The phone is empty, the tablet battery needs charging urgently and the games console won't last much longer either. Now a power socket and the beautiful day on the water would be saved. Installing a fixed 230-volt supply on board that is independent of the shore power supply usually involves a lot of effort.
However, there are inexpensive alternatives for small everyday consumers. Mobile inverters turn the direct current from the vehicle electrical system into alternating current for the socket. Many battery-operated devices require 230 volts for the charger, but only relatively little power.
And this can often be provided with very small inverters. Such converters do not require complex installation, but only a socket, as we know it from the cigarette lighter in the car. This is available as standard on many boats or can be retrofitted with reasonable effort.
If you limit yourself to operating a single 230-volt consumer at a time, there is no need to set up a complicated network on board. At first glance, the choice in this class is huge. On closer inspection, it quickly becomes apparent that although there are many names, there are only a few different devices.
Our selection was based on which models are widely available for water sports and we always tried to contact the actual manufacturer or its main importer. LTC (Waterloft Schleswig), Sterling power, Voltcraft (Conrad Elektronik), Cotek and Xantrex (both Vmax24) and Dometic-Waeco took part.
All test subjects supply alternating current at 230 volts and 50 hertz, which is standard in Europe. They are designed for operation on 12-volt systems and are usually fitted with a plug for the cigarette lighter (like in a car, 21 millimetres in diameter). For the smaller socket, which is sometimes installed in marine control panels, you have to fit a plug yourself.
System question
Alternating current in the land grid is generally sinusoidal, and this is how many consumers expect it to be. In the lower price range, however, it is common for inverters to generate a rectangular voltage curve. Manufacturers like to sweep this important difference under the carpet and advertise the signal as a "modified sine wave".
However, there are a lot of devices that need to be powered that are not indifferent to this, often those with a lot of electronics in them. Simple light bulbs can cope with both versions, but they are unlikely to be operated using an inverter.
To avoid any confusion about the signal shape, we have divided the test devices into two groups: those that supply rectangular current and those with a true sine wave, as in the shore power network.
Performance
The test includes devices with rated outputs between 100 and 350 watts. How much of this you can actually use on board is determined by connecting a mobile inverter via the cigarette lighter socket. There are two common types, identical in appearance but with different specifications for the maximum permissible current: 7.5 or 15 amps.
You cannot recognise which one is fitted from the outside, unless the boatyard has kindly attached a corresponding sign. If in doubt, the fuse to which the socket is connected will give you an indication of the type installed.
You can draw around 90 watts from the weaker version, while the larger version can draw up to 180 watts. The inverter needs a small part of this for itself, leaving around 80 or 160 watts available for the connected consumer. You can connect inverters with more power there, but only draw a maximum of this wattage without the fuse tripping.
If you need more, you have to modify both the socket itself and its wiring. Simply replacing the fuse with the larger version should never be considered as the sole tuning measure! Because if the socket or cable are not designed for the higher current, there is a risk of melting housings and cable fires - which in turn results in consequential damage to the inverter plug and, in the worst case, to the boat.
Alternative connection
The inverters from Xantrex and Cotek could never utilise their nominal output of 300 watts on a 12-volt socket. That's why these manufacturers supply two connection cables: one with the usual plug for the cigarette lighter and a second, stronger set that connects directly to the battery terminals with pole clamps and allows the inverter to deliver its full power. You will need a Phillips screwdriver to replace and connect these cables for the first time.
This allows you to access the power without having to tamper with the electrics, but the clamps are not a permanent solution. When connecting them quickly in a dark battery compartment, it is easy to reverse the polarity. The manufacturer rightly recommends a fixed connection if full power is required.
Personal protection
All test devices have exactly one socket. There is a deeper meaning behind this, which unfortunately not all manufacturers point out: It's about your safety. It is well known that alternating current at 230 volts is not without danger. At this voltage, enough current flows in the human body to disrupt the heart muscles due to the periodic changes in current direction.
This condition must therefore be prevented at all costs, even if a defective device is connected to the inverter. There is a defined earthing in the shore-based grid or in a permanently installed on-board grid. The prescribed protective devices such as fuses and residual current circuit breakers (RCCBs) need this earthing in order to recognise when current is not flowing in the intended way, but possibly via a person.
Earthing is difficult with mobile inverters and is usually "forgotten" in practice. Other protective devices must therefore be used: The output of the inverter represents an isolated grid and is completely unearthed in normal operation. For this reason, a phase tester on the inverter does not work either.
