Twice As Nice

Just ask anyone who’s inadvertently drained their battery at anchor while listening to the stereo, found themselves unable to start the engine, and needed to call TowBoatU.S. for a jump. There are a number of benefits to adding a second battery to boats with only a single battery installed. With two batteries, the turn of a switch provides a fresh battery to start the engine and be on your way. Here’s a look at basic ­planning and installation tips for installing a second battery.

Know your type

The first choice you’ll face is deciding on the type and size of battery to install. If providing reserve cranking power when needed is the primary goal, one option is to simply mirror the existing engine starting battery.

If the goal is to power electronics and equipment while sailing or at anchor, a larger capacity, deep-cycle battery with enough amperage to start the engine is a better choice.

While simply adding a second battery in parallel to your existing battery would increase system capacity ­(effectively creating a single, bigger battery), installing a separate “house bank” provides the most benefit and greatest flexibility. This allows you to select the house bank while at anchor and isolate the engine start battery, keeping it fully charged and ready to be selected when needed. Keep in mind, though, that if the house bank battery is discharged, you should select the start battery rather than combining it with the discharged battery when attempting to start the engine.

A manual dual-battery selector switch (such as those available from Perko or Blue Sea Systems) costs around $50 and allows you to select an individual battery for use, combine both batteries (for maximum capacity when needed), or disconnect both when shutting down your 12-volt DC system. When installing a new battery selector switch, ensure that any vital equipment that may be wired directly to the existing battery (e.g., bilge pumps) continue to receive power regardless of selector switch position.

More complex switches and systems with features such as automatic switching or charging regardless of switch position are available, but this basic setup is simple to install, reliable, and easily upgraded if needed.

Location, location, location

Next up is figuring out where to install your second battery. Ideally there will be enough room to mount it beside your existing battery. If that’s not possible, look for a location that keeps battery cable runs as short as possible, is protected from the elements, is reasonably cool, and has good ventilation.

Although engine compartments are probably the worst location with regard to coolness and ventilation, manufacturers commonly locate batteries there due to reduced cable length requirements and convenience. If that’s the case on your vessel, the best location is usually low in the engine compartment but well above the normal accumulation of bilgewater. This typically keeps the batteries away from mufflers or exhaust manifolds while providing some benefit from the engine compartment ventilation system.

Other considerations include never mounting a battery directly above or below a fuel tank, fuel line connection, or fuel filter, or directly below a battery charger or inverter.

Installation basics

Section E-10 of ABYC’s (American Boat & Yacht Council) “Standards And Technical Information Reports For Small Craft” covers battery installation requirements in great detail. Although there may be some minor differences in installation requirements between the various types of batteries (e.g., wet cell, AGM), the following are some general requirements to keep in mind:

• Batteries should be installed in acid-proof, leak-proof battery trays or boxes. These containers can be purchased or homemade, as long as they meet the above requirements. Also keep in mind they do not have to be able to contain all of the battery electrolyte – only spillage from boil over or a minor leak.
• Batteries are required to be secured against movement (1 inch maximum in any direction for at least 1 minute) when exposed to a pulling force twice the weight of the battery.
• All positive terminals must be covered to prevent accidental shorting (from dropped tools, for example), a requirement that can be satisfied by using rubber terminal caps or boots, nonconductive covers, or the lid of a battery box. >> For wiring, you can purchase premade battery cables or fabricate your own, the benefit of the latter being you can customize their length as needed. When making your own cables, use marine-grade, low-resistance tinned copper wire and connectors to reduce electrical resistance and corrosion.
• Terminal connectors must provide secure mechanical and electrical connection, meaning no spring clips or alligator clamps. Wing nuts are also prohibited for battery cables and other conductors size 6 AWG and greater – they’re difficult to properly torque and can work loose due to vessel movement. If wing nuts are provided by the battery manufacturer, replace them with marine-grade locknuts.
• If making your own cables, do yourself a favor and purchase a proper crimping tool to install them. Not only will the installation look more professional than using a set of locking pliers or a hammer to pound them on, but you’ll also avoid the problems associated with loose connectors later on. Spray a corrosion inhibitor (e.g., Boeshield) into the connector prior to crimping, and add a length of adhesive-lined heat-shrink tubing afterward to also reduce corrosion and future issues.

Final tips

Stay consistent! When installing a second battery, don’t mix different types, such as having an AGM for battery one and installing a wet-cell or flooded type for the second battery.

Safety first! Batteries contain a huge amount of potentially dangerous stored energy. When installing a second battery (or any time you’re doing major work on the DC electrical system), always remove battery cables from the battery terminals.