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Updating A Battery Switch

The four-position master switch that controls a boat’s 12-volt system has been the standard for years; it’s simple to install and reliable, but it has several drawbacks. First, it requires the operator to remember to change the position of the switch to use either battery bank one, or battery bank two, or combine the two banks to start the engine and power the 12-volt lights and electronics. Since an alternator is usually wired to one of the battery banks to charge both battery banks, the switch must be turned to the “combine” or “both” positions. Also, whichever position the switch is in, the electrical loads from the 12-volt lights, navigation instruments, and engine starter draw from the same battery bank or combined banks. The engine starter may even draw enough current to cause a momentary drop in voltage deep enough to shut down the navigation instruments.

Most new electrical panels have several master switches—one for the starting bank, one for the house bank, and one to combine or parallel the banks if either gets discharged. A battery combiner automatically connects the banks when charging, so only the master switches have to be turned on.

Upgrading master panel battery switches could be a big project until Blue Sea introduced the Dual Circuit Plus battery switch, an innovative replacement for the four-way switch. “This switch was designed to simplify battery management on any boat with two banks of batteries,” says Wayne Kelsoe, vice president of research and development at Blue Sea Systems. Dual Circuit Plus has only On, Off, and Combine Batteries positions. Instead of connecting each bank to a common output terminal like a four-way switch, this switch provides a separate output terminal for each bank. The On position turns on both the starting and house banks, but the switch keeps both circuits separate; when switched to the Combine Batteries position, both banks are joined together. This separates the house loads from the engine-starting loads so the start bank won’t be drawn down when at anchor and so navigational equipment will not be subject to any voltage drop during starting. Kelsoe says, “If one of the battery banks has a faulty battery, it should be disconnected before the banks are combined. With a four-way switch the good bank can be selected without having to combine the banks.”
Since the battery banks are isolated, unless the master switch is turned to the Combine Batteries position, only one bank can be charged by the alternator. To make the system truly hands off, a battery paralleling device must be installed. Blue Sea sells a package system with the Dual Circuit Plus Switch and a 120-amp SI-Series automatic charging relay (ACR). The SI-Series ACR is used to connect the banks whenever the alternator is producing power.

Swapping out the master switch is straightforward—these switches are the same diameter and share the same mounting screw spacing as most of the older, four-position switches. Before starting this project, to reduce the likelihood of an short circuit or arcing, disconnect the batteries by removing the positive cable from both battery banks. As you disconnect the battery banks, positively identify each cable running from battery bank one and two at the master switch and mark them (wear safety glasses when working around batteries). Then remove the cables from the back of the master switch. Note the other wires connected to the switch. Some may be connected to the common stud on the old switch and are turned on and off by the switch, like the feed for the DC main panel. Wires leading to the bilge pump or from the battery charger are usually connected to the individual battery terminals and are not switched.
Loosen the bolts that hold the old switch to the panel and remove it. Drop in the Dual Circuit Plus Switch and bolt it in place. You may need to purchase longer bolts depending on how the old switch was mounted. Attach the cables from the battery banks to the top set of studs on the new switch. In order to remain ABYC compliant, avoid exceeding four ring terminals per switch (or any) stud. You probably will have to bend the old cables a bit to align with the new stud locations. Depending on how the old switch was wired, connect the existing wires that may power the bilge pump to the top terminal of the house battery to directly connect the pump to the battery. Wires from a battery charger should also be connected to the line or battery side of the switch so it is always connected. Then connect the cable for the starting circuit to the lower stud below the starting bank. Connect the wires leading to the house panel to the stud below the house battery bank. Thread on the nuts and loosely tighten to hold the cables in place.

The easiest way to install the ACR is to wire it directly to the new master switch. Marine suppliers sell high-amperage fuse blocks and ready-made cables with preinstalled lugs that will fit the studs on both the switch and ACR. The output of your alternator determines the size of these wires and fuses. The higher the amperage output from the alternator, the heavier the wiring. Always read the installation instructions before you begin.
A house battery bank usually requires more charging than the starting battery bank (especially after a night at anchor) so the alternator is connected to the house bank. Most stock engines have the alternator output connected to the starter terminal, which is connected to the starting battery. In this case you may wish to modify the wiring so the alternator output is directed to the starting battery bank.
When charging the house bank, the ACR is installed to sense the voltage of the house battery bank. As the alternator begins to charge this bank, the voltage rises to over 13.5 volts. The ACR senses this and parallels the banks.
While your alternator regulator plays a dominant role in this process (make certain it’s appropriate for your battery type), the SI-Series ACR comes with default settings for flooded batteries. In the off chance that the house batteries are AGM and the start bank is flooded, consider using the Digital Duo Charge made by Balmar ( . On the rare occasions where this may be needed, this device has built-in adjustable voltage regulation to accommodate different charging voltages required by each battery type.

Installation is easy. Connect a properly sized fused wire from the top stud of the master switch that the alternator charges to terminal A of the ACR. Run another properly sized fused wire from the B terminal of the ARC to the other top master switch stud. ARC requires a ground to allow it to sense the voltage, so run a yellow 16 AWG f wire from ground terminal located on the bottom of the unit to the 12-volt negative bus.
An LED can be installed to indicate when the batteries are combined. Connect the negative (yellow) wire of a remote LED to the remote LED terminal on the ACRand the positive side (red) of the LED to any 12-volt source like the terminal A of the ARC. A good place to install the LED is in the main panel or close to the amp meter to signal when the ACR combines the batteries.

This upgrade was completed in a couple of hours on a 1984 single-engine Grand Banks 36 for less than $250. Depending on how a twin-engine installation is wired it, it may require a bit more work and additional wiring to complete the upgrade. Now, to get under way, simply turn on the batteries—battery management is automatic.

Includes edits from our Technical Editor, Steve D'Antonio