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Surge Protection

Both the owners of monster-hunting sportfishing boats equipped with a plethora of fish-finding and navigational equipment and of large yachts with onboard theaters that rival IMAX cinemas share more than just their passion for good times on the water. They each have a serious need for electricity.

To meet that need for electrical power, their boats have batteries—and usually lots of them. Most experts consider a battery bank to be large if it gets more than 750 amp-hours at 12 volts from three paralleled 8D-series batteries or 500 amp-hours at 24 volts from four 8D batteries.


Boaters who want to add batteries to their boats’ battery banks have many considerations. One big one is security. A commercially made battery tray or box is the ideal housing, but if you’re in a retrofit situation and can’t gut the dedicated battery area aboard your vessel,your most important consideration will be to make sure that the batteries stay put, even in the roughest conditions. American Boat and Yacht Council (ABYC) standards say that a battery shouldn’t be able to move more than one inch when a force of 90 pounds or twice the battery’s weight, whichever is less, is applied. The image of a group of three or four batteries wiggling even an inch during a storm is unsettling. So you need to do your best to ensure that those batteries don’t move at all.

If your batteries are stored on a shelf, it must have a retaining lip that will keep them from sliding out when your boat pitches or rolls. The lip also will also help retain any spilled electrolyte if a battery leak should occur. Because electrolyte contains acid, the shelf must be fabricated from an appropriate acid-resistant material such as aluminum or fiberglass. 

Another way to hold batteries in place is to clamp them down using sturdy insulated metal alloy or molded fiberglass retainers that have threaded, through-bolted rods at each end. Make sure you can get to the fasteners at both ends. If rigid-style retainers won’t work, try synthetic web straps. Again, whatever material you choose must be impervious to acid, so something like a stainless steel positive-ratcheting strap would be a good choice. It should attach to the shelf or tray with strap eyes rather than with screws piercing the strap material, as they can weaken the material and work loose over time. Securing the strap with through-bolts is an even better option.

Using the shelf-and-retainer-style approach has the added benefit of ensuring that your batteries will have adequate ventilation. If you choose to employ a battery box with a cover, you must make sure that it can vent potentially flammable gases that might build up during charging to the outside. All batteries will release a little bit of hydrogen and oxygen during the charging process. Because hydrogen is lighter than air, a battery box or locker must be ventilated at its highest point. Batteries also need to dissipate the heat that is generated during normal charging. When multiple batteries are grouped together, there should be a gap of at least a quarter inch between each of them.

Most battery manufacturers agree that the maximum number of batteries you should wire in parallel is four. It’s a must to periodically inspect these interconnecting cables for security and corrosion. When attaching them, make low-resistance connections and ensure that large-gauge cabling does not overload the terminals. No more than two-cable ring terminals should be installed on a battery post. Common lead battery posts require some form of conversion to studs that will accept a ring terminal. Look for adapters that utilize through-bolt fasteners. Bus bars and other termination points should be made of pure electrical-grade copper.

Finally, whether you design your own battery bank or a yard does it, make sure everything is clearly and permanently labeled to simplify service when the time comes. And make a diagram of the bank that shows the gear installed and the cable/wire routing, and keep a photo of it on your phone.

This article originally appeared in the March 2012 issue of Power & Motoryacht magazine.