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A battery-powered setup gives you air conditioning without the noise of a genset. Just be careful about the details.
Owners of boats with multiple air conditioners may need to choose one unit to run on battery power alone, unless the boat has room for even more batteries, plus the charging equipment to keep the batteries healthy.

Owners of boats with multiple air conditioners may need to choose one unit to run on battery power alone, unless the boat has room for even more batteries, plus the charging equipment to keep the batteries healthy.

More people these days are asking if they can set up their boats to run air conditioning from their batteries. A few years ago, this wouldn’t have been possible, but with powerful inverters, efficient a/c and the right size battery bank, boaters are now able to enjoy cool, generator-free silence at anchor.

There are just a few details that have to be discussed first.

The Good Ol’ Days

The amount of electricity we expect our boats to store has steadily risen over time. Fifty years ago, a boat might have had a few lights and fans, a depthsounder and the need to start the engine. One or two small car batteries worked fine for this.

DC electrical systems used to run lights, fans, electronics and a few other low-amperage devices. Any larger loads were typically handled by an AC generator. Then, DC refrigeration systems proliferated, creating a need for larger battery banks. It was not uncommon for the DC refrigeration to triple the boat’s previous DC requirements.

The next big step came with powerful inverters that could provide AC power from the battery bank. This setup allowed more creature comforts, such as microwaves and TVs, to be brought aboard. It also increased the need for larger battery banks. Bow and stern thrusters brought another need.

Today, far more high-amperage electrical equipment is available and expected on boats. We also have faster-charging batteries that do not require adding water, and batteries with more usable energy (and sophisticated equipment to charge them).

Now, we’re adding another big amperage load: the ability to silently run a/c from a battery bank at anchor.

Math To Be Cool

A 10,000-Btu marine air conditioner might draw around 900 watts while running, not counting the higher start-up wattage. You can calculate amperage at different voltages using this formula: watts divided by voltage equals amperage.

If an air conditioner uses 900 watts, that would be 7.5 amps at 120-volt AC, or 75 amps at 12-volt DC. Note that to convert DC into AC with an inverter, we need to apply about a 10 percent voltage conversion loss factor. (Some inverters need more or less loss factored in, but there will be some loss.) We then need to calculate how long we want to run the air conditioning on the inverter, and make an assumption about its duty cycle. A good guess would be about 30 percent to 40 percent run time, especially if the boat is already cooled by running the air conditioning on the generator. We also need to factor in our other normal DC loads.

Let’s say we’ll want to switch to silent inverted mode between 8 p.m. and 8 a.m. Our normal, non-air-conditioning loads are mostly lights, TV, a few chargers and refrigeration. Using the formula above, our needs might average out to 25 DC amps an hour, multiplied by 12 hours for 300-amp hours. Add the air conditioning loads with inverter loss at 75 amps times 10 percent, and you get 82.5. Multiply that by 12 hours, divide it by a 40 percent duty cycle, and you’re at 396 amps.

Added together with our 300-amp hours for house loads, that 396 amps for the a/c brings us to 696 DC amps that we’ll need to have ready in storage in our batteries.

Most high-quality inverter/chargers have optional remote panels with in-depth battery monitoring capabilities. Knowing battery state and charge rates is imperative for long-lived battery banks and optimum performance of the systems. 

Most high-quality inverter/chargers have optional remote panels with in-depth battery monitoring capabilities. Knowing battery state and charge rates is imperative for long-lived battery banks and optimum performance of the systems. 

Types Of Batteries

The batteries that were traditionally used on boats were offshoots of the automotive and heavy-equipment industries. Wet-cell, lead-acid batteries have been with us since the early days of transportation and have not changed much. It was discovered early on that thin plates of lead immersed in an electrolyte (sulphuric acid) inside the batteries transferred energy quickly and worked great for start-up batteries. Thicker plates did slower energy transfers, but made the batteries much more rugged and able to carry deeper discharges without premature failure. These deep-cycle batteries made up the bulk of our house banks.

Lead-acid batteries do have a shortcoming: They shouldn’t be discharged more than 50 percent of their capacity because it shortens (sometimes dramatically) their lives. That means if you have a 1,000-amp-hour battery bank, you only have 500 amp hours that you should use. To note it on a voltmeter, a fully charged (but not charging) lead-acid battery rests at 12.6 volts with no loads present, is 50 percent discharged at 12.3 volts, and is dead at 11.9 volts.

