How to Use a 1-2-Both Battery Switch and Run Two Batteries

Open the battery compartment and figure out which battery is "1" and which is "2" by following the cables from the switch. Label each battery and the switch: for example, "1 = Start" and "2 = House." Note the type of each battery (starting, deep-cycle or dual-purpose, and flooded, AGM or lithium). Do not touch the terminals with tools while you trace cables.

Look for a small automatic charging relay (ACR), battery combiner or isolator near the batteries, and check your boat's wiring diagram if you have one. If your boat has one of these devices, it charges both batteries automatically and you mostly use the switch for on and off. If you are not sure, a marine electrician can tell you in a few minutes.

Some newer boats have a dual-circuit switch instead of a 1-2-Both dial. Blue Sea Systems describes it this way: in ON, the start battery feeds only the engine circuit and the house battery feeds only the house circuits, so running the stereo can't flatten the starting battery. A "Plus" version adds a COMBINE position to tie the batteries together for an emergency start. On its own, a dual-circuit switch only charges the house battery when you select COMBINE, which is why it is usually paired with an automatic charging relay.

Turn the switch to the starting battery (for example, 1) before you turn the key. Starting on a fresh, dedicated battery is the whole point of having two.

With a simple switch and no charging relay, the alternator charges only the selected battery. Many owners move the switch to BOTH once the engine is running so both batteries charge, then back to a single battery before they shut down. Perko notes that running on BOTH may not fully charge both batteries, so an occasional full charge with a shore charger still helps.

When you move the switch with the engine running, turn it so it never passes through OFF. On a typical dial the order is OFF, 1, BOTH, 2, so going 1 to BOTH to 2 avoids OFF. Check the order on your switch.

Owners often ask whether turning from one position to the next drops power for a moment. Many selector switches are "make before break" (the new contact closes before the old one opens), but not all are. Look for it in the switch's label or instructions; if you can't confirm it, make your changes with the engine off.

When you are anchored, drifting or fishing with the engine off, select the house battery (for example, 2) and run your accessories from it. BoatUS warns that when the batteries are tied together, the accessories drain both, and you can end up without enough power to start. Do not sit at anchor on BOTH.

When you are ready to go, switch back to the starting battery and start the engine.

Disconnecting the battery while the alternator is charging can cause a voltage spike that damages the alternator diodes, the voltage regulator or an outboard's rectifier/regulator. Some switches include an "alternator field disconnect" (AFD) feature to protect certain alternators, but it must be wired in to work, and it is not a reason to do it. Blue Sea Systems notes that an AFD only works on alternators with an external regulator, so it does nothing for most outboards or for alternators with a built-in regulator. Make "engine off, then switch off" a habit and tell your crew.

If the starting battery is too weak to crank, turn the engine key off, select BOTH (or the house battery) and try again. Once the engine is running, charge as usual and have the weak battery load tested soon: a battery that went flat once often fails again.

Turn the switch to OFF to stop small drains from flattening the batteries. Before you do, find out whether your automatic bilge pump is wired directly to the battery (it should keep working with the switch off) or through the switch. BoatUS recommends wiring bilge pumps and high-water alarms directly to the battery with their own fuse so they stay powered when the battery switches are off. If yours goes through the switch, a boat left in the water with the switch off has no automatic bilge protection: have it rewired.

With the engine off and the boat rested for a few hours, a fully charged 12-volt lead-acid battery reads about 12.6 volts across its posts. Then start the engine (in the water, or with cooling water supplied: flush muffs or a test tank for an outboard, not the built-in flush port unless your manual allows running on it), set the switch the way you normally run, and bring the engine to a fast idle. Measure each battery again: a battery that is being charged reads noticeably higher, typically somewhere around 13.5 to 14.5 volts (check your engine manual for its charging spec).

If one battery stays at resting voltage while the engine runs, it is not being charged in that switch position. Find out why before your next trip.

An automatic charging relay (ACR) or battery combiner connects the two batteries only while a charging voltage is present, then separates them when charging stops. BoatUS describes this as better than an old-style diode isolator, which drops the voltage reaching the batteries by about half a volt or more (Blue Sea Systems puts the diode drop at about 0.7 volts). On outboard boats this matters even more: Blue Sea explains that an outboard uses the same cable to charge the battery and to draw starting current, and a diode isolator blocks current flowing back the other way, so an isolator is the wrong choice there. With an ACR you no longer have to remember to move the switch to charge. It must be sized and wired to the maker's diagram, so have it installed by someone qualified if you are not confident with high-current wiring.

• Do not mix battery chemistries (flooded, AGM, gel, lithium) in the same bank or on a charger set for one type. One will be overcharged. Lithium batteries need their own charging setup.
• Secure every battery in a tray or box with a strap, and fit a non-conductive cover over each positive terminal, as Mercury recommends.
• Use the threaded terminals and replace wing nuts with nylon-insert lock nuts, which do not vibrate loose.
• Keep both negative cables connected to the common engine ground. A second battery with no proper negative connection will not start the engine or charge.
• Have aging batteries load tested; a resting voltage alone does not prove a battery is healthy.

Tape a short card next to the switch: "Start on 1. Never OFF with engine running. House on 2 when engine is off." A guest who flips the switch to OFF to "save the battery" while the engine runs can cost you an expensive repair.