Stuart Cunningham – Lead Electrical Technician

Sitting in a peaceful bay, sun going down with a cheeky G&T on the aft deck. There is a gentle lapping sound on the swim platform and in the distance you can hear some kids laughing while they swim in the clear waters. Then the generator kicks in. 


It is that exact moment you wish you had specified a much bigger battery bank.

As vessels become more and more advanced, the demands on a suitable power supply are rapidly increasing. Luckily the same time, advances in battery technology is also advancing with leaps and bounds. Gone are the days of a standard car size battery would suffice for the DC supply on board. With the advent of widescreens, ships PCs, better and more powerful communication equipment touchscreens, and a million other proprietary devices all sucking power out of the system, a well designed and specified DC battery system can eliminate a lot of problems on board, leaving you to enjoy the real reason for having a boat.

So what do i need?

A battery! A Charger! But how big do they need to be?

 The specific requirements vary from vessel to vessel, dependent on physical size, equipment aboard, required quiet running and charging times.

 Whilst it all seems like the usual minefield of conflicting information from multiple sources, it can really be broken down into FIVE main questions.

How long do i want to run on batteries for?

How much power do I use?

How quickly do i need to recharge the system?

What physical space is available?

And probably the most important, budget!!

But first… a quick word on power calculations

 There are only a few calculations needed to get a rough idea of your power requirements aboard. Most information surrounding service batteries refer to amp-hours (Ah), which is the amount of power which can be stored, or used over a certain time.

 If an appliance drawing 5A was to run for 1 hour, its consumption would amount to 5Ah.

This would be the same as an appliance drawing 1A running for 5 hours – again the consumption would be 5Ah.

The Ah ratings for batteries are normally easily found and usually expressed in Ah, however ratings for equipment and loads might require some investigation into their manuals, and will be normally expressed in terms of power (measured in watts) or current draw (measured in amps). The relationship between power and current is expressed as:

Current (A) = Power (W) / System Voltage (V)

For example, a 6 watt navigation light bulb in a 12 volt system will draw 0.5 amps – which, if it’s switched for ten hour each day when underway will have consumed 5 amp-hours (Ah).

Using this theory, we can calculate the total average Ah required by the vessel each 24 Hrs by calculating the power for each piece of equipment on board and the duration of its use. 

We can also figure out how long the batteries are going to last (Our silent run time) for a given load by using this equation:

Silent Run Time (h) = Battery Capacity (Ah) / DC Load (A)

Note here that we’re using DC Load, which will of course include the current going to the inverter if there are any AC loads.

A look at your loads

Far and away the biggest load on most power or sail yachts is in heating and cooling.  Air Conditioning, water heating, and cooking appliances. If you can do away with these loads, or are happy to run a generator only when they’re in use, then silent running becomes a much more manageable proposition. Adding air conditioning and galley loads and putting this through and inverter is theoretically possible on many yachts, but simply not practical from a space and cost perspective. This gets even easier when using an automatic generator controller – the generator will turn itself on when it senses it’s needed, or the battery voltage gets too low.


How long do you want to run on batteries for?

Looking at the equation in the  previous section, there are two ways to increase the battery (silent) run time. Increase battery capacity, or decrease load.

Think of it as a car and its fuel tank in relation to the vessel and its battery bank. Do you want to drive really fast stop every twenty miles for fuel? Would you rather an economical car that plods along with a big fuel tank or do you want foot-to-the -floor performance lightweight racing car? 

One interesting example of a series of boats with almost indefinite quiet running capability is the Gunboat Catamarans. 10 x 12v 225Ah AGM Batteries give a  2250Ah battery bank, which is supplemented by solar panels and charged through both a Mastervolt Combi unit and dedicated chargers. The system has been designed to be to operate without connecting to sketchy shore power, allowing them to cruise more remote areas without risking their power system. The System is controlled by Mastervolt mass combi ultras, coupled with digital control of the shore power inlets and and all in one power control station, it allows continued use whilst in areas with questionable VAC supplies.

