Battery Bank Upgrade

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"I want to add a few batteries"and "I think I need new batteries; the ones I have aren't holding a charge"are phrases I hear from boat owners on a regular basis. My response often begins: "There's more to designing and managing a battery bank than adding more batteries or simply replacing what you have because they aren't working."

Battery banks are among the most neglected systems aboard cruising boats. They are often undersized for the loads they carry or oversized for the charging sources. Many battery banks are poorly secured and improperly wired, and they frequently suffer from poor ventilation.


There are essentially two types of battery banks aboard most boats: house and starting. The house bank supplies power to all of the DC loads, from lighting and pumps to entertainment-system electronics. A house bank typically is designed to supply small to moderate amounts of energy over long periods, ideally with 12 hours or more between charge cycles. When under way, while dockside, or when your generator is running, the house bank is being charged and, hopefully, keeping up with the loads. When you are at anchor, unless your generator is running or you have a solar array, you are using power from this bank to supply the vessel's power needs.

House batteries and banks should be chosen based on their ability to deliver the maximum number of amp hours during their life span-and, if it's important to you, how quickly they can be charged. Intuition may lead you to believe that the larger the house bank, the better off you are. More power is always better, right? Not so fast. Stand by; we'll revisit this equation later.

Starting battery banks, on the other hand, work quite differently. They are designed to supply Brobdingnagian quantities of energy for very brief periods. When starting a diesel engine, for example, it's possible to draw hundreds of amps, but only for a few seconds. (Your engine should start within three seconds of cranking; any more indicates a problem is afoot.) Thus, the starting batteries should be chosen based on their ability to deliver the start-up wallop: the greatest number of cold-cranking amps (CCA) for a given case size. Don't be fooled into thinking that a larger case is necessarily desirable; when it comes to CCA, bigger doesn't always mean more.

Leaving exotic chemistries aside, marine batteries generally fall into one of three categories: conventional flooded, which have free electrolyte inside that you can see when a cap is removed; absorbed glass mat (AGM), in which the liquid electrolyte is soaked up in a glass cloth or fabric, making them extremely efficient and rugged; and gel, which immobilize their electrolyte by mixing it with a fine sand called fumed silica. The latter two styles, AGM and gel, fall into a subgroup called SVRLA (sealed valve-regulated lead-acid). This simply means the batteries are designed to operate with a slight head of pressure inside the case, usually a little over 1psi. Their caps cannot and should not be removed; doing so dooms the battery to a quick death. Pressure helps the battery's chemistry work more efficiently during recharging.

The primary advantages that gel and AGM batteries have over their conventional cousins are an increased charge-acceptance rate and the fact that they require no electrolyte maintenance. There is a price to be paid for these advantages. On average, when comparing like quality, gel and AGM batteries cost more than flooded batteries and, in many cases, don't last as long.


With just a little effort, you can inspect and assess your battery bank to determine whether it utilizes the right type of battery and is correctly sized for your loads and charge sources. Begin by assessing your energy demand. Add up all of your DC loads (and loads that operate from an inverter). In many cases, it's easiest to convert all figures to amp hours (Ah), the number of amps a particular device uses in the amount of time it's operating. For instance, if your lighting load averages 15 amps and you use your lights for an average of 6 hours in a 24-hour period, your lighting load is 15 amps x 6 hours, or 90Ah. (You can use other time periods, such as 12 or 18 hours, if this represents your recharging-period goal.) Refrigeration and lighting often account for the greatest energy consumption. If you use a DC refrigerator that draws 5 amps and it runs for 30 minutes out of every 60, its consumption over 24 hours is 60Ah. Now, throw in your entertainment system, various water pumps, a watermaker, and a microwave via an inverter, and you can easily exceed 300 or 400Ah per 24-hour period.

Because of the manner in which batteries operate, the greatest number of amp hours can be derived from a bank over its life if it's cycled from 100 percent charged through 50 percent charged and back to 100 percent charged. A single cycle is from 100 percent to the start of the next charge regimen, preferably 50 percent, and then back to 100 percent (or wherever you stop charging-85 percent, 90 percent, etc., which may be the case when charging via a generator or engine alternator).

Thus, in order for you to use your bank to supply, say, 400Ah over a 24-hour period you'll need 800Ah of capacity to stay within the 50 percent discharge threshold. You will likely not recharge to 100 percent with every cycle, particularly when relying on a generator to effect the charge (a battery's resistance increases as it charges, so putting that last 10 percent back into a battery is a slow process and is unrealistic under some circumstances). Therefore, you will need to add approximately 20 percent to the amp-hour capacity calculation, putting you at about 500 usable amp hours, or a 1,000Ah bank.

I'll discuss charge sources for battery banks in an upcoming issue of The Newsletter. However, for the purposes of this discussion, I'll provide a brief overview of the requirements. There are two primary sources from which a house battery bank may be charged: a stand-alone charger (often an inverter/charger) and the engine alternator(s). When you're dockside and shorepower is available, the charger will be used to recharge and maintain the batteries. While under way, the inverter/charger may be used via the generator (if one is installed) and/or the alternator.

