Know How: Battery Chargers

If you’ve decided to live on your boat, or if you spend a lot of time in marinas where shore power is readily available, you probably either have a battery charger on board or are thinking seriously about installing one.

If you’ve decided to live on your boat, or if you spend a lot of time in marinas where shore power is readily available, you probably either have a battery charger on board or are thinking seriously about installing one.

Perhaps, like a typical thrifty sailor, you’re thinking the automotive battery charger sitting on a shelf in your garage can do the job just as well as an expensive marine unit. Not so. For one thing, marine battery chargers are built to higher specifications, are ignition protected and water-resistant, and have isolated internal wiring so that stray current won’t leak into the surrounding water, eating up your zincs, nibbling away at your propeller and through-hulls, and possibly shocking anyone unlucky enough to be swimming near your boat.

Deep-cycle boat batteries also require a more sophisticated charging regimen than automotive batteries—whose job consists solely of storing enough energy to start an engine—and their needs can’t be met by a simple ferro-resonant charger of the sort sold in automotive stores. Batteries are fickle creatures that essentially have to be conned into accepting a full charge, and this calls for a multi-step “smart” charger that can monitor the battery throughout the process and tailor the charge regimen accordingly.

What’s a Smart Charger?

A basic ferro-resonant charger is a primitive beast that uses a low-frequency magnetic control system. Such chargers are heavy, bulky and slow, and generate a magnetic field that can upset adjacent electronics. They also have poor or non-existent float-charge control, which means they can easily overcharge your batteries, causing the cells to dry out. In short, it’s a dumb charger. Linear chargers also have a low-frequency transformer to reduce the input voltage to somewhat above nominal charging voltage, and electronics to control the voltage fed to the batteries. This technology is used in some multi-stage chargers, but they still tend to be bulky and inefficient—slightly less dumb.

Virtually all marine chargers nowadays are switch-mode chargers—they convert the AC input into very high-voltage AC, which is then rectified into DC. The high-frequency transformers that reduce the voltage to the level needed to charge a battery are more efficient and a fraction of the weight of the low-frequency transformers used in ferro-resonant and linear chargers, so switch-mode chargers are much lighter, more compact and, with their sophisticated electronic control systems, quite smart, though some are smarter than others.

A smart charger can sense how much current a battery can absorb at each stage of the charging cycle. There are three stages:

Bulk (fast) charge: The charger pumps out maximum amperage—10 to 20 percent of the battery’s capacity in amps, or as much as it will accept, but usually at no more than 14 to 14.8 volts—until the battery’s voltage reaches the bulk charge level, typically 14.6V. This brings the battery up to about 75 percent of capacity.

Absorption charge: The battery voltage is kept at the bulk charge setting (between 14.1 and 14.8V), but the amount of current delivered drops as the battery approaches full charge and internal resistance increases. This can take several hours.

Float phase: The battery is kept at a constant predetermined level of charge. This again prevents it from being overcharged, while compensating for losses from self-discharge and supporting DC loads like lighting, refrigeration and stereos. Voltage in the float stage varies from around 13.2 to 13.8 volts.

Some chargers also provide a fourth step, called equalization, that specifically benefits flooded lead/acid batteries. This is a controlled overcharging of the battery that forces voltage up to around 15.5V, stirring up the electrolyte to break down any lead sulfate crystals that have accumulated on the plates and are compromising the battery’s ability to hold a charge. Lead/acid batteries can be equalized once a month during the sailing season, but AGM and Gel batteries should not be equalized at all. Battery chargers with equalization or “de-sulfation” capability are marketed as 4-step chargers. Some, like the charger we installed, can do this automatically if the flooded-battery setting is selected.

Choosing a Charger

The charger you buy should be determined by the size of your battery bank and the kind of sailing you do. A rule of thumb when sizing a charger is that its output should be 10 percent to 25 percent of battery capacity, depending on your needs. If you keep your boat hooked up to shore power so that your batteries are always in float-charge mode, you can get away with a smaller charger—perhaps one of the numerous and inexpensive 6 to 10 amp chargers on the market.

Our 34-footer, which is kept on a mooring, has a 200Ah domestic bank with a pair of flooded deep-cycle batteries and a 65Ah flooded cranking battery for the engine. Leaving the starting battery out of the equation, because it is always at or near full charge anyway, this suggests that a 20-amp charger (10 percent of 200Ah) should be adequate, and it would be if we spent a lot time in a slip. But if we visit a marina during a summer cruise, our DC refrigeration is running around the clock, the stereo is playing most of the time, fans are probably running, the laptop is in use for a few hours each evening, and our batteries were probably already deeply discharged when we arrived.

If you factor in those loads and assume the boat won’t be connected to shore power for more than 12 hours, the case for a higher-output charger becomes stronger. It takes time to bring a depleted deep-cycle battery back up to full charge: if your 400Ah house bank is 50 percent discharged, it could take the best part of a day to put that 200Ah back into it with a 20-amp charger, or even longer if there are heavy ongoing DC loads. Even for our 200Ah house bank, a 40-amp charger—20 percent of capacity—is a better bet, especially considering the limited time we spend connected to shore power. I’d want the batteries—which may be almost dead flat—fully charged in under 8 hours. In this case, a 40-amp or even 50-amp charger for a 200Ah bank is not overkill. Note however that charger manufacturers recommend that you don’t exceed 25 percent of total battery capacity because too much charging current can cause batteries to overheat. If your boat has a very large total battery capacity, you may have to think about installing two chargers.

