On a recent run from Charleston, S.C., to the Chesapeake Bay, a low-voltage alert flashed on the multifunction display. The 12-volt power supply was slowly dropping. The crew went to the tender, stripped its battery and charger, and used them to replace the failing battery as a temporary solution, but the smaller charger could not keep up with the big boat’s demand. As night approached, voltage slowly dropped until the system crashed.

This failure took out the chartplotters, depth, speed, radar, AIS and VHF radio, leaving the crew flying blind.

Here’s what happened, how they handled troubleshooting, and how they repaired the system to add redundancies for the future.

High Voltage

There are many excellent reasons for utilizing higher voltages in a DC system. With 24 volts rather than 12 volts, smaller wiring can be used, and electrical efficiency increases. Lighter-gauge wire also reduces weight, sometimes significantly on a larger vessel.

Unfortunately, all equipment cannot run at the higher voltages. And most, but not all, navigation electronics still require 12 volts.

Higher voltages are achieved by wiring batteries in series. If you have two 12-volt batteries, then one positive and one negative from each battery are connected. The 24-volt power is drawn from the remaining negative on one battery and positive on the other battery.

Twenty-four volts can also be created from six 4-volt batteries or a dozen 2-volt batteries. On larger boats, there will often be a combination of series and paralleled batteries. When you parallel batteries, voltage stays the same, but capacity (stored amperage) increases with each battery added. With series batteries, the amperage capacity stays the same, but the voltage multiplies with each battery added.

Today, there is a push to increase system voltages because of the increasing numbers of electric vehicles and boats. Electric cars may be utilizing 300- to 400-volt propulsion batteries, with some newer models jumping to 800-volt packs. The advantage of the higher voltage is the same for boats: Smaller wiring with less weight, faster charging and a reduction in heat. Lights and electronics that require 12 volts can still be used with a converter.

There is, however, a downside to higher voltage, and it dates back to the origins of commercial electricity, when inventors Nikola Tesla and Thomas Edison fought about whether we should use alternating current or direct current. Edison favored DC at lower voltages (110 volts DC) because it was relatively safer. Tesla said AC at higher voltages (440 volts and higher) would use smaller wiring with less line loss and heat. Edison famously arranged for a New York murderer to be killed in an AC-powered electric chair, to show how dangerous the power source is, but AC won the war because at higher voltages, it can be efficiently transmitted over much longer distances. Still, it is undeniable that higher-voltage AC comes with increased electrocution risk.

Cars have much shorter distances for the electricity to flow, so 12-volt DC was settled upon as a safe standard for energy storage and use. Boats followed suit, even with longer wire runs that carry more amperage in large cables. Higher voltages were tried; Hatteras Yachts, for instance, was a proponent of 32-volt systems.

How high can we go with our voltage and still be somewhat safe? DC danger depends on atmospheric conditions. High humidity or wetness greatly increases the shock danger. Anything over 50-volt DC has the potential to be deadly.

There is a push now to use 48 volts as a boat power source. It is right under the deadly threshold, and can be created by readily accessible 12-volt (or 4-volt) batteries in series. Several companies are making 48-volt gear such as windlasses and thrusters. Integrel Solutions, a company using a high-amperage alternator and a 48-volt battery bank, is reducing or eliminating the need for an AC generator in some cases. The 48-volt bank is used as a power tank of sorts, and voltage converters either step down the voltage for 12-volt DC equipment or step up the voltage through inverters for 120-volt AC equipment. Any 48-volt equipment can use the voltage directly, saving wiring weight and line loss.

Back to the Boat

There are several ways to set up multi-voltage boats. In our case, the manufacturer installed two 24- to 12-volt converters that pulled power from the house bank. The manufacturer also installed a 12-volt battery backup. The theory was that the voltage converters would power the electronics, and the battery would be there if both voltage converters failed. The battery could offer a buffer if the 24-volt system was taxed by a temporary high-amperage load, such as a thruster, windlass or davit. (On some systems, these heavy loads can momentarily drop the system voltage below the navigation electronics’ voltage threshold, causing dropouts and reboots.)

By installing two converters, the manufacturer might have intended for one to serve as a backup. Multifunction displays and the other transient loads on the circuit, however, produce a high current draw, and both converters were required to be on to maintain voltage. It is unlikely that both failed at once; more likely, one failed earlier, and the remaining one masked the problem but didn’t trip because only some loads were being used.

Fortunately, the captain had a headlamp and a multimeter, and started troubleshooting power to the converters. All the fuses were removed and checked for continuity with the multimeter set to resistance. That doesn’t sound too difficult, until you add pitching seas on a darkening night in a hot, tight space beneath the helm. It’s a job for someone with an iron stomach.

It didn’t take long for the captain to confirm that both converters had power coming in, but were not producing any 12-volt output. These particular units had no diagnostic lights and were buried in a location that isn’t normally inspected.

Lights Out

There is another point to note about using 24- to 12-volt converters to charge batteries: Good battery chargers use multistep charging profiles fine-tuned to the specific battery chemistry. Better ones use temperature compensation to tune the charge to the ambient temperature.

Some voltage converters supply a constant voltage of around 12.8 volts. This voltage is too low to fully recharge or float a battery, and will hasten its demise. Better voltage converters can regulate the supply voltage tailored to the batteries’ needs, and can include temperature sensors to help adjust the charging voltages based on the battery temperature. Using the wrong voltage will overcharge or undercharge a battery and can dramatically shorten its life.

There are several ways to set up multi-voltage systems. An alternative would have each system independent of the other, with each voltage having its own charging source(s). One method would be to have multiple alternators, with a smaller, 12-volt alternator for the 12-volt system and a larger, 24-volt alternator for the house bank. Supplemental charging can also come from independent solar panels or wind generators for each voltage.

Rube Goldberg Would Be Proud

Navigation is a mission-critical system. In this case, the crew had multiple tablets and phones with navigational programs, as well as a paper chartbook. Running closer to shore kept the devices within cell-service range, but risk increased with no depthsounder and shallower waters.

Fortunately, there were excellent wiring diagrams for the boat and electronics system. Interestingly, while the radar display was set up to run on 12 volts, it could be powered by up to 30 volts. After a call to the installer to confirm, the 12-volt NMEA 2000 network was unplugged, and the power for the radar display was transferred to the 24-volt bus with spare wire. This solution wasn’t pretty, but it worked. Unfortunately, NMEA networks run on 12 volts, which meant there would be no display of depth, speed or navigation data, but at least the crew could see and avoid radar targets.

Ultimately, a portable, backup 120-volt AC charger for the electronics’ 12-volt system will keep the battery charged if both the DC converters fail again. The charger can be powered from a generator, an inverter or shore power. It is sized to carry the expected loads and keep the battery topped off.

During troubleshooting, the crew also learned that additional 12-volt devices on the bus could draw more amperage than a single converter could provide. This setup is probably what led to the demise of the converters.

So, the converters themselves were upgraded. Now, each one provides double the amperage and will run one at a time, to give some warning if there is a failure, and to have a spare as a full backup.

Backups for the Backups

As boats become more complex, we’ll probably do less fixing of individual equipment underway, and more switching to backup systems. It is far more relaxing to flip a switch for the backup rather than to crawl into a dark, hot place to address a broken part.

Having an easier time on the water means understanding how things work and planning for redundancy before you get underway.  

This article was originally published in the October 2023 issue.