The problem
Most single engine boats built in the last 40 years are equipped with two nearly identical (and undersized) marine batteries, used interchangeably for starting or house loads. Prior to starting the engine, the operator would turn the OFF–1–2–BOTH battery switch to the BOTH position to have both batteries’ full cranking power. With the engine now running, the switch would stay in the BOTH setting, and both batteries would be charged while powering to the day’s destination.

Once the engine was turned off (while anchored or under sail) the operator would (in theory) return the switch to the 1 or 2 position, and reserve a battery for starting. When it was time to crank the engine again, the battery switch would be turned to BOTH, or possibly to the reserve battery, and the engine would be started.

The problem is that this requires a lot of thought by the operator, who is trying to relax in the first place. Inevitably, the operator (or the operator’s Brother-in-Law) may leave the switch in the BOTH position, resulting in a very dead battery. Statistically, dead batteries are one of the most common reasons for calling a towing service. "I can’t crank my engine!" is right up there with "I’m out of gas!" for ruining a day on the water.

We actually experienced this dreaded condition last year, with the In-Laws aboard, of course, when sailing our new-to-us Newport 30Mk II for the first time. Sadly, we alone were responsible for leaving the switch in the wrong position, but fortunately we were able to impress our relatives by sailing smartly back into our slip. We then began to look for an idiot-proof solution to help protect us from ourselves.

The goal
Simply stated, the goal is to always be able to start your engine. One simple path to this destination is to have properly sized batteries. If your boat has lots of added devices like an electric windlass, bow thruster, downrigger or subwoofer system, your Original Equipment battery system may need an upgrade. Another battery added in parallel to your House bank may be all you need. Using a dedicated starting battery to turn over your engine, instead of a Deep Cycle battery, will better generate the burst of amps required to crank the engine. See the article in the You Can Do It section of this site called Adding a Second Battery.

Combining and separating automatically
Solving the "operator error" problem, the need to remember to flip the battery switch off of BOTH, requires another approach. One solution involves the Dual Circuit Plus battery switch, which simplifies multi-bank battery switching with a single ON/OFF switch. Here’s how the system works.
When the Switch is set to ON current flows from the start battery bank to the start circuit, and from the house battery bank to the house circuits.
When the switch is set to COMBINE BATTERIES current flows from both house and start battery banks to house and start circuits. The COMBINE BATTERIES function offers the ability to combine the two battery banks in the event of a low start battery.

Switch/Auto Charging Relay (ACR) Combo = automated battery management
The Dual Circuit Plus battery switch, combined with an Automatic Charging Relay fully automates the charging of two battery banks. Battery management is now simple. You simply turn the battery switch to the ON position when arriving on the boat, and turn it to the OFF position when leaving. You no longer have to worry about which batteries are charging or discharging. More than just a battery isolator, the ACR automatically connects batteries during the charging cycle and disconnects them under discharge.

• Closed when voltage is high (the ACR’s COMBINE voltage). When the sensed voltage is high (default setting of 13.5 volts for the CL-Series ACR) and has remained at that level for at least 30 seconds, the batteries are combined together. When combined, both battery banks are charged.
• Open when voltage is lower (the ACR’s UNDERVOLTAGE voltage). When the sensed voltage is below the COMBINED voltage level (CL default: 13.5 volts), the relay is open. The batteries are not connected and all electrical loads are on one battery while the second battery bank is isolated. When the sensed voltage drops below the UNDERVOLTAGE level (CL default: 12.7 volts) for 60 seconds, the relay opens. The house battery bank is not being charged. If the house battery bank is being discharged through loads connected to it, the start battery bank is isolated and will retain its charge.

Auto Charging Relays are useful for many functions
Automatic Charging Relays have lots of other uses in addition to isolating the Start and House batteries:

• CL7600 Current Limiting: Great for automatically charging a remote battery for a windlass or bow thruster, for combining or isolating two banks on a smaller outboard, I/O or inboard. Connect it to your high-powered stereo to prevent the subwoofer system from draining the House bank. Handles 60 continuous amps. The CL-Series ACR disconnects when the voltage drops below the charging voltage to prevent accidental discharge of an unintended bank.
• SI7610 Starting Isolation: Designed for isolating a larger House battery, and rated for 120 continuous amps. Allows temporary isolation of house loads from engine circuit during engine cranking to protect sensitive electronics.
• ML7620: New high-amperage ACR is great for preventing an inverter from discharging your house bank.. High-current 300A continuous current rating allows emergency cross-connecting of larger battery banks. Magnetic latch draws no current, and is rated for high G-forces. Specs for this new unit are still under development as we go to press. Check www.bluesea.com for the complete details on this bluewater ACR product.

• View our current selection of Batteries