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It’s not just an inverter/charger

Outfitting your boat with an inverter/charger like the Mass Combi may allow you to downsize your genset

Outfitting your boat with an inverter/charger like the Mass Combi may allow you to downsize your genset

Read the other story in this package: Selecting the right power inverter

Simplicity on boats always seemed to me to be a virtue. Occasionally, I’ve seen cruises descend into an obsession to get this or that to work; when I started cruising simplicity was all there was. A compass, alcohol stove, insulated box and mechanical head was about the extent of the outfitting.

However, with the exponential advance of technology, every year the gadgets get better and better, and at a certain point this or that becomes so reliable, so power efficient and so nice to have that, what the heck, give in and enjoy. So it is with inverter/chargers, and this past season I installed the latest 2-kW Mastervolt Mass Combi inverter/charger (model 12/2000-100/120V). Now the galley on Maramor, my Grand Banks 42, hums with a microwave.

What brought me to the conclusion that an inverter/charger was worth a couple grand and hours of running cable? Certainly not just the convenience of a microwave. A powerful 100-amp battery charger and “Inverter Power Support” to supplement shore power or the generator — preventing overloading during surges and allowing for a smaller genset and lighter-duty shore power cables — are very desirable features for any cruising boat.


The Mass Combi uses high-frequency technology that eliminates the need for large transformers. It is compact, lightweight and efficient, and on Maramor it fits nicely in the space previously occupied by the 50-amp battery charger. It weighs only 24 pounds and measures 19.5 by 12.5 by 6.1 inches. The LED indicator on the unit and a plug-and-play remote (ICC, or Inverter Charger Control) panel that connects on both ends with a telephone-style modular jack indicate DC load, state of charge, inverter or charger mode, failure and AC power availability. The remote has an on/off switch.

The Mass Combi is designed to be mounted on a bulkhead, which, given its small size, light weight and convenient attachment points, was easy for one person working alone. The electrical connections are sturdy, logical and convenient. However, it was awkward to feed the 125 V cables through the AC cable glands, probably because the unit was designed originally for230 V/50 Hz service. Half the voltage and you must double the current for the same power (P = IV). But energy loss in the form of heat in the wires varies as the current squared (P = I2R). Thus, heavier gauge wires are needed for 125 V service; I used 8 AWG. I had to run the grounds through ventilation holes in the case rather than through the cable glands, a small flaw in an otherwise beautifully designed unit.

The battery charger

Maramor is kept on a mooring, and when cruising we anchor whenever possible. Last season Maramor did not berth alongside until the fall, when we took her up the Hudson River and through the ChamplainCanal. We like peace and quiet, so we run the generator sparingly. A powerful battery charger for Maramor’s two 8D AGM house bank batteries (245 amp-hours each) shortens the run time.

The three-step, electronically controlled battery charger in the Mass Combi delivers 100 amps on the bulk charge to the house bank, double the bulk charge capacity of the charger it replaced. In addition there is a secondary 5-amp output for the starting battery. I installed a battery selector switch on this secondary output to direct it to either the engine starting battery or to the generator starting battery. Both of these batteries normally are fully charged by their respective alternators, but the selector switch provides a link between the generator battery and the charger should the need arise.

The main power switch on the Mass Combi has a charger-only position, so you can choose to disable the inverter mode from automatically engaging should there be a shore power outage. This is very useful if you leave AC equipment running when you are away from the boat, as it will prevent the batteries from draining.

The unit comes with a battery temperature sensor so that the charge voltage is adjusted for temperature to prevent overcharging and gassing. You can connect your laptop to the Mass Combi’s QRS232 port and, with an optional “PC-Link” converter and free software, adjust numerous parameters, including the preset charging voltages.

The pure sine wave inverter

An inverter shines when it must power a heavy AC load for a brief period, such as using a microwave. Take a typical 1,300-watt microwave (AC input, not cooking power), which at 125 volts is theoretically drawing about 10.4 amps. If you cook for 10 minutes you have used about 18 amp-hours from your 12 VDC house bank.

Light AC loads, such as a laptop, are more efficiently powered with a DC-to-DC power supply. These power supplies, which are inexpensive and readily available, look just like the AC-to-DC power supply that comes with your laptop, but instead of an input of 125 V they use the input of 12 V from your batteries. Consequently, there is no power loss in the conversion from DC to AC in an inverter and then back to DC in the laptop power supply. Also, if your laptop is your navigational computer, the insertion of an inverter in the circuit introduces another possible failure point and should be avoided.

An inverter is not suitable to run heavy loads for long periods — for example, air conditioning — unless the main propulsion is running and is fitted with alternators of sufficient size to keep the batteries from being quickly drained. Thus, when away from shore power, air conditioning is run by an on-board diesel generator running continuously.

To account for the heavy starting loads of electric motors, the generator has a power output far in excess of the normal running load. These starting loads also require the shore power connection to be much more heavy duty than the running load requires. This is a problem the Mass Combi solves, allowing for the fitting of a smaller diesel generator that can run efficiently, instead of a large generator running at greatly reduced load, and lighter-duty shore power connections. More on this later.

Dual power transfer

The Mass Combi has one AC power input (50 amps max) and two AC power outputs, “Short Break/Inverter” and “Power.” The Short Break/Inverter output passes through up to 25 amps from shore power or the generator. If no AC power source is available, the Mass Combi switches to inverter mode, and the Short Break/Inverter output is then energized by your batteries. When an AC source fails, the inverter instantly picks up the load. The Power output, on the other hand, is just a pass-through (50 amps max) from the AC power source.