If your boat has a generator, you want to minimize its running time, so you’ll need a charger with as much output as your batteries will accept. The alternating current produced by some malfunctioning generators—and also found in some marina AC supplies—is “dirty,” i.e., instead of being a smooth sine wave the current appears as a series of jagged spikes. These surges can wreak havoc on sensitive electronics and can also affect the operation of switch-mode battery chargers, which need clean electricity. One symptom of dirty power is that the charger switches itself off repeatedly. If you’re using a generator as a source of current for your charger, make sure the latter has a power factor correction circuit (PFC) to smooth out any peaks and spikes. PFC is also a boon when dealing with the dodgy shore power in some marinas.

Many marine chargers have three outputs, letting you charge two house banks and a starting battery at the same time. The output may be split evenly between the three banks, but some chargers will sense each bank’s state of charge and tailor output to suit.

Charger selection will also be influenced by battery type. Flooded, sealed, Gel, AGM and TPPL batteries each require different charging protocols. AGM and Gel batteries must be charged at lower voltages, for instance. Many sophisticated chargers can be programmed to optimally charge any of these battery types. Some can be programmed to charge different types of battery at the same time. Some can sense battery temperature and adjust charge rates to suit, as temperature is a critical factor—the colder the battery, the higher its resistance to charging. A few chargers even have a sophisticated float mode that cuts current down even more when no loads are sensed.

The unit we chose, a 40-amp 4-stage Promariner ProNauticP charger, had all these desirable features, along with a few more that had a bearing on installation and operation. For instance, it can accept voltages from 95 to 250, an important feature if you intend to go long-distance cruising.

Whichever make of charger you choose, remember it is better to err on the side of caution and choose one a size up from the one you think you need.

Install a Charger in 10 Steps

Location, location, location.

It can be difficult finding a convenient place to install a battery charger, and the problem is magnified on a smaller boat like our Norlin 34. Luckily, there was a small area outboard of the nav station where some instrument had been removed in the distant past, and it was perfectly sized for our Promariner charger. Unfortunately, it was also above the batteries. This is generally not recommended, as the corrosive gases emitted from flooded batteries during bulk charging can damage sensitive electronics. However, the Promariner is ignition protected and its circuitry is resistant to corrosive vapors, and this was a major factor in our choice. That aside, the charger should be mounted as close as possible to the batteries to minimize voltage drop in the cables.

Watch those volts.

Voltage drop along undersized DC cables can really affect the rate at which your batteries are charged. Your installation manual should contain a table showing what gauge cable to use, depending on the charger’s distance from the batteries, and I strongly advise you not to cheat. We could probably have gotten away with 10 gauge cable, but I used 6 gauge, because I had some left over from another project. Voltage drop was less than 1 percent; it should be no more than 3 percent, and preferably much less.

Start your DC wiring at the charger and finish at the battery. Here the two positive cables and the negative cable are in place. Get yourself a proper cable-crimping tool, and make sure to put heat shrink on the terminals. I also had to run a separate ground cable from the unit’s case to the DC negative bus bar just in case there is ever a fault in the DC side of the system.

Don’t forget the fuses.

Overcurrent protection (OCP) is mandatory for a battery charger’s DC positive leads—a short circuit could be disastrous. ABYC regs state you must place the OCP within 7 inches of the positive battery terminal, but for us that wasn’t possible—there just wasn’t enough room in the cramped battery box for the bulky fuse boxes. There is however a “fudge factor” in the ABYC regs. You can place the fuses up to 6 feet from the battery so long as the cables are sheathed, so I figured that 18 inches instead of 7 inches was neither here nor there. As far as fuse size goes, it shouldn’t be greater than the ampacity of the cable—which for our 6 gauge cable, over this short distance, was 72 amps. I settled on 50 amp fuses.

Highest loads first. Now it was time to connect the DC feed cables from the charger to the batteries. I placed the +ive and -ive cables at opposite ends of the two-battery house bank in hopes of getting the most efficient charge possible.

The starter battery is located in a separate compartment a few feet away, and I found I did not have enough cable to reach, so I connected its feed to the same stud as the starte battery +ive cable on the 3-position battery switch—not ideal, but good enough.

Time for the AC wiring.

The charger installation was part of a complete electrical system makeover that included a new Blue Sea AC distribution panel. This had already been connected to the shore supply as per ABYC recommendations.

Do not use solid-wire household electrical cable. I used 12-gauge stranded tinned copper wire (sized as per the table in the instructions) to connect the charger to the 15-amp shore power breaker, crimping on spade or ring terminals. Do not ever use wire nuts on marine wiring! If you need to extend AC wiring, use butt connectors and seal them with heat shrink or liquid plastic.

Watch out for overheating. I also connected the battery temperature sensing wire to the house bank’s negative terminal. Should the battery overheat, the sensing wire will cause the charger to shut off.

And that was about that. After one final check to make sure the connections were all good, I plugged in the shore power, switched on the charger, and programmed in the battery type using the charger’s intuitive menu. Initially, the Promariner’s voltmeter showed 15 volts going into the battery bank in the absorption stage. This was a little too much, so after a quick call to tech support I was able to access the unit’s Custom menu and altered the absorption voltage to 14.8 volts.


AC voltage can kill you. Installing a marine battery charger is well within the capabilities of a savvy DIY boat owner, and there are a number of good books that cover this subject—but if you are in any doubt at all about your ability to work with AC wiring, engage a professional.


Charles Marine, charlesindustries.com

Guest, marinco.com

Mastervolt, mastervolt.com

Newmar, newmarpower.com

Promariner, promariner.com

Sterling Marine, sterling-marine.com

Victron, victronenergy.com

Xantrex, xantrex.com


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