How to Optimize Your Boat’s Power Systems
Boatyard: Power to Play
More electrical gadgets on board mean higher demands for power, but simply adding batteries is rarely the answer.
Most captains like adding more stuff to their boats, and much of that stuff is electric. Fancy stereo systems, multiple satellite TVs, wireless routers, elaborate nav-station electronics, power-hungry consumers like air conditioning and gyrostabilizers—all these things need juice, and plenty of it. But where do you get the juice, and how do you squeeze it efficiently? It takes more than just adding bigger batteries. You also need a power-management system to keep the electrons flowing in the right direction.
Don’t expect to read this column and learn how to custom-build your own power-management system. This is not one size fits all: Every boat has different appliances, different power sources and different usage. A powerboat that lives in a slip at the end of the yellow cord needs a simpler set-up than one whose skipper likes to hang on the anchor for days at a time with the air conditioner running, but hates listening to—or smelling—a genset. Some people prefer complicated systems that give them the comforts of home but demand the skills of a NASA engineer to operate, while others want a basic setup that works without issues. I strongly recommend hiring a professional for this, one who’s up to date on the technology, which is changing and improving all the time.
Do research yourself to learn the basics of power management so you don’t get hosed by an “expert” who’s really not. To understand how current flows through your boat, there are, of course, plenty of books out there, but one particularly easy-to-understand yet very deep dive on the subject can be found in Nigel Calder’s bible: Boatowners Mechanical and Electrical Manual. Every boating magazine nowadays too has covered the switch to lithium batteries again and again, so you’ll have no trouble reading up on the subject. I found many sources online for information on advanced boat electrics. I like the videos from Pacific Yacht Systems, available on YouTube or the company website. PYS owner Jeff Cote and his technicians cover a wide variety of electron-related topics and the videos are free. Not free, but worth the tuition, are two marine electrics courses from BoatHowTo.com (another Calder project). The first covers, in minute detail, DC systems, while the second adds AC and advanced topics like power generation and storage—just what you want to know before investing in new batteries and associated gear. Cost for both courses is about $400, much cheaper than buying the wrong batteries. I took both BoatHowTo courses and learned plenty from each one. Nevertheless, I’d still call in a pro to build my new power-management system.
Lithium, or Not?
Everybody’s talking about lithium batteries nowadays, how great they are, how they let us omit the genset and live off the inverter and how we’ve all gotta upgrade from lead-acid. (By “lithium” I mean lithium iron phosphate, LiFePO4, batteries.) But do you need the extra power storage lithium batteries provide? They are expensive and usually require changes in associated components, rewiring with heavier cables, etc., so the first thing is to determine if you really need more juice. Maybe new lead-acid (today that usually means AGM) batteries will do the job just as well—particularly combined with more efficient chargers and/or alternators—and save you many bucks, especially if your boat is plugged into shore power most of the time, and you spend your underway hours with the engines running and the alternators pumping out juice.
Whatever batteries you choose, install battery monitors so you can keep track of things in the DC department; a good monitor tells you voltage, current, state of charge, remaining capacity, etc. Most monitors have alarms to warn you when the battery needs attention. Some models connect via Bluetooth with your smartphone, or can hook into your NMEA 2000 system. Most monitors require installing shunts to measure amperage draw, a simple add-on. Managing your batteries via a monitor can add years to their life while optimizing your DC system.
Lithium batteries are ideal if you’re off the grid for days—they store much more power in the same space as AGMs, weigh less than AGMs on a power-for-power basis and can be discharged deeper than AGMs without damaging the battery. Some lithium users often take their batteries down to 20 percent charge; anything deeper than 50 percent will shorten the life of a lead-acid battery. A couple hours of running the engines after spending a night at anchor—typical usage when harbor-hopping around New England or the Bahamas, for example—will shoot some juice into the batteries, but maybe not enough to recharge them fully. Lithium batteries don’t mind not being fully recharged—some experts say they prefer it, and recommend not charging lithiums above 90 percent—but a constant state of partial discharge will weaken a lead-acid battery. For hardcore cruisers, lithiums are the choice.
Lithium batteries are more demanding than lead-acids. They don’t like it too hot (don’t put them in the engine room) nor too cold—although when it’s cold enough to affect a lithium battery, it’s too cold to go boating for fun. They need good ventilation. They don’t like to be overcharged, nor over-discharged. They need a battery management system (BMS): a computer brain that keeps everything copacetic, prevents overheating and disconnects the battery when something goes awry. Every lithium battery that’s approved for marine use has a built-in BMS, but some power-management pros prefer a remote BMS, especially in complex systems. A remote BMS provides more data and can be mounted where it’s easy to use, or connected to NMEA 2000. It’s an upgrade to a built-in BMS, but adds cost and complexity. Some battery builders tout their lithiums as “drop-in,” but it’s rarely that straightforward. Hire a professional to set up your lithiums, even if the swap looks simple.
