Battery Isolators Compared: Diode, ACR, Combiner and DC-DC Charger

Open the battery compartment and follow the heavy cable from the engine's charging output (or the battery switch) toward the batteries. Look for:

• A finned aluminum box with three or more large studs, often labeled with a "1," "2" and "A" or "ALT": a diode isolator.
• A compact sealed relay with two big studs and a small ground wire, sometimes with an LED or a manual-override knob: an ACR or combiner.
• A box labeled "DC-DC," "battery to battery" or "charger" with input and output terminals and often a Bluetooth logo: a DC-DC charger.

Photograph the label and model number, then look up that model's manual on the maker's website.

Blue Sea Systems explains that the diodes let current flow from the charging source to each battery but never backward from one battery to another. The price is a voltage drop of about 0.7 volts across the diodes. If the alternator's regulator only senses its own output, every battery behind the isolator sees about 0.7 volts less than it should, and over time the batteries are chronically undercharged.

Diode isolators can work well when the alternator or charger has a remote sense wire connected at the battery, so the regulator raises its output to make up for the drop. Without that, Blue Sea calls the performance poor. They also get warm in normal use, so they need open air around the fins.

With the engine running at a fast idle (outboard in the water or on external flush muffs, never dry; inboards in the water), set the meter to DC volts and measure:

1. Across the isolator's input (alternator) stud and battery negative.
2. Across each battery's posts.

A difference of several tenths of a volt to about a volt between the input and the batteries is the diode drop doing its job. If the batteries read noticeably low while charging (compare with the charging voltage your engine or battery maker specifies), the setup may need a regulator with remote sensing, or a switch to an ACR. Our charging-system test guide covers what normal charging voltages look like.

On an outboard, the same cable carries charging current out to the batteries and starting current back to the starter. A diode won't let current flow backward, so Blue Sea Systems warns that outboards (and inverter/chargers, for the same reason) can't be connected through an isolator. That is why most outboard boats use an ACR, a manual switch, or charging leads built into the engine.

Some larger outboards have a separate charging lead meant for a house battery. If yours does, your owner's manual shows how it's meant to be used, so you may not need an extra device at all.

An ACR is an automatic switch. Blue Sea Systems says it typically closes when one battery rises to roughly 13.2 to 13.7 volts, meaning a charger or the alternator is working, and opens again when the voltage falls to roughly 12.4 to 13.1 volts, depending on the model. While it's closed the batteries are effectively one bank, with almost no voltage drop. When it opens, your house loads can only drain the house battery.

Advantages: no voltage loss, works with the alternator and charger you already have, and works on outboards. It's the usual choice for lead-acid starting plus lead-acid house batteries.

Blue Sea points out one quirk. If a multi-stage shore charger feeds only one battery directly and the second through an ACR, the charger may drop to its lower "float" voltage, which can be too low to keep the relay closed. Meanwhile the other battery, the one powering the fridge or bilge pump, slowly runs down. The fix is to connect the charger to the battery that carries the loads, or to use a charger with a separate output for each bank.

An ACR that clicks on and off over and over is a related problem Blue Sea calls cycling. The relay closes, the big or deeply drained second bank (or a heavy load on it) pulls the voltage down faster than the charger or alternator can supply, the relay opens, the voltage recovers, and it closes again. It isn't dangerous, but the second bank never really charges and the relay wears. Blue Sea lists three usual causes: a load bigger than the charging source, a very large or very flat second bank, and sensing wires connected far from the battery being charged. Their suggestions: on a boat with a big house bank and a small start battery (many sailboats), feed the charger into the house bank; on a fishing boat that starts and stops all day, keep it on the start battery; and fix long sensing runs with closer connections or remote sensing. Note the clicking and tell your technician which battery the charger feeds.

Some ACRs also separate the banks while you crank. Blue Sea notes that starting an engine while the banks are combined can sag or spike the house voltage enough to reset a GPS or other electronics. Their "start isolation" models take a signal from the key switch's start wire and open the relay just before the starter engages. If your chartplotter or stereo reboots every time you start the engine, ask whether your combiner has this feature and whether it's connected.

A DC-DC charger is a charger in its own right. It draws from the starting battery side while the engine runs and charges the second bank with a controlled current and its own charge profile. Boating Magazine, quoting a Navico technical manager, describes it as doing the isolating job of an ACR while also converting the alternator's output into the profile a lithium battery needs.

They're also useful when a long cable run would starve the second bank, or when the two banks are different voltages: Victron suggests DC-DC converters for feeding 12-volt gear from a higher-voltage system, and Boating Magazine lists models in 12-to-12, 12-to-24 and other versions. The trade-offs are cost, heat, and a charge rate limited to the charger's rating.

This is the big one. The same Navico expert says a plain ACR won't adequately charge a lithium iron phosphate (LiFePO4) house bank, and if the lithium battery's management system shuts the battery off while the alternator is pushing current into it, the sudden disconnect can damage the alternator. BoatUS adds that outboard alternators and regulators generally aren't made for lithium recharging, so a common layout keeps the engine on a lead-acid starting battery and charges the lithium bank through a DC-DC charger.

Also plan on a lithium-compatible shore charger: Boating Magazine notes the DC-DC charger alone usually won't bring the bank to full between trips. Our lithium conversion guide covers the rest of that project.

A rough guide to start the conversation with your technician:

• Small outboard boat, two lead-acid batteries: an ACR or a properly used 1-2-BOTH switch.
• Outboard with a lithium house or trolling bank: a DC-DC charger fed from the starting battery, plus a lithium shore charger.
• Inboard or sterndrive, lead-acid banks: an ACR, or a diode isolator only if the alternator senses voltage at the battery.
• Inboard with a big house bank and upgraded alternator: an ACR, or a DC-DC charger or lithium-capable external regulator for lithium. This is design territory.
• Trolling-motor bank: usually kept completely separate and charged by its own onboard charger, so it never pulls on the starting battery.
• Older small outboard and an AGM or gel second battery: check the engine manual first. Some older outboards have simple charging systems, and AGM, gel and lithium batteries each have charging-voltage limits set by their makers. If the engine's output doesn't fit, a DC-DC charger (which makes its own controlled charge) or a shore charger at the dock is the safer route.

With the engine off and no charger on, switch on a house load (cabin lights, stereo) for a while and measure both batteries. The house battery should drop and the starting battery should stay where it was. If both fall together, the batteries are still connected: an ACR stuck closed, a switch left on BOTH, or a wiring change someone made. Note it for your technician.

While you're in there, check without touching:

• Cables and lugs are tight, clean, not discolored, and every positive stud has a boot.
• Each cable to the device has a fuse near the battery, sized for that cable (Blue Sea summarizes ABYC's distance rules; most are within 7 inches of the battery).
• A diode isolator has open air around its fins; a DC-DC charger isn't buried in a sealed locker where it overheats.
• Nothing is mounted in a gasoline engine compartment unless it is rated ignition-protected for that space.

If you decide to change from a diode isolator to an ACR or DC-DC charger, or to add lithium, give the technician your photos, model numbers, alternator size and battery types. The cables, fuses, battery switch layout and charger settings all have to be redone together. BoatUS insurance data show batteries and engine-area DC wiring behind a large share of boat fires, so this is not a place to improvise.