The owner of a 46-foot power catamaran called after finally getting back to the boat from an extended delay of six weeks. The AC shore power circuit breaker was tripped at the marina dock, and the boat’s DC voltmeter showed 7 volts. The boat was dark.
After the dock circuit breaker was turned back on and AC power was reestablished, the batteries still wouldn’t charge. With more than 800 amp-hours in the house battery bank, the owner wondered how the batteries could be dead after such a short amount of time.
We could answer that question, but first, we had to get the batteries charged back up.
Back from the Dead
Many smart, electronically controlled battery chargers need a minimum DC voltage (usually 8 to 10 volts) in order to turn on. Ironically, the dead batteries prevented the not-so-smart charger from charging.
To solve this dilemma, we had the owner parallel his isolated and charged start batteries with the house batteries (the boat has a paralleling battery switch). This allowed the charger to sense a higher voltage and begin charging.
These AGM batteries were only a year old. A deep discharge like this was bad for them and potentially removed some life span. How much life span is debatable, and their longevity will depend on how they are treated in the future.
Another solution would have been to start the engines and let the alternators charge the house batteries enough that the shore charger could take over. But it was late at night, and we were trying to solve the problem quietly.
If the start batteries had not been fully isolated or were not fully charged, there were a few other options. A jumper pack or fully charged battery could have been temporarily connected with jumper cables. Or, an old-school portable ferro-resonant charger that operates regardless of battery voltage could have brought the voltage up to a usable level so the larger, installed charger could take over.
Where Did the Amps Go?
Once the batteries were topped off, a bigger puzzle needed to be solved. What had caused the batteries to go dead?
Obviously, leaving the battery switch on and loads running would have done this, but our owner is fastidious, and the switches were left off. A leak that caused the bilge pump to run constantly also could have run the battery flat. This also wasn’t the case, but was worth exploring.
The main battery switch is installed in a DC system to disconnect loads from the batteries when there is no one aboard the boat. But there is a compelling reason to have some circuits that are not switched by the main battery switches. These are circuits that supply important safety and monitoring loads that you wouldn’t want turned off when you leave.
A bilge pump float switch is one of these circuits. The idea is that if a seawater hose were to pop off a hose barb, the float switch would turn on the bilge pump. If the bilge pump could not keep up and the boat had a high-water alarm, that circuit would also turn on, hopefully to alert someone that the boat was sinking.
Some float switches, depending on the type, have an amperage draw when they are in standby mode. These draws may be no more than a few milliamperes (mA). In our case, there were four pumps with a draw of 2 mA each, along with the breaker indicator lights that draw 1.5 mA each.
In the business, we call these types of loads parasitic. They quietly feed off the batteries even with the main switches turned off.
There can be quite a few different feeders. Let’s explore a few of them.
Types of Parasites
This boat was equipped with a liquefied petroleum gas (LPG) range and barbecue. The control and alarm system for gas detection was correctly wired to bypass the main battery switch and be on all the time, so that if a leak were to occur inside the boat, the owner wouldn’t find out about it as he turned on the battery switch, possibly creating a spark from some device in the bilge that would cause an explosion. (It’s always better to have an alarm go off while you are away, and hopefully have someone be bothered by it enough to find out why it was going off.)
These gas-detection systems have a small amperage draw; this one used just 163 mA in detection mode.
Our boat also had five carbon-monoxide (CO) detectors installed, each one drawing 108 mA, or 540 mA total.
It is a good idea to have CO detectors aboard boats with staterooms for sleeping. Carbon monoxide is colorless, odorless and poisonous. It is more of a concern from gasoline-powered engines and generators, but even diesel-powered equipment produces some CO, as do LPG appliances. Some boats utilize home CO detectors, which can work as long as the batteries are maintained. A better approach is to purchase marine-rated units that run on ship’s DC power, and then wire them in each stateroom. Like the LPG detectors, the CO detectors should be wired to be on 24/7, bypassing the main battery switch.
Another common parasite is the modern marine stereo, which can be programmed to save settings and stored stations. Stereos do this by way of a third DC wire (usually yellow) that is wired directly to the battery. This wire does draw a small amount of current all the time, with different stereos having different amperage draws. The memory with this boat’s stereo drew 180 mA.
On some boats, owners tie the yellow and red positive wires together at the breaker, allowing the memory wire to turn off when the power is cut. This means that each time the stereo is reactivated from the breaker, any programming will have been lost. It also means that one parasitic load has been removed.
Another consideration is modern battery-monitoring systems. They can show granular detail about the state of the battery banks: voltage, amperage, state of charge, time to discharge and more. Sophisticated programming can take into account that the bigger the load taken out of a battery, the less capacity the battery will have. Some can even account for the fact that batteries lose capacity over time.
One of the things our owner questioned was why his complicated battery-monitoring system did not seem to show the parasitic amperage. This was a twofold problem. The monitor only had resolution to tenths of an amp (100 mA), so it didn’t accurately show the draw. Additionally, several of the loads were bypassing the monitor’s shunt.
