Solar Cruising

Tech Bights by Jerry Culik "Solar cruising boats tend to be long and narrow, reminiscent of the low-resistance hulls originally developed for steam launches and for the early “one lunger” engines.."

Devlin’s Solar Sal 27 design

Solar cruising boats tend to be long and narrow, reminiscent of the low-resistance hulls originally developed for steam launches and for the early “one lunger” gas engines. Unlike the high-powered planing (and now foiling) electric boats that are headlining the power boat shows, solar cruising boats operate at less than the theoretical hull speed. And solar cruisers are typically capable of traveling considerable distances—as long as there is some sunlight. For example, Joe Grez, the founder of PropEle Electric Boat Motors, completed the inaugural Salish 100 cruise in a salvaged Jet 14 hull powered by one of his EP Carry electric outboards and a couple of solar modules. Recently Doug Wade described building and operating his solarized 18-foot Nomad cabin cruiser in Small Craft Advisor #120 (Nov/Dec 2019). And during the Covid summer of 2021, David and Alex Borton completed an epic 100% solar-powered trip up the Inside Passage to Alaska in their 27-foot Sam Devlin-designed “lake boat.” Since the Canadian border was closed, they were unable to land anywhere for the twenty days it took to cruise from Bellingham to Ketchikan. “Wayward Sun” was propelled by a 4-kW Torqeedo pod drive with 21 kilowatt-hours of lithium-ion batteries—and 1,700 watts of solar panels to charge the batteries. According to Alex, they had only two clear, sunny days during their voyage, yet they managed to average 3.7 knots using whatever sunlight they got.

Just like the wind, the sun is free; and if you’re using it for power, you can never have too much of it.

Besides a slippery hull, lithium-ion (Li-ion) batteries are the common denominator in today’s solar cruisers, and they have many advantages over traditional deep-cycle lead-acid batteries. Li-ion batteries can be discharged nearly 100% without damage. They can be charged at least twice as fast as lead-acid batteries. They can accept all the power that solar modules produce; and they can accept a partial charge if the sun decides to be uncooperative. They do not need to be fully recharged or require a lead-acid battery’s current-tapering “absorption” stage that wastes solar power; and they do not need to be float-charged during off-season storage. And they are lighter—a lot lighter! While more expensive than deep-discharge AGM batteries, prices for Li-ion batteries have been decreasing year by year, and if you aren’t in a big hurry to buy one, they will become even more cost-competitive as the supply chains recover and manufacturing volumes skyrocket. Some of a Li-ion battery’s higher cost results from its sophisticated “battery management system” (BSM), which provides charge control and protection from unsafe usage conditions. And many Li-ion batteries now also include a Bluetooth connection for wireless real-time monitoring, several features that are rarely, if ever, found in a lead-acid battery.

Most of the electric outboard suppliers mentioned in my previous Tech Bights column sell proprietary Li-ion batteries designed to play nicely with their motors and controllers. For example, Torqeedo’s 3-kW Cruise outboards run on their 24V battery, while the 6-, 10- and 12-kW motors use Torqeedo’s higher-capacity 48V battery (or two 24V batteries connected in series). Both batteries can be connected in parallel to increase storage capacity. Like the batteries in electric cars, where high output current, fast charging, and heat control are critical requirements, Torqeedo designed theirs around hundreds of small, cylindrical NMC (nickel-manganese-cobalt oxide) Li-ion cells instead of “pouch” cells (like those used in computers) or higher-capacity “prismatic” cells. Torqeedo’s batteries all have internal thermal and overcurrent protection, and the packaging is rated IP67, that is, they are protected against dust, splash and even immersion.

