Installing a Second Battery

Need more juice?

If your boat came from the factory with only one battery, or with two batteries of inadequate size, consider installing a second battery bank. Boats often need two banks to have a second "reserve" bank to be sure the engine will start, or to have a large "house" bank so you can run bigger DC loads for longer time periods. If you are installing a reserve starting battery, you can usually ensure enough amps of cranking power by duplicating the type and size of your original battery. To install a "house" bank, you should probably invest in a quality deep cycle battery.

Marine Batteries

Starting batteries are similar to automotive batteries. They can supply lots of current for a short period of time, but like to be recharged almost immediately and are not tolerant of deep discharges. They are appropriate as the single battery for ski boats, runabouts, personal watercraft, and other boats with minimal DC loads where the engine is always running.

Deep–cycle batteries can endure repeated deep discharging and recharging without damage. They are appropriate for sailboats, fishing boats, and all other boats with larger DC power requirements. Any battery used to power onboard DC loads (lighting, trolling motors, inverters, etc.) should be a quality deep–cycle battery, like our SeaVolt brand.

Safety Precautions

  • Batteries contain a tremendous, and potentially dangerous, amount of stored energy. Whenever you are working on your electrical system, remove the positive battery cables from the battery terminals to eliminate the chance of a short circuit and/or possible electrocution.
  • The American Boat and Yacht Council (ABYC) publishes safety standards for circuit protection, wire sizes, and other related issues in electrical systems. Where appropriate, we have provided information on circuit breakers and wire sizes which are in accordance with these guidelines. The complete text of these standards is available in reprints from the ABYC.
  • Since the connections made in the battery circuits can conduct hundreds of amps, it is imperative that you have low resistance connections. This means having clean metal–to–metal connections, the right size terminals which are properly crimped and waterproofed, and secure mechanical fasteners.
  • For outboard engine applications we recommend replacing wing nuts on the tops of the battery with nylock nuts or hex nuts and lock washers that are tightened to at least 10 foot pounds of torque. For inboard engine applications you should use clamp–on battery terminals connected to the correct gauge of battery cable.
  • Be sure to wear safety glasses when working on batteries and have a water hose or other available method of flushing battery acid out of the eyes and off the face. Be careful of using un–shielded droplights or any open sources of heat or flame around the batteries. Be very careful with uninsulated tools that might short across the battery terminals.

Location, location and location

This credo works as well for batteries as it does for real estate. Batteries are temperature sensitive, needing a temperature that is comfortable for a person, and high temperatures dramatically shorten their life. If the proposed battery location is too hot for you, it is also too hot for your battery. This presents a dilemma for many battery installations, since you want short runs of battery cable to your engine’s starter or alternator to prevent a loss of voltage, but your battery won’t last long in a hot engine room and must be ventilated. Batteries, especially the flooded type, give off lighter–than–air explosive hydrogen gas when charging, so they must be vented from the top directly to the outside of the boat. If there is not sufficient natural ventilation you may have to add a sparkproof exhaust fan to eliminate gas buildup and to lower the temperature. Batteries, especially the flooded type, must also be easy to service.

Use quality battery cables

We recommend low–resistance, tinned copper wire in all applications. Moisture, vibration, heat, abrasion, and chemicals are major enemies of all electrical system components, and only marine rated wire (like that from Ancor Marine) is built to withstand these conditions. Be sure to use tin plated copper connectors as well, for the least electrical resistance and the best corrosion protection. We recommend double crimped terminals, with solid metal–to–metal connections and heat shrink tubing for waterproof protection.

Pre–manufactured battery cables are available in various lengths with well–constructed connectors, properly covered with adhesive–lined heat shrink and terminated with closed barrel ring terminals. However, the lengths available may not be appropriate for your installation so you may have to make your own battery cables. To do this requires a proper crimper. There is no way to shortcut this, since terminals smashed on the cable with a pair of Vise Grips or a few hammer blows will certainly fail, perhaps with catastrophic results. You must use a heavy–duty crimper designed for the purpose. Use closed–barrel terminals with holes the right diameter for the bolts to be used at the terminations. Positive and negative terminals often have different diameters. Spray a corrosion inhibitor such as Fluid Film, Lanocote, or Boeshield into the connector before crimping. Cover the joint between the cable insulation and the connector body with a three–inch section of adhesive–lined heat shrink tubing. Using a heat gun is the best way to apply heat shrink, although you can use a propane torch with a flame spreader if you are very careful. Be sure to have a fire extinguisher on hand. You can also tape the joint with several layers of quality electrician's tape and finish it with a small nylon tie where the tape ends to keep it from un–wrapping due to heat and solvents. Remember to cover positive terminals with a boot (or the battery box cover) to prevent shorting.

Battery Boxes

The batteries must be secured so there is no more than one inch of movement in any direction. A loose battery skidding about (weighing about 3/4lb per amp hour, or 150lb for a 200Ah battery) is both a physical hazard and a fire hazard. Pre–manufactured acid–proof battery boxes, available in various sizes, are the best way to safely anchor the batteries. Use more than one box if your batteries require you to make a series or parallel arrangement. Home–built battery boxes must be ruggedly constructed from marine grade plywood, covered inside and out with fiberglass cloth saturated with several coats of epoxy, and secure even in the event of a capsize. Do not mount batteries directly above or below a fuel tank, fuel line or filter, or directly below battery chargers or inverters.


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 for detailed technical papers.


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.