Charge sources are where many installations, particularly those with large house banks, go awry. In order to reasonably recharge a large house battery bank, the charge source's output should be approximately 25 percent of the bank's amp-hour capacity. Using our previously mentioned example of 1,000Ah, the alternator and/or inverter/charger's output would be approximately 250 amps. While that may sound like a lot, consider this: if you run the bank down to 50 percent (and that's our goal), then replacing 500Ah will take between three and five hours of charge time, depending on a number of factors such as battery type, regulator configuration, and temperature. Single 250-amp high-output alternators are available, and two alternators may be used, one on each engine or two on a single engine, provided they are properly regulated and electronically synchronized. Single 250-amp chargers, on the other hand, are rare, which means it may be necessary to parallel or "stack"chargers, and few charger manufacturers offer a stacking option.


The battery bank aboard PMM's test vessel had seen better days, so Marine Technical Services (MTS) of Tracys Landing, Maryland, was hired to install a new one. The original battery bank aboard the test boat was made up of conventional flooded, automotive-style Group 24 batteries, and the security for these batteries and the shelf on which they rested were inadequate at best. The shelf was made of high-density polyethylene, a material which, because of its durability and ease of installation, has become exceedingly popular aboard recreational cruising vessels. The variety of products now manufactured from this material-everything from hatches and drink holders to cutting boards and shelving-is nothing short of amazing. There are, however, some tasks for which high-density polyethylene is ill suited, and one of these is supporting heavy objects or highly loaded structures such as batteries. The original battery shelves were sagging, and the batteries were supported in trays that lacked tie-down supports.

I met with Ryan McQueeney, proprietor of Marine Technical Services (, to discuss the refit of the vessel's batteries and their overall installation. I began by impressing upon him the importance of the installation meeting all applicable ABYC guidelines. However, this was probably unnecessary, as Ryan and his crew are ABYC-certified marine electricians and are accustomed to operating under this self-imposed mandate.

Because we were looking to shorten the battery bank's required charge time, I told Ryan I had decided to go with AGM batteries, in this case Odyssey PC2150s (see Each one of these batteries is rated for 100Ah, which meant five would give us about 200 usable amp hours-more than adequate overnight support for the vessel's modest house loads of an inverter, refrigerator, lights, and so forth. Thanks to their TPPL ("thin plate pure lead") technology, Odyssey batteries are extremely energy dense and are capable of impressive charge-acceptance rates that exceed their own amp-hour capacity.

While the ABYC Standards are clear on the subject of battery storage and installation (an entire chapter, E-10, is dedicated to batteries), it must be pointed out that ABYC guidelines are recommendations, not requirements, and they often represent a minimum requirement. I believe that battery installations aboard all seagoing vessels should be completely immobilized and installed in such a way that under no circumstances, in any possible sea state the vessel might experience, will they come adrift, move about on their platform/tray or inside a box, or suffer from damage or compromised electrical connections.

Toward this end, Ryan's crew removed the test vessel's old batteries and inadequate shelves. They built new supports using a product called GPO, a pre-manufactured structural fiberglass-reinforced plastic that's ideally suited to high-load structural installations such as battery shelves. Individual purpose-made battery trays were then through-bolted into the shelf. The trays support the batteries, and, even though AGM batteries are leak-proof, they are designed to contain any spilled electrolyte (in the event that "wet" batteries are installed in the future). The support mechanism for the trays was deemed inadequate, so MTS designed new "strong backs"that traverse multiple batteries, securing them in place using a clamping screw. With this arrangement, the batteries are completely immobilized, and because there is ample space between the batteries, they can dissipate the heat that is a normal part of the charging process. Installing batteries in a tightly packed fashion inhibits air flow, invariably leading to overheating and diminished life. When and wherever possible, batteries should be kept cool and well ventilated.

The original batteries had been wired in a daisy-chain fashion. In this arrangement, the battery closest to the primary connection suffered from the greatest loading and wear, while each successive battery (farther away from this connection) cycled less and less. This led to a bank whose individual batteries suffered from uneven use and diminished performance. Ryan and I discussed this, and he agreed that he would connect the primary supply leads "across" the bank- i.e., positive to one electrical end of the bank and negative to the other electrical end of the bank. Then, the battery charger and alternator output (which will be discussed in a later Newsletter article) was also connected at opposite ends of the bank, but opposite the connections of the output cabling. With this "X"arrangement, heavy loads are buffered by the bank when a charge source is present.

With the installation complete, the test vessel now has an ample supply of DC energy for quiet, on-the-hook relaxation time-with no generator or engines running. The batteries recharge quickly, minimizing engine and genset run time, and their new home is rugged and secure and exceeds ABYC guidelines.


Steve owns and operates Steve D'Antonio Marine Consulting (, providing consulting services to boat buyers, owners, and the marine industry.