Toqueedo Power 24

The metrics used to compare different battery technologies are based on energy density— specifically, “storage capacity per pound” and per cubic-foot—and bigger numbers are better. Torqeedo’s 24V battery has 3.5 kilowatt-hours (kWh) of storage capacity in a compact, 1.1 cubic-foot rectangular enclosure that weighs 56 pounds. The energy density works out to 63 watt-hours (Wh) per pound and 3.2 kWh per cubic foot. For comparison, the energy density of deep-cycle lead-acid batteries is only about 18 Wh per pound—or less than one-third, and around 2.9 kWh per cubic foot. One last metric is a battery’s cost per kilowatt-hour of storage capacity (that is, dollars per amp-hour rating times the voltage). At a list price of $3,000, Torqeedo’s 24V battery sells for $860 per kWh. For comparison, deep-cycle AGM batteries cost $250 to $300 per kWh. However, if you derate a lead-acid battery’s nameplate storage capacity to avoid reducing its working lifetime (typically by 50%), the difference in cost per kilowatt-hour gets a lot smaller, and lead-acid’s energy density numbers become much worse. The bottom line: if you deeply discharge a lead-acid battery frequently, the replacement costs over time can turn it into the more expensive choice.

Toqueedo Power 48

Torqeedo’s 48V battery is rated at 5 kWh of capacity, and it lists for $5,200, or $1,040 per kWh. Why is it priced so much higher than their 24V battery? This particular battery uses proprietary technology developed by BMW for their electric cars. And the “lifetime” of the 48V battery is rated at more than 3,000 recharging cycles, rather than only 800 charge-discharge cycles for the 24V battery (which is comparable to a deep-cycle lead-acid AGM battery used at 50% depth-of-discharge). The high-voltage 5-kWh battery weighs only 80 pounds—a little more than a 100Ah lead-acid battery that has only 1.2 kWh of capacity (nameplate, not derated). And because Torqeedo’s 48V battery is designed for automotive applications, it’s much more compact (4.5 kWh per cubic foot of space) and is capable of integration and monitoring using CAN communication and control bus components, features that are required in electric cars.

Unlike Torqeedo, ePropulsion designs all of their electric outboards to operate at 48 volts, which keeps the size of the power cables more manageable. And they offer three battery options, with 2, 4, and 9 kWh of storage capacity selling at $1,400, $2,300, and $4,400, respectively. And in contrast to Torqeedo, ePropulsion’s batteries are built using lithium-iron-phosphate (LFP) cells, which is considered to be a safer technology than NMC, albeit a bit heavier and slower to charge. Their mid-capacity battery weighs 117 pounds and takes up 1.8 cubic-feet of space (35 Wh per pound and 2.3 kWh per cubic-foot). Like Torqeedo, ePropulsion batteries can also be connected in parallel (up to sixteen of them, along with a daisy-chained CAN communication cable) to increase storage capacity.

EPropulsion’s lithium ion phosphate battery

In addition to proprietary batteries from the motor manufacturers, most electric outboards can also be powered by “drop-in” Li-ion batteries. BattleBorn Batteries , based in Nevada, designs and manufactures stand-alone 12V LFP batteries for solar, marine and RV applications. Each battery has its own high-voltage BSM so that multiple batteries can be connected in series, and in parallel, to build higher voltage, and higher capacity, battery banks. BattleBorn’s batteries are sealed and have IP67 rating (dust tight, resistant to water spray and brief immersion). Since they are used extensively by RVers, they are designed to take abuse; and their warranty is considered one of the best. Many more suppliers of drop-in batteries are importing and distributing batteries made in China, where battery-powered vehicle manufacturing is a major industry initiative. For marine applications, look for LFP batteries with “sealed” plastic cases and at least IP65 (dust tight, water resistant) enclosure ratings, such as those from ReLion, MillerTech, AmpereTime, Chins, SOK, and EG4’s WP models. And verify that they can be series-connected (many of the 12V drop-in batteries use BMSes that are not capable of handling the higher stacked voltages). Some LFP suppliers also sell 24V and 48V drop-in LFP batteries that can be conveniently bussed together in parallel to increase storage capacity. This ensures that the batteries are “matched” and it saves a bit of wiring. Unfortunately these higher-voltage batteries don’t seem to experience any pricing discounts, at least at the present time.

