Issue: January 2009

Constantly worried about draining your cranking battery by running bait wells and electronics? Consider the straightforward upgrade of adding a "house" battery.

by Tom Richardson

The whole thing started with fishing. Big menhaden had invaded the harbors near my house, and I saw an opportunity to catch my own bait. But how to keep the menhaden alive while fishing? Or, more specifically, how to keep the bait well running while drifting or at anchor without draining the boat's only battery? Clearly the solution was to add a "house" battery—and so began my spring boat project of 2008.

It's not uncommon for small boats like mine, a 21-foot Mako, to have a single cranking battery that also powers the onboard electrical system. This works fine as long as the alternator is charging the battery or if the electrical needs are so small that they don't draw many amps while the engine is shut down. Problems arise when you start adding items like bait wells, wash-down pumps and powerful deck lights, or if you simply want to play your stereo and keep your fishfinder, VHF and other electronics running while at anchor, on the mooring or at the dock.

Which Battery?
The first step was selecting a battery that would suit my electrical needs. After consulting with electrical guru John Levelle, whose company Merke Marine ( in Annapolis specializes in rigging complex electrical systems on cruising boats, I decided on a traditional wet-cell, deep-cycle battery, which Levelle says still gives you the best bang for the buck. While these batteries need more care and feeding than gel cell or AGM (absorbent glass mat) batteries, a good and properly maintained wet-cell battery will typically provide years of service at low cost.

I wanted the biggest battery my boat could accommodate, so I wouldn't have to worry about running out of juice or discharging the battery below the recommended 50-percent capacity (discharging a deep-cycle more than 50 percent shortens its life). That meant a so-called Group 31 battery, which has a capacity of 100 amp-hours. That's enough to power a pair of 5-amp livewell pumps for 5 hours without discharging the battery below 50 percent. To figure out the best battery for your boat's electrical load, visit the West Marine website and read the article on deep-cycle batteries ( Nigel Calder's Boatowner's Electrical and Mechanical Manual also contains a wealth of information on batteries, as well as a handy chart for choosing the proper battery size for your boat's electrical load.

The next step for me was figuring out where to put my two batteries—no small consideration on a 21-foot boat with limited storage space. And it's not merely a space issue; the added 60 or 70 pounds of a Group 31 battery can dramatically affect trim and performance on a boat this size. In my case it made the most sense to place both batteries inside the console. It would be a tight fit, but the weight would be concentrated in the center of the boat, where it might even improve stability and planing ability.

The two major drawbacks to this arrangement were that I would need a long run of cable from the battery to the engine, and the batteries would take a lot more abuse from pounding than they would if located farther aft. To help solve the latter problem I made a battery tray from a sheet of polyethylene and attached it to the deck beneath the console with stainless steel screws. Then I anchored the batteries to the tray with heavy-duty straps.

Cable Guy
Now my batteries were solidly installed inside the console, but I still had to make the long cable run (about 14 feet for the total circuit) from the batteries to the engine. According to Levelle, I needed a minimum of 2-gauge cable to carry enough charge to crank the engine. Quality, tinned-copper marine cable is expensive—2-gauge retails for around $6.50 per foot. Fortunately, the leads from the engine were fairly long, so I didn't need the new cables for the whole round trip. Instead, I installed a pair of heavy-duty terminal posts in an aft compartment. The posts served as the connecting point for the engine cables and the cables from the cranking battery.

If you have access to a heavy-duty lug crimper (Ancor,, makes a good one that sells for around $85), making your own battery cables is a lot cheaper than buying ready-made ones. But you have to do it right—that is, make sure all terminals are well protected from moisture by adhesive-lined heat-shrink tubing. Levelle also taught me a neat trick for snaking heavy cable through tight spaces below the deck, where you can't always reach: Lubricate by pulling it through a rag treated with silicone. It's a bit messy, but it makes the wire as slippery as an eel.

Making the Switch
There are many kinds of battery switches on the market, and a simple setup like mine didn't require anything too high-tech. That said, I decided to go with the 716 single-engine/dual-battery maintenance cluster from BEP Marine ($190; This is a "smart" switch system with a microprocessor that automatically monitors the battery voltage and allows both batteries to charge while the engine is running. When the engine is off, you don't have to switch off the cranking battery to prevent it from draining, as the system automatically isolates that battery to keep it fully charged. And if you accidentally forget to turn off the switches at the end of the day, no worries; the computer will continue to monitor and isolate both batteries to prevent them from draining. Also, if the cranking battery ever runs too low to start the engine, the "emergency parallel" switch can be used to combine the voltage of the two batteries to start the engine. Another plus: The 716 cluster design allows you to replace any individual unit without having to buy a whole new cluster.

Finding a spot to mount the switch proved tricky, given the limited space under my console and my desire to keep the switch close to the batteries, but I eventually found a spot for it on the inside of the console hatch, where it was both easy to reach and reasonably well protected from the elements.

Putting It All Together
With the batteries installed and the cables hooked up to the switch cluster, the next task was running power from the house-battery module to the fuse panel, then running the ground to a negative bus bar inside the console and, finally, back to the negative terminal of the house battery. At this stage I replaced all of the original wiring in the console, which was showing signs of corrosion around the terminals, with new Ancor tinned-copper wire and adhesive-lined heat-shrink ring terminals.

With the new batteries, cables, wires and connectors installed, I coated all exposed metal terminals, lugs and posts with plenty of dielectric grease to thwart corrosion, then crossed my fingers and turned the battery switch to "on." Trust me when I say that there is nothing quite so beautiful as hearing the sound of your engine-trim motor after a rewiring project like this.

After a season of using my new dual-battery system, I'm happy to report that it works, and I can now run my bait well for hours without having to worry about draining the cranking battery. Maybe it's time to add that wash-down pump and stereo I've been thinking about. . . .Do these boat-improvement projects never end?

A Word on Wire
It's critical to choose the proper type and size of wire or cable when tackling any kind of electrical project. According to Merke Marine's John Levelle, you should use only marine-grade tinned-copper wire, terminals and connectors for boat projects. The marine-grade tinned-copper wire is less vulnerable to damage from saltwater corrosion and vibration than automotive-grade wire, and the insulation is designed to withstand exposure to petroleum, water and sunlight. Additionally, always keep wires away from bilge water and secure them every foot or so (ABYC says 18 inches) with a wire tie or cable clamp. Finally, make sure the wire is protected against chafing where it passes through an opening in the deck or bulkhead or makes a sharp bend. Even a smooth edge can eventually chafe through wire insulation, creating a breach where moisture and salt can enter.

When buying crimping terminals and connectors, make sure they have adhesive-lined heat-shrink tubing to completely seal out moisture, and always use a professional electrician's crimping tool, which will apply even pressure to both sides of the terminal or connector sleeve. This ensures a perfect crimp—without ripping the heat-shrink tubing, a common cause of corrosion. As an added measure of protection, you should coat all exposed terminals and battery posts with dielectric grease.

The required amperage and the length of the circuit will affect which gauge wire or cable you'll need for your boat. The longer the circuit and the more amps being drawn, the thicker the wire you'll need to carry the charge. The websites and catalogs of West Marine ( and Ancor ( offer simple charts for calculating the correct wire or cable size based on these factors. When in doubt, choose wire or cable in a heavier gauge than you think you need, although it will take up more space and be more expensive.

Lastly, when purchasing wire, always buy a bit more than you measured for, in case you underestimated the length of the run or need to re-crimp a connection. You can always add an extra length of wire with a butt connector, but an unbroken run is preferred.