Since all current from the AC source (shore power or generator) passes through the Mass Combi, it can measure the total power being consumed. This permits so-called “Inverter Power Support” and “Power Sharing” so the AC source and battery-fed inverter can function together as a system to handle peak loads without tripping a breaker.

On Maramor the Power output energizes the air conditioning and the hot water heater, and the Short Break/Inverter output energizes everything else. For flexibility, I installed an AC power source selector switch on the leg feeding the air conditioning bus so that it can be energized by either the Power output from the Mass Combi or directly from shore power or the generator.

AC Power Control (APC)

This is the feature that motivated me to make the investment in time (considerable) and money (also considerable) to fit Maramor with a Mass Combi inverter/charger. Since I knew I would be keeping my Grand Banks on a mooring or at anchor, I ordered her outfitted for air conditioning but without air conditioning compressors and evaporators (in case I changed my mind), and with a propane stove and oven. A GB42 usually is fitted with three-zone air conditioning (hence, the three air conditioning breakers in the schematic at right), and an all-electric galley supplied by a 50-amp/250 V AC shore power input. The 250 V input is wired just like your house; there are two 125 V single-phase wires, a midpoint common neutral, and a ground (split phase). Typically one 125 V wire powers the AC outlets, galley and miscellaneous equipment, while the second powers the air conditioning. The 125 V inputs from a 250 V shore supply aren’t in phase and thus can be combined to power 250 V equipment.

For convenience and economy I connect a 30-amp/125 V shore power cable (one 125 V positive wire, one neutral and one ground) to an adapter that bridges the cable and the 50- amp/250 V shore power connection on Maramor. The net effect is that both 125 V buses on Maramor are energized, and both singly or together can draw a total of 30 amps/125 V from ashore. There is no phase issue in this setup.

Now to the air conditioning. Last summer I installed one Cruisair 16,000 BTU air conditioning/heating compressor supplying two evaporators (12,000 BTU in the saloon and 4,000 BTU in the aft stateroom) that are energized by one of the “aircond” breakers in the schematic. (The other two aren’t used.) Because of the AC Power Control feature of the Mass Combi, I can run the air conditioning, charger, microwave, etc., without risking tripping the shore power breaker. This is achieved through the Power Sharing and Inverter Power Support features.

Power sharing

On the optional AC Power Control remote panel, I set the Mass Combi for maximum 30 amps input to match the capacity of Maramor’s shore power connection. This also can be done via a dip switch on the unit itself.

Suppose I have just arrived at the marina and plugged in. The battery charger is putting out 100 amps DC at 14.4 V. The microwave is heating a bottle for the grandchild, and the air conditioning compressor comes on. The Mass Combi Power Sharing feature constantly measures the total AC load, and when the total load approaches the set value (in Maramor’s case 30 amps) and the breaker is in danger of being overloaded, the Mass Combi instantly reduces the battery charger output, thereby lowering power consumption.

Inverter Power Support

The Mass Combi limits the draw from shore to the set value and makes up the difference with power from the inverter. There are two ways to accomplish this, the “Generator/Mains Support” function and Power Support function, which can be enabled separately or together to prevent an overload.

The Generator/Mains Support function parallels the inverter with the AC source. To parallel two AC sources they must be synchronized — in other words they must have the same frequency and be in phase so that the voltages reach peak values of the same polarity at precisely the same instant. For example, paralleling would allow a small generator on a sailboat to power an air conditioning compressor with a surge load in excess of the capacity of the generator.

The Power Support function supplements the AC source by transferring the loads on the Short Break/Inverter output to the inverter when the set value is in danger of being exceeded. At that point the AC source is supplying only the loads on the Power (pass through) output from the Mass Combi. When the total load is below the set value for about 10 minutes, the Short Break/Inverter output is connected automatically back to the AC source.

On Maramor I enable both Inverter Power Support functions to allow the use of a 30-amp or 20-amp/125 V shore power connection. However, since Maramor’s powerful generator needs no support, when it is online the AC power source selector switch described earlier provides a direct path from the generator to the air conditioning bus, bypassing the Power output from the Mass Combi. This bypass also will be used should Maramor ever connect to shore power with a 50-amp/250 V connection.

What combination you use on your boat will depend on your circumstances, and the flexibility provided by the Power Sharing and Power Support functions surely will provide an appropriate solution. Should your power requirements exceed one unit, two Mass Combis can be paralleled using the Masterbus connector shown in the photograph.


The Mass Combi is very carefully designed, engineered and manufactured, but its pedigree is obviously a 230 V unit for the European market. Two minor adjustments for the North American 125 V boat are larger AC cable glands and a Power Sharing setting for 20 amps instead of 25 amps. The other choices of 50, 30, 15 and 10 amps are spot on.


With Power Sharing and Inverter Power Support, generators no longer need to be oversized to account for electric motor starting loads. If you outfit your boat with an inverter/ charger like the Mass Combi you can then size your generator (and shore power connection) to handle the normal running loads. This has many advantages, not the least of which is that the generator is then “loaded up” so that it runs efficiently as designed.

Maramor came standard with a 9-kW Onan generator, way more than what I consume on board. If I were ordering Maramor today I would specify a much smaller generator that matched my running loads, supplemented by Inverter Power Support. In addition to being more efficient, a smaller generator would have the added benefit of being more accessible for service and less in the way of other equipment that needs to be reached for inspection and maintenance.

The combination of an inverter like the Mass Combi, with its Power Sharing and Inverter Power Support features, and a generator sized to handle just the “running load” is the way of the future, in my opinion, particularly for boats with limited space for a generator. For more information go to .