REDARC Smart Start SBI24D24V 100A dual-sensing smart battery isolator

Overview

The SBI24D is the 24V, 100A dual-sensing Smart Start isolator. It closes when either battery exceeds 26.4V, so the start battery of a truck or commercial vehicle can be topped up from a solar regulator or AC charger on the auxiliary battery, while the alternator still charges the auxiliary battery when driving. Below 25.4V the batteries are separated. Size is 75×70×80mm, weight 200g, inrush rating 400A and standby current under 5mA.

• 12V and 24V models rated 100A or 200A continuous
• Connects at 13.2V (26.4V), separates below 12.6V (25.4V)
• D models sense both batteries for solar or AC top-up
• LED fault codes for over-voltage and contact voltage drop
• Standby current below 5mA, silver solenoid contacts
• Optional override wire for start assist and remote LED

How it works

The Smart Start SBI is a solenoid priority isolator for vehicles with two battery banks. A microprocessor watches the start battery: once the alternator has raised it to 13.2V (26.4V on 24V models) for five seconds, the solenoid closes and the auxiliary battery charges in parallel. When the start battery falls to the off threshold for ten seconds, the auxiliary battery and its loads are disconnected, so fridges and lights cannot flatten the battery needed to start the engine.

Dual-sensing D models monitor both terminals and close the solenoid when either battery exceeds 13.2V (26.4V). A solar regulator or mains charger on the auxiliary battery can then top up the start battery as well. Unlike a diode isolator, the solenoid path does not add a diode voltage drop to the charging circuit.

Key features

The 100A models (SBI12, SBI12D, SBI24, SBI24D) measure 75×70×80mm, weigh 200g and accept 400A inrush. The 200A models (SBI212, SBI212D, SBI224, SBI224D) measure 90×95×100mm, weigh 600g and accept 600A inrush, which suits heavy-duty and commercial vehicles or winching from both batteries. All draw under 5mA in standby, suppress solenoid voltage spikes and use silver contacts that REDARC rates as suitable for marine use. The SBI12KIT bundles an SBI12 with pre-terminated cables and fuses.

Installation and compatibility

Mount the isolator close to the start battery and away from direct engine heat, and fit fuses or circuit breakers at the battery end of both positive cables. REDARC's cable table for 0–3m runs gives 60A fuse and 8mm² cable without override or 100A and 21mm² with it on 100A models, and 125A with 21mm² or 200A with 35mm² on 200A models. The control terminals must never be connected, as this voids the warranty.

REDARC limits the SBI to lead-acid types (standard, calcium, gel, AGM, SLI and deep-cycle) and recommends start and auxiliary batteries of similar chemistry. The blue override wire can drive a dash LED or, through a momentary push-button, link the batteries manually to assist starting. Positive-earth vehicles are supported after contacting REDARC.

Monitoring and fault indication

A red LED shows when the solenoid is closed and both batteries are linked; it may stay lit for hours after the engine stops until the start battery drops to the off threshold. Two flashes signal over-voltage above 15.5V (31V), in which case the isolator opens to protect the other battery. Three flashes mean more than 1V has been measured across the contacts for over 30 seconds, typically excessive current or poor wiring, and the auxiliary battery is disconnected.

Questions and answers