isolator # 331-SBI212D that you referenced would work well for you since you need to pull 200 amps through it from your starter battery. I attached a diagram that shows how this wires to your setup as well. This has a max continous current of 200 amps and can be used with LiFePO4 batteries.
    view full answer...
  • Recommended Fuse to Use with Redarc Smart Start Battery Isolator # 331-SBI212D
    I spoke with my contact at Redarc regarding the Smart Start Battery Isolator # 331-SBI212D referenced in your question. He stated that with the override, you will want a 200 amp fuse. Without the override you can use a 120 amp fuse. Also note that the cable size will be determine by the cable run length. I have attached the complete written instructions manual for this isolator that you can also check out.
    view full answer...
  • Isolator Recommendation for 2018 GMC Sierra 3500 with Second Battery
    You'll might want to check the RPO codes for the alternator size you have in your 2018 GMC Sierra 3500 to determine the amperage rating for the alternator so that you pick a properly rated isolator. K68-105 amp KG8-130 amp KG3-145 amp KW1-160 amp Most likely you have the highest rated alternator so you'd want to go with the Redarc Smart Start Isolator part # 331-SBI212D which is a dual-sensing 12 volt isolator rated for up to 200 amps.
    view full answer...
  • Charging Batteries in Truck Bed with Solar Panels and Alternators
    For a way to charge your service batteries with your alternator and your solar panel you'd want the CTEK D250S Dual Battery Charger # 331-SBI212D. This will give you a way to charge the batteries from both your solar panels and your alternator without killing your vehicle battery.
    view full answer...
  • What Would Allow Solar Panel Charge Kit to Charge Trailer Batteries as Well as Charge From Outlet
    You can use the solar panel charge kit # 34272628 with the battery bank of your camper and still charge the batteries with a separate charge source (vehicle alternator, power outlet) but you'll also want the Redarc # 331-SBI212D which balances the charge between the two sources and prevents any backfeeding or other issues like you mentioned. So basically you'd run your two charge circuits to the CTEK and then it would charge the batteries. See attached picture.
    view full answer...
  • What is the Difference Between Redarc Battery Isolator # 331-SBI212 and # 331-SBI212D
    The primary difference between the Redarc Smart Start Battery Isolator # 331-SBI212 and # 331-SBI212D is that the Isolator # 331-SBI212D is actually a dual-sensing battery isolator, which means it can transfer power in either direction when connected in-line with 2 batteries. When connected in this way with 2 batteries, 1 that is charged by your alternator and another that is connected to an outside source, it will transfer power in either direction to keep both batteries charged. The...
    view full answer...
  • Is Circuit Breaker Needed When Wiring Deka Battery Isolator # DW08771
    When wiring up the Deka Battery Isolator # DW08771 the main/first battery will connect directly to position one on the isolator without the need for a circuit breaker. The second battery though should be attached to the second position on the isolator via a circuit breaker that is rated appropriately for the battery. The alternator will go to the A post on the isolator. I have attached a diagram that you might find helpful as well. You should not have to worry about the second battery...
    view full answer...
  • Can Trailer Battery Be Charged from Vehicle, Solar Panel, and Wall Outlet Charger
    Basically you'd want the Redarc Dual Battery Charger # 331-SBI212D to cover the solar panel and alternator charging of the battery and then when wanting to charge the battery from shore power you'd want a charger designed for that like the # CTEK56353. You can install the quick connects of this kit to the battery and easily connect the charger when needed. This would not require the removal of the D250S either. Basically it would just be a matter of letting the trailer charger setup charge...
    view full answer...
  • Redarc Smart Start Battery Isolator Recommendation for 150 Amp/Hour Alternator
    Since the alternator is 150 amp/hours you'd want the Redarc Smart Start Battery Isolator # 331-SBI212D which is rated for 200 amps. The 100 amp kit would be too low rated for your alternator.
    view full answer...

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Info for this part was:

Edited by:
Lindsey S
Installed by:
Jeff D
Expert Research:
Jameson C
Video Edited:
Chris R
Written by:
Brian T
Updated by:
Kristina F

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