Glad I can give ya some useful info here in the clean room.....
The following is from the link at the bottom. It spells it out better than I can...
Battery Switches
Old–school OFF–1–2–BOTH battery switch: Most single engine boats built in the last 30 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
Dual Circuit Plus™: A simpler alternative to isolate your Start and House circuits is available in the Dual Circuit Plus Battery Switch (Model 7808801) . In the ON position, it keeps your batteries isolated, so both can't be fully discharged by accident. In the COMBINE position, it parallels your batteries in the event of a low Starting battery. Used with the BatteryLink ACR, it automatically combines or isolates your batteries when in the ON position.
The most affordable way to purchase the Dual Circuit Plus switch and SI7610 ACR is to buy a kit that includes both, the “Add–a–Battery” Dual Circuit System, Model 8646275.
Charge Management
Battery Isolators used to be the best method of distributing charging current to multiple battery banks while assuring that they remain electrically isolated during discharge. These devices are electrical "one–way check valves" that allow current to flow to, but not from, the battery. Their disadvantage is that diodes cause a voltage drop that wastes charging energy, creates heat and causes batteries to be undercharged. Alternators with external voltage sensing can correct for the undercharging problem, but voltage drop and the heat generated remain a problem.
Automatic Charging Relays (ACR) are becoming a popular method for achieving the same goal as isolators, but they work on a different principle. Instead of using diodes to block current from flowing in both directions, ACRs use mechanical relays combined with a circuit that senses when a charging source is being applied to either battery. When a charge is being applied, the ACR closes; and when the circuit senses that the charge is no longer present, the ACR opens (after a short time delay to assure that the ACR doesn’t open due to temporary voltage sags caused by load startups, like a refrigerator turning on).
Automatic Charging Relays have lots of other uses in addition to isolating the Start and House batteries, and Blue Sea Systems now produces two relays for two kinds of jobs:
* 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. User–adjustable open and close voltage settings, for 12V systems. Includes Current Limiting feature.
* 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. Waterproof rated IP67. 12/24V systems.
* ACRs can be used for load shedding, pilot house navigation battery isolation, and charge current limiting for a remote battery. See
http://www.bluesea.com for detailed technical papers.
Conclusion
Adding another battery is a good idea if you want to add a powerful stereo subwoofer system, cabin lighting, electric windlass, wakeboard ballast pump or any of the other electrical and electronic accessories we all want. The installation can be as simple as connecting two batteries in parallel to make one larger battery bank. You can build in redundancy and failure protection with two battery banks, battery switches and charge management devices like ACRs. That way, when you drop anchor and use all your new gadgets, your engine will still start at the end of the day.
Remember that you must match battery chemistries between your banks. If your starting battery is of the lead–acid type, stick to a deep cycle battery of the same chemistry, instead of choosing a gel or AGM battery with different charging requirements.
http://www.westmarine.com/webapp/wc...r/10001/-1/10001/Batteries-Second-Battery.htm