Another fact discovered by Wilhelm Peukert in 1897 is that larger loads drain batteries faster. Since batteries are typically rated with a 20-amp load, you can expect even less capacity if you are draining your batteries at a higher rate.

A typical wet-cell lead-acid 8D battery weighs about 135 pounds and is rated around 225 amp hours. If we’re trying to store 696 amp hours, we’re going to need seven batteries. Why? Because we only want to use 50 percent of the capacity of a lead-acid battery, and we don’t want smaller batteries mixed with larger batteries in our bank.

Finding space for seven 8D batteries is a challenge on most boats, not to mention what the weight will do to the waterline.

sLithium batteries offer more usable amp hours, quick recharge times and lighter weight. Built-in battery management systems keep the batteries’ individual cells balanced and protect the batteries from overcharging. 

sLithium batteries offer more usable amp hours, quick recharge times and lighter weight. Built-in battery management systems keep the batteries’ individual cells balanced and protect the batteries from overcharging. 

The Next Generation

In a quest to design a better battery, manufacturers tried making the liquid acid into a gel by adding silica dust. These gel batteries showed improved energy density, but they could be susceptible to vibration damage in certain conditions. Improving on the concept, absorbent glass mat batteries (AGM) were introduced. These put the plates and acid in little packets with a fiberglass mesh that had better energy density. AGMs are also considered more rugged.

The downside to both of these battery chemistries is the way they’re sealed so that you do not have to check or add water to them. The design is called valve-regulated lead acid (VRLA) or sealed lead acid (SLA). As long as they are never overcharged, any evaporation that may take place gets reabsorbed back inside the battery case. Unfortunately, if the battery is overcharged and the vents release moisture, there is no way to put it back. The battery will have a loss of capacity, sometimes dramatically. This same thing can occur if a sealed wet-cell lead-acid battery is overcharged.

A Lifeline AGM 8D battery is rated for 255 amp hours. That means we would need at least six of these batteries to power our a/c and house loads. That still seems like a lot, and at 150 pounds each, we’re still talking about 900 pounds of weight to deal with and secure. At 50 percent depth of discharge, these batteries are rated for 1,000 to 1,100 cycles.

Firefly Batteries

In an effort to further improve battery design, Firefly Energy created a battery with microcell carbon foam technology. For our application, if these batteries are more deeply discharged, or if voltage gets low over a period of downtime, it’s easier to bring the batteries back to life. The downside is that the available battery sizes are limited, and they are still VRLA. They will be damaged if they are overcharged and moisture gets vented.

Firefly batteries come in a Group 31 size and (less useful for most of us) a 4-volt size. The Group 31 is rated at 116 amp hours at 20 hours with a 5.8 amp load. Note that the industry standard for rating deep-cycle batteries is 20 hours with a 5 amp load.

Firefly says that at 80 percent depth of discharge, you can expect 1,000 to 1,300 cycles. At 50 percent depth of discharge, you can expect 3,600 to 4,200 cycles. You can decide if you’d like fewer batteries and a deeper discharge, or more batteries and longer life.

If we are willing to accept a rated life similar to that of an AGM battery, then you can reduce the physical size of the bank to eight Group 31 batteries—a significant space and weight saving. At 76 pounds each, a bank of eight would weigh 608 pounds.

Large-frame, high-output alternators with remote three-step regulation can rapidly replace amperage in a deeply discharged, high-capacity battery bank. These alternators are designed for the high heat and run times necessary to replenish a depleted bank. 

Large-frame, high-output alternators with remote three-step regulation can rapidly replace amperage in a deeply discharged, high-capacity battery bank. These alternators are designed for the high heat and run times necessary to replenish a depleted bank. 

What About Lithium?

While it is certainly possible to run a/c from wet-cell or AGM batteries, the size and weight of the bank can be limiting. Here’s where lithium batteries start to make sense for these applications.

Prices are coming down rapidly, but you should expect to pay at least 2.5 times the cost of a high-quality marine AGM battery for a similarly sized, high-quality lithium battery. There are significant benefits, though.

Lithium batteries are significantly lighter for similar capacity: AGM 8D at 150 pounds versus lithium 8D at 80 pounds. While rated capacities for size are similar, you can expect a small increase with the lithium batteries. They can also recharge much quicker, and they have longer expected lives.