If, due to a defect, the housing you are holding in your hand makes contact with one of the wires, you will not be "wiped" but will instead act as an earth connection. If the other wire also makes contact with the housing, a short circuit occurs and the inverter switches off. In both cases, nothing will happen to you.
However, this principle only works with a single connected load. If two are connected to the same inverter and each has a different wire in contact with the housing, there is no safety measure to protect you if you touch both. A particular problem with this system is that the individual fault is not noticeable and can be present for years.
More sockets
In order to supply more than one consumer, a standardised 230 volt network with earthing and circuit breakers would have to be installed. There is only a reference to this connection in the instructions for the SIB 12200 from Sterling.
This device can be switched between isolated and earthed mains via a jumper. This manufacturer also points out that additional protective devices must be installed downstream of the inverter for the latter case.
However, setting up a 230-volt network on board is not for the layman. Proper earthing alone is a science in itself and can lead to serious problems with corrosion. And that would be a shame for your propeller.
If there is to be more than one shore-independent socket on board, the cheapest way is to use several small inverters, one per socket. And without any earthing or corrosion problems.
Contact question
The cigarette lighter socket is often a problem child on board. In terms of material, most versions are designed for use in dry cars and quickly have contact problems with the humid air on board - especially if you have a berth by the sea.
Treatment with a little contact spray can help - temporarily. The surfaces should be wet, but not dripping wet. Multiple plugging and unplugging can also overcome contact problems in the short term.
In continuous operation with high power, these sockets nevertheless heat up quickly. This leads to poorer contact and therefore even more heat. You should therefore only operate an inverter on sockets with a metal body. Versions consisting of a plastic socket with simple metal contact strips often cannot withstand the high currents for long.
More power
If the existing sockets do not fit or are not sufficient for the desired power, the only option is to retrofit them. On the 12-volt side, this is also feasible for the experienced layman; fuse protection, cable cross-section and connection technology must be taken into account.
For the new socket, look for a place that allows short distances to the battery and also offers a well-ventilated location for the inverter. A maximum of three metres of cable should be used on the 12-volt side.
A cross-section of six square millimetres is sufficient for all devices presented here. The new cable is fused with 30 amps in the positive terminal as close as possible to the battery or directly after the main switch. Do not use any material from the car section of the DIY store, even if this appears to be sufficient.
Seek advice from a specialised marine electrical company and make your purchase there. Although this is more expensive than the entire inverter, the pleasure of a smoothly functioning system made of good, compatible material will last much longer than a cheap purchase. Philippi, for example, has a cigarette lighter socket in its range that is specified for 20 amps. Even 15 amp versions are not a common product in DIY stores.
If you're already doing it yourself, you don't have to stick with automotive technology. There are plug connections available from specialist dealers that are much better suited to high currents than the cigarette lighter plug. There are no technical reasons against a conversion, only the loss of the ability to continue using the device in the car.
Location issue
It is already clear that the inverter should be located a short cable distance from the batteries. However, it should not be located directly in the battery box or engine compartment, as all devices like to be cool and airy.
As can be seen in the table, the operating range is quite narrow, at least for the sunny south. There is no need to worry about damage, as all test subjects switch off automatically in the event of overheating. But in a warm environment, the possible output power quickly decreases.
The fan in some models makes it advisable to place it slightly away from the bunks. You can always reach the output with a standard 230 volt extension, and because the losses there are low, its length is not critical.
One task remains to be solved: All inverters draw juice from the battery in standby mode. This is why almost all of them have a main switch, which should be used. However, remote, cool, airy locations near the battery are usually difficult to reach. In this situation, the option of remote control means a significant gain in convenience.
The Cotek device has two standby modes. In economy mode, it checks every second whether a load is connected; if this does not draw any power, the inverter does not supply any voltage. This reduces consumption from 8 to 1 watt in standby mode.
However, this behaviour can cause problems if only a mobile phone charger that requires less than 20 watts is connected: The energy-saving mode can be deactivated via a tiny switch.
Conclusion
Solutions for mobile sockets are already available for little money. However, whether the inexpensive devices with square wave voltage fulfil your requirements depends on the connected load and is difficult to predict. All inverters with a true sine wave output, often labelled "pure sine", are unproblematic.
For frequent use, you should choose a device either without a fan or with large dimensions due to the noise level. Before buying, check whether a suitable 12-volt socket is available on board; it must be the model with an internal diameter of around two centimetres. If not, plan to retrofit one straight away. It is obvious that the inverter is not connected to the starter battery but to the service battery: Driving remains the most important part of the hobby.
(This article comes from BOOTE issue 7/2017.)