A shunt is a device that is typically installed in the main negative cable, as close to the batteries as possible. It sends amperage information to the monitor. The cable is cut, and the two ends are crimped with new ring terminals to fit the terminals on the shunt in the correct direction of amperage flow. A shunt’s terminals are always labeled, such as “load” and “line” or “panel” and “battery,” or sometimes just with an arrow. There are also small terminals for the measurement wiring. Between the two terminals are thin sheets of metal. This metal has a known resistance, and the measurement wiring reads the difference between the two terminals. It represents this difference as amperage shown on the monitor.
Shunts are also rated for the amperage they expect to have flow through them. It is not uncommon to bypass large DC amperage consumers such as engines, windlasses or thrusters because they could exceed the rating of the shunt. It is important to know what loads have bypassed the shunt; since each terminal will read as a ground on a multimeter, an unknowing electrician may choose the wrong stud to connect additional equipment. Any loads directly connected to the battery posts will also bypass the shunt.
This happened with our boat because the CO alarms and stereo memory grounds were attached to a battery post. We moved these back to the load side of the shunt.
All of the battery-monitoring capability does come at a cost, albeit a small one: 9 mA. But if you are keeping track, we’re starting to add up quite a few milliamps of constant current draw.
More on that in a bit. First, let’s talk about some other systems that can pull from the batteries when you’re not around.
Distributed Power
There is a growing trend of doing away with the standard electrical panel that has breakers and wiring that’s led throughout the boat. The replacement is a remote-controlled distributed power system.
These systems run a set of trunk DC cables to control pods with electronically controlled breakers that are located closer to equipment. Small control wiring runs back to a central hub. These systems can save a huge amount of wiring and weight on a boat, and can allow for a plethora of features not available on a conventional system.
Early systems had dedicated displays, but many newer systems can be controlled right from the multifunction displays at the helm, or via Wi-Fi with a computer or tablet. Since these systems need to be able to run all the time, there is also a standby amperage draw. Depending on how many devices are being controlled and how many displays are used, this draw can add up to several amps.
Outside of the proprietary distributed power battery switches used in many of these systems, several manufacturers produce their own remote-controlled battery switches that can be used in conventional electrical systems. These switches can solve the problem of placing the battery switch close to the battery, yet still allow owners to turn them off without going into the engine room or machinery space.
This setup can be an issue in the event of a fire in those spaces where being able to shut off power remotely is a safety concern. Depending on the style, these switches can have a small amperage draw of 8 mA each when off.
Whole-Boat Monitoring Systems
Have you ever woken up at night worried that your boat was sinking? I occasionally have this nightmare. A number of remote-monitoring systems can remove some of that worry.
These systems can monitor all sorts of things aboard the boat and send a text to your phone or a note to your computer based on set parameters. Alerts can be programmed for everything from a door or hatch being opened to a bilge pump running to the freezer warming. Virtually any information you might want to know from afar can be provided with a sensor.
These systems operate via cell service or Wi-Fi, and come with an amperage cost. One popular system draws 20 to 70 mA at idle, depending on whether the unit is sending out alarms.
In the case of our owner, a system like this could have alerted him that the AC power was lost, and that the DC power was dropping. This alert could have presented the opportunity for a call to be made to the marina for someone to walk down the dock and flip the breaker to restore power.
Malfunctioning Equipment
All the systems described above are known, quantifiable parasitic draws when working as designed. There also can be draws from malfunctioning equipment that operates on the 24/7 bus, or from faulty battery switching or breakers, allowing some amperage to be used even when turned off.
Tracking down these problems can be tricky because they are often intermittent and have to be caught as they are happening. In some cases, we will set up recording amperage meters in line with the battery negative to prove whether there is an amperage draw. Then, we move the meter to different circuits to track down the culprit.
The Solar Solution
Once it has been established that all the systems are working properly and all the parasitic loads are tallied up, a decision needs to be made. Either the boat will need to be tethered to shore power, a monitoring system will need to be installed, or an alternative power source will need to be installed.
The alternative power option is where a solar panel can really shine. As long as the panel can provide enough power during daylight hours to overcome the parasitic loads, plus charge back the batteries from the previous night’s losses, the system can be silent and self-sufficient.
If there is enough deck space, additional panels can be installed to help with normal DC loads aboard the boat. Panels should always be installed with regulators/charge controllers. Different types of controllers have differing levels of efficiency, but all will help prevent the panels from overcharging the batteries, something an unregulated panel can easily do.
Add It Up
Since the parasitic loads on our catamaran included a stereo memory of 180 mA, bilge pump float switches and indicator lights at 14 mA, a battery monitor at 9 mA, and LPG and CO alarms at 703 mA, there was 906 mA being taken out of the batteries each hour. Multiply that times 24 hours in a day and 30 days in a month, and we’re at 652 amps.
Without knowing when our owner’s shore power breaker tripped, it is easy to see how it would be possible for the boat to drain the batteries given enough time without an AC power source.
To mitigate the problem in the future, our owner added 100 watts of solar panels and installed a monitoring system that would alert him by phone if the shore power disconnected, or if the battery voltage ever dropped again.