Bottom line: If you’re a dayboater, AGM batteries are probably fine; if you like to cruise and anchor out, but don’t have a genset, consider lithiums. Even if you switch to lithiums for house batteries, you’ll probably want AGMs for starting and powering the thruster and windlass; they’re better than most lithiums when massive short-term current flow is required—a lithium battery’s BMS can misinterpret the sudden high demand for a short circuit and shut down the battery, which can fry your alternator in milliseconds if the engine’s running. An AGM starting battery in the circuit will protect the alternator should the BMS disconnect the lithium battery for any reason. Install a DC-to-DC charger to charge both the AGM and lithium batteries from the alternator. This practice might soon change, though: Dual-purpose starting/deep-cycle lithium batteries are coming onto the market. For example, the DL+ batteries from Dakota Lithium combine lots of cranking amps with the deep-cycle performance that lithiums provide.
Charge ‘Em Up
No matter what batteries you choose, if your boat doesn’t have an up-to-date AC multistage charger, install one; the newest models are smarter than those of just a few years ago. Most folks who switch to lithium add lots of amp-hours to their house battery bank—more power stored in a smaller package is one of lithium’s biggest advantages—and add a bigger charger to match, or sometimes more than one. Some chargers have remote control, so you can monitor and regulate them from your phone. (Is this really necessary?) Select a charger with multiple outlets so you can charge several battery banks simultaneously.
To illustrate, the 120-amp Xantrex XPLORE 120/12 charger can be connected to six battery banks, including banks with different chemistries, e.g., AGM and lithium. It’s programmed through an LED display, automatically determines which battery bank is most in need of charging, has temperature compensation and can be connected to your boat’s NMEA 2000 system for easy monitoring and configuration. Many modern chargers, including the Xantrex, can operate on both 120- and 240VAC, and 60 or 50 Hz, so if you’re planning on cruising in Europe, you’ll still be able to charge your batteries dockside—assuming you can find the appropriate converter for your shore-power cord.
Temperature compensation is important, especially with lead-acid batteries, so pick a charger that can accept temperature sensors. The temperature of the battery affects the charge voltage required at any point during the recharge cycle. A battery that’s too cold will tend to overcharge without compensation; a hot battery won’t charge to 100 percent. A temperature sensor attached to the battery, usually taped to the side, signals the charger to add or cut back the juice as necessary. This makes charging more efficient, and prevents overheating from prolonged high charge rates when the battery is hot. Alternators benefit from temperature sensors, too.
Many folks lust after lithiums because they want to install an inverter to power 120VAC appliances while underway, but don’t want the inconvenience and added maintenance of a genset. If you have serious 120V requirements, you’ll need lots of battery amps on hand: Converting 12V to 120V demands 10 times as much input as output current, plus roughly 20 percent to make up for inverter inefficiency. If you want to depend on an inverter for 120V, upgrade to lithium batteries that take up less room than AGMs, so more of them can be shoehorned into the available space. Lithiums recharge faster than lead-acid batteries; they can accept a constant recharge current until fully topped-up, whereas lead-acid batteries cut the charge current back as charge level increases. Some lithiums can recharge at 100% of their amp-hr rating, but most max out at around 50%. Check the battery specs before buying. Most inverters incorporate battery chargers, but with high-amp-hour house banks, plus starting batteries, thruster batteries, etc., you might want to install a second, and maybe even a third charger. Consult with a pro to determine how much AC capacity you need, and how to engineer your house banks and recharging system.
Maximize Your Alternator
Fast-charging a lithium battery might be fine for the battery, but it might not be good for the alternator. An alternator is happiest generating about 50 to 60 perceny of its maximum rated output. Replacing an AGM house battery with a substantially larger lithium bank—which can probably accept as much current as the alternator can generate—might cause the alternator to run near full capacity, which could lead to overheating or failure. You’ll want to upgrade to a higher-capacity alternator to match the new battery banks, and add an external regulator with a temperature sensor to cut back the output when either the alternator or the battery gets too hot. This isn’t a bad idea for any alternator—an external regulator does a better job of charge-managing than a built-in. Charging the house bank through the starting battery with a DC-to-DC charger will also regulate current to the lithium batteries, thus controlling alternator output, but that setup does cut back a bit on the charging current reaching the lithium bank.
While I was writing this column, I received a press release from Wakespeed announcing a new variant of their WS500 external regulator, the WS500 Pro Bluetooth Alternator Regulator. Like most things today, this new regulator connects wirelessly to your smartphone or tablet; once you download the appropriate app you can configure, update and monitor the alternator remotely. The non-Bluetooth WS500, virtually the same as the Pro in everything but Bluetooth, can be configured with a Windows computer and USB cable, or with its onboard DIP switches. Both regulators work with 12V, 24V and 48V systems, and are pre-programmed with charge profiles for eight battery types, including LiFePO4 lithiums.
Designing a power management system takes knowledge, experience and skill. It’s not a project for amateurs. Based on my experience, electrics cause more problems than anything else, and the more complex they get, the more gremlins they hide. My advice: Minimize the electrical equipment on your boat, and therefore the amount of juice you need while underway, then call in a pro to build an efficient, reliable system that won’t leave you powerless.
This article originally appeared in the November 2024 issue of Power & Motoryacht magazine.
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Source: https://www.powerandmotoryacht.com/maintenance/how-to-optimize-your-boats-power-systems