The biggest benefit comes from the fact that they can output much more of their rated capacity without causing them damage. Depending on the manufacturer, a typical lithium battery can be cycled down to between 10 percent and 0 percent of its capacity.

Lithium batteries also suffer less from Peukert’s law: They can supply higher loads without diminished capacity. This means it is no longer necessary to have double the batteries for the same effective capacity. An 8D lithium can be rated for 270 amp hours. We can use three to supply our 696 amp hour loads with a bank weight of 240 pounds.

Lithium batteries can still be fire hazards if they are installed incorrectly, or if they don’t have well-designed battery management systems integrated in their designs. There are two dangers.

The first danger is from the chemistry itself, which, if not carefully charged and discharged, can catch fire—and this is a fire that cannot be easily extinguished. As with most new products, it is wise to look to the established names or companies that have been making the batteries for several years, rather than buying unknown brands that may not have the research or warranty budgets to carefully design and stand behind their batteries, or understand which chemistries are going to be safer and better able to handle the stresses of a marine environment.

The second danger is from the massive discharge potential of the batteries, especially when they’re formed into a large bank. Installation of wiring and overcurrent protection must be fastidious, compliant with American Boat & Yacht Council standards, and checked frequently for loose or damaged connections.

Multiple chargers for the same banks can shorten recharge times, as long as the batteries can take the higher charge rates. Check the batteries’ specification sheet to determine the optimum charge rate.

Multiple chargers for the same banks can shorten recharge times, as long as the batteries can take the higher charge rates. Check the batteries’ specification sheet to determine the optimum charge rate.

What Goes Out Must Be Put Back

A simple car battery has to discharge enough energy to turn the motor over and start the engine. Ten seconds of cranking might only drain this battery a few amps. Once the engine is running, so is its alternator, which is sized to be able to keep up with the running loads of ignition, gauges, lights and a stereo with a little extra for charging. Plenty of airflow keeps this lightly used alternator cool.

On a boat, we have a different scenario. We’re asking the alternator to keep up with current house loads and recharge any amperage used the night before. The next morning, we want to run the engines for as short a time as possible to recharge the batteries (if we aren’t traveling that day). This calls for a larger alternator that can supply a sustained high output without overheating. A sophisticated regulator and a battery bank that can take that larger charge quickly are also necessary, especially if we’re trying to recharge 696 amps from our example above.

Most standard marine alternators are rated between 70 and 100 amps. With two engines running 100-amp alternators, the math says we’re looking at four hours of engine run time. But even that is not as rosy as it appears: There are also efficiency losses in recharging batteries that will increase the charging time.

The internal structure of lead-acid batteries limits the amount of charge they can take at one time without overheating. Larger alternators are available, maxing out around 300 amps, but there are downsides: They’re expensive, they take horsepower from the engine, they produce heat, and they need sizable cables.

Sophisticated multistep regulators with temperature sensors on the batteries and alternators are highly recommended for managing battery and alternator longevity. We should look at other alternatives as well.

While running air conditioning silently overnight on DC is possible, nothing short of being tethered to shore power can produce the kind of wattage necessary for powering multiple a/c units for the long term other than an AC generator. 

While running air conditioning silently overnight on DC is possible, nothing short of being tethered to shore power can produce the kind of wattage necessary for powering multiple a/c units for the long term other than an AC generator. 

Back To The Genset

Since we have a generator, we can also factor that in for recharging the batteries. However, a generator doesn’t charge batteries directly, but rather through an AC battery charger. That’s why we’ll want to make sure the charger is properly sized.

The good news is that we’re already going to need an inverter to run the a/c, and inverters that double as chargers are readily available. Inverter/chargers with 100-amp chargers are not uncommon. With 775 amps to replace in our bank, we may decide that a second charger also makes sense to reduce the recharge time.

We’ll likely be running the generator for our a/c loads during the day, so the longer charge times are less significant.

Easy Living

We’ve talked about the theoretical issues that have to be considered when running a/c from a battery bank. The specifics of your boat, its electrical system and its a/c will dictate which components need upgrades or rewiring. There are many variables, from your boat’s heat load to your boating location, which could require a larger or smaller bank of batteries and commensurate charging sources.

Regardless, having the ability to run the a/c at night without the rumble of the generator means that boating life can be more relaxing than ever.

This article was originally published in the September 2022 issue.

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