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TL;DR
- An isolator joins a second battery to the charging system while the engine runs and separates it when the engine stops, so camp loads can't drain the starting battery.
- A voltage-sensitive relay switches on voltage: Blue Sea's SI-ACR combines at 13.0 V for 90 seconds and opens at 12.75 V for 30 seconds.
- Diode isolators lose about 0.7 V, which can leave the second battery undercharged.
- Smart alternators vary between about 12.5 V and 15 V (Victron), so a relay may not stay closed.
- For lithium or a smart alternator, a DC-DC charger that sets its own voltage and current is the usual choice.
The Problem an Isolator Solves
Running a fridge from the starting battery works only until the fridge's own low-voltage cutoff stops it. Dometic's CFX3 manual, for example, tells owners to use the MEDIUM or HIGH battery-protection setting on a starter battery, which switches the fridge off at 11.2 V or 11.8 V on a 12 V supply. That protects engine starting but cuts fridge runtime short. Blue Sea Systems describes the goal of isolators and charging relays as distributing charging current to all batteries while leaving them isolated during discharge, so each battery is affected only by its own loads.
How a Voltage-Sensitive Relay Works
The most common type is the voltage-sensitive relay (VSR), also sold as an automatic charging relay (ACR) or combiner. Blue Sea Systems explains that it closes when the voltage on one battery rises to a level showing it is being charged — normally 13.2 to 13.7 V — and opens again when the voltage drops to a lower level, normally 12.4 to 13.1 V depending on the model. It needs no ignition wire. Blue Sea's SI-ACR is a representative example:
| Blue Sea SI-ACR (12 V) | Rating |
|---|---|
| Combines (connects) the batteries | At 13.6 V for 30 seconds or 13.0 V for 90 seconds |
| Opens (separates) the batteries | At 12.75 V for 30 seconds or 12.35 V for 10 seconds |
| Current rating | 120 A continuous; 210 A for 5 minutes |
| Cable size to meet the ratings | 1/0 AWG |
| Its own current draw | 175 mA combined; 15 mA open |
| Lockouts | Stays open below 9.5 V or above 16.0 V |
The SI-ACR senses both batteries, so a charge source on either side, such as a solar panel on the auxiliary battery, can close it. It also offers start isolation, which briefly separates the house loads while the engine cranks to protect electronics from voltage sags and spikes.
Diode Isolators and Their Voltage Drop
A diode isolator uses two or more silicon diodes as one-way valves: current flows from the alternator to each battery but never back from one battery to the other. Blue Sea Systems says the cost is a voltage drop of about 0.7 V across the diodes. If the alternator's regulator senses its own output rather than the battery, the battery charges at a lower voltage and ends up undercharged. For example, a regulator holding 14.4 V at the alternator would leave about 13.7 V at the battery, well below the 14.3 V absorption voltage Lifeline specifies for its AGM batteries at 77°F. Diodes also block reverse current, so Blue Sea notes that a device that both charges and draws power on one line, such as an inverter/charger, can't be connected through a diode isolator.
Why Smart Alternators Cause Problems for Relays
Many newer vehicles use engine-computer-controlled "smart" alternators. Victron says these produce a variable voltage, from about 12.5 V to 15 V, rather than the fixed 14 V or so of a regular alternator, with the widest swings in vehicles that use regenerative braking. A relay that waits for 13.0 V or more before combining may stay open, or switch in and out, while the alternator is running at the low end of that range. Victron's Orion-Tr Smart chargers handle this with separate settings: by default they start charging at 14 V, or at 13.3 V after a two-minute delay on a smart alternator, and decide the engine is off when the input stays below 13.1 V for a minute.
Isolator vs DC-DC Charger: The Real Difference
| Voltage-sensitive relay | DC-DC charger | |
|---|---|---|
| What sets the charge voltage | The alternator | The charger, set for the battery type |
| Current limit | None beyond the relay and cable ratings (120 A on the SI-ACR) | Fixed output, for example 18 A or 30 A on Victron's 12 V Orion-Tr Smart models |
| Works with a smart alternator | Only if the voltage stays above the combine level | Yes; its input range is 8–17 V on 12 V models |
| Efficiency loss | Minimal through a relay; about 0.7 V through a diode | About 13% (87% efficiency on Victron's 12/12 models) |
A relay is the simpler choice for a lead-acid or AGM second battery on a vehicle with a fixed-voltage alternator. For a lithium second battery or a smart alternator, Victron's position is that controlled charging is needed: it calls it indispensable with smart alternators, and says it protects the alternator in lithium systems, where the battery's low impedance can overload the alternator when it is connected directly.