The cost per kilowatt-hour for premium, US-made, drop-in LFP batteries from Battleborn currently runs around $730 per kWh (as of August 2022). For comparison, the cost of ePropulsion’s 4-kWh battery works out to $560 per kWh, which coincidentally is equal to the cost of a deep-cycle AGM battery when derated by the usual 50% of nameplate capacity. Good news: Li-ion battery prices are decreasing with capacity and time. ePropulsion’s larger-capacity 9-kWh battery already breaks the $500 per kWh “cost barrier.” And while some of the imported LFP drop-in batteries now sell for less than $400 per kWh, it’s entirely conceivable that large-capacity Li-ion battery prices will fall below the price for deep-cycle AGM batteries, and potentially less than $200 per kWh, within a year or two as manufacturing volumes ramp up.

Relion’s 12.8V battery

To charge the batteries, on sailboats, where shadowing by sails is an issue, it’s common to use multiple smaller solar modules with individual charge controllers. And since the direction of the sun is always changing, the best controllers are capable of maximum power point tracking (MPPT). On a solar cruiser, however, shadowing should not be much of a problem, especially if the solar array is also used for shade. Therefore a single MPPT charge controller (or maybe two, for redundancy) with larger, series-connected modules can simplify the wiring and reduce the cost per watt of the panels. The KID controller from Midnite Solar, for example, can accept series-connected modules up to 150 volts; and it can be programmed to charge 12, 24, 36, and 48 volt batteries. Its maximum output current is 30 amps—or almost 1.5 kW into a 48V battery; and multiple controllers can be paralleled to increase the output. There is a “marine” version of the KID, but both models are rated IP64 (dust tight, splash resistant), are UL listed, and have a very good performance record. Victron also produces a series of charge controllers (plus inverters, batteries, and power monitoring systems) that have a good track record of use in RVs and on sailboats. The charge controllers are rated by maximum input voltage and maximum output current, and are priced accordingly. The latest generation of their small SmartSolar MPPT controllers includes a Bluetooth connection and app so that it’s very easy to monitor the system operation, and even modify the charging parameters. Both of these manufacturers have free on-line design tools to correctly match the charge controller to specific solar modules and configurations, and to the battery type and storage capacity.

Are there any safety concerns with using Li-ion batteries on boats? The Li-ion battery’s integrated BMS is designed to shut down the battery if a fault condition such as incorrect charging or a short circuit occurs. Once the problem is corrected, the battery BMS can usually be reset. Although as yet there is no specific standard for installation on boats, lithium-iron-phosphate is generally accepted as the safest of the Li-ion chemistries. And some batteries, from KiloVault, Lithionics, and SimpliPhi, are certified to UL-1973 (“Batteries for Use in Stationary, Vehicle Auxiliary Power, and Light Electric Rail Applications”). ReLion’s InSight batteries are certified to UL/ULC-2271 (“Standard for Batteries for Use in Light Electric Vehicle (LEV) Applications”). Nevertheless, if you are insuring a high value boat, it may be prudent to check with your agent to make sure there are no issues with using Li-ion batteries. While a sudden loss of power is always challenging, a Li-ion battery bank shut-down could become dangerous. Therefore, having a backup power source for critical electrical loads (e.g., navigation lights and radio) is a good idea. One solution is to add a small, “dumb” 12V AGM battery for these loads, and to keep it fully charged using the primary 24V or 48V battery bank.

“Power cruising,” when compared to sailing, can be rightly criticized as boring. Describing his initial reaction in BoatSense, Doug Logan says, “We were level, shaded, and comfortable…There seemed to be nothing to do but steer and watch the gauges…” However, the reports from those who have pursued solar-based cruising suggest that following the sun and managing your battery power can be just as challenging as “playing the wind.” Just like the wind, the sun is free; and if you’re using it for power, you can never have too much of it. And even better than that, you’ll never have to find an open gas pump for your solar cruiser. •SCA•