Cables and Fuses
A relay passes the full charging current, so the cable between the batteries must carry it without much voltage loss. Blue Sea specifies 1/0 AWG cable for the SI-ACR's full 120 A rating, and Victron's Wiring Unlimited guide recommends keeping voltage drop to 2.5 percent or less, which is 0.3 V on a 12 V system. Each battery end of the cable needs a fuse sized to protect that cable. Victron's own example shows how quickly losses add up: a 5-meter run (10 meters out and back) of 16 mm² cable has about 11 mΩ of resistance, which drops about 0.39 V at 35 A.
Adding Solar Later
A relay doesn't regulate solar power. A panel needs its own charge controller wired to the auxiliary battery; with a dual-sensing relay like the SI-ACR, the batteries then combine when solar charging raises the voltage. Some DC-DC chargers build the solar controller in. REDARC's Manager30, for example, combines a 30 A alternator charger, a mains charger, an MPPT solar regulator and a battery isolator in one unit.
When an Isolator Is the Right Choice
A voltage-sensitive relay suits a lead-acid or AGM second battery on a vehicle whose alternator holds a steady charging voltage, with cable sized for the current. A DC-DC charger suits lithium batteries, smart alternators and long cable runs. The dual-battery and portable power hub covers sizing the battery itself.
How We Research This
This explainer draws on Blue Sea Systems' relay specifications and technical articles, Victron Energy's charger manual and Wiring Unlimited guide, and the battery manuals cited below. We do not fit or test this equipment ourselves; see how we rank for the site's full method.
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We research products from manufacturer specs and owner feedback; we don't test them ourselves. How we choose.
Sources
- Blue Sea Systems — Battery Isolators and Automatic Charging Relays — How diode isolators work, their ~0.7 V drop and its undercharging effect without remote sensing, why inverter/chargers can't run through an isolator, and how ACRs combine (13.2–13.7 V) and open (12.4–13.1 V).
- Blue Sea Systems — SI-ACR Automatic Charging Relay 12/24V DC 120A (7610) specifications — SI-ACR combine/open voltages and times, 120 A continuous and 210 A 5-minute ratings, 1/0 AWG cable, 175 mA/15 mA operating current, lockouts, dual sensing and start isolation.
- Victron Energy — Orion-Tr Smart DC-DC Charger Isolated manual: Introduction — Controlled charging is indispensable with smart alternators and protects the alternator in lithium systems.
- Victron Energy — Orion-Tr Smart DC-DC Charger Isolated manual: Engine shutdown detection — Smart alternators vary from 12.5 V to 15 V; default start (14 V), delayed start (13.3 V smart alternator, 2 minutes) and shutdown (13.1 V, 1 minute) levels; the 5 m / 16 mm² / 35 A cable voltage-drop example.
- Victron Energy — Orion-Tr Smart DC-DC Charger Isolated manual: Specifications — 12 V Orion-Tr Smart input range 8–17 V, 18 A and 30 A continuous outputs, 87% efficiency.
- Dometic — CFX3 Mobile Cooling Operating Manual (CFX3 25/35/45/55IM/75DZ/95DZ/100) — CFX3 advice to use MEDIUM or HIGH battery protection on a starter battery, and the 11.2 V and 11.8 V cutoffs.
- Lifeline Batteries (Concorde) — Technical Manual for Lifeline AGM batteries, Doc. 6-0101 Rev. F — Lifeline AGM absorption voltage of 14.3 V at 77°F.
- Battle Born Batteries — 100Ah 12V LiFePO4 battery (BB10012) manual — BB10012 50 A maximum charge current and 14.2–14.6 V absorption voltage.
- Victron Energy — Wiring Unlimited — 2.5% (0.3 V at 12 V) voltage-drop guideline and fusing to protect the cable.
- REDARC — The Manager30 — Manager30 combines alternator DC-DC charging, AC charging, an MPPT solar regulator and battery isolation, rated 30 A.