The best arrangement for a battery system with silent running in mind

The best arrangement for a silent running boat is to have a large 24V battery bank, connected to one or more Mastervolt Mass Combi Pro or Mass Combi Ultra Inverter/Chargers. The generator output is also connected to the Mastervolt Inverter/Charger and switching between inverter and generator is automatically and seamlessly handled internally. The Mass Combi can even start the generator automatically if a large load is detected (such as the air conditioning or cooking hob). The entire AC switchboard can be connected to the output so there’s no confusion over which plug sockets can be used and when, and because they can be paralleled to provide up to 35kW of inverter output (more if using generator / shore power output) there’s always plenty of power available. Perfect for making a nice cup of tea after your 2am watch without waking anyone up! 

Add to this as much DC charging as possible, such as large alternators, or solar panels applied directly to the batteries and you’ve got a very effective system.

Of course not every boat is going to be able to run this particular system, but in the 60-110ft sailing yacht range, our experience tells us it’s an extremely good starting point to start from.


What are my options to recharge the system?

90% of the boats we deal with, in the 60-110ft range have a generator aboard. As mentioned before, if you want to run the air conditioning on anchor or at sea, then it’s basically a prerequisite. It then makes sense to add a decent charger on the AC circuitry to charge the batteries while you are cooking or have the air conditioning on

Although a generator is handy for charging the batteries, it certainly isn’t the only option. The effectiveness of a couple of alternators on the main engine(s) can not be overrated. By hijacking the main engines power, it can greatly reduce the amount of space required in the engine room, as well as reducing the complexity and servicing costs aboard. For bonus points on a sailing yacht, driving the alternator from the prop shaft instead of the crankshaft can allow electricity production when the boat is sailing – meaning an indefinite quiet period, so long as the wind is good!

And what about wind and solar? If you’ve not seen a wind installation close up on a yacht, you will have definitely heard one in a marina. Although they can be effective, they can also be quite noisy, which for our purposes defeats the purpose of running silently!  And although solar can be very handy for charging the batteries when the boat is not in use, unless the yacht has been designed to allow a large area of solar panels (such as many sailing catamarans), they just don’t provide enough power to be a serious charging system.

With the continuous developments into different construction of battery cells and the materials they are produced out of the recharge time is considerably less than it was just a decade ago, with the advent of Li-ion batteries allowing almost unlimited charge and discharge currents possible without the risk of a fiery explosion. The limit is now no longer the batteries, but the charger or alternator used. 

How much space and weight do you have available?

The physical size and weight is another contributing factor to the choice of a battery bank as the power available has a direct correlation to the size required. As mentioned before, the technological advances in battery construction have certainly minimised the weight gain to Ah ratio in recent years, with the Mastervolt MLi-Ultra 12/5000 having an average 6Ah available per kilogram of mass, compared to a standard lead acid 12v battery of a similar capacity having on average only 1.8Ah available per kilogram of mass. This allows you to fit more power than ever before into the available space or increase the capacity of an existing bank without having to sacrifice more space than currently available.

One 30M sailing yacht we dealt with recently has taken advantage of the smaller size of lithium ion batteries. 14 Mastervolt MLi-ion batteries are used in parallel to produce a 2800Ah battery bank at 24V – easilly enough to run for 12 hours with all the bells and whistles running. She also utilises C-Zone and Mastervolt digital switching and control for the whole power generation and control allows seamless operation of all systems involved. As Mastervolt and C-Zone are owned by the same company, the integration is smooth and reliable.


How about different battery types?

As mentioned at the start of this article, there has been some amazing increases in battery technology in the past few years. 

Battery Technology

Price per capacity (24V)

Capacity per weight (24V)

Max charge rate


€6 per Ah

1.7Ah per Kg

40-50% of capacity

Lithium Ion

€32 per Ah

3Ah per Kg

Approx 150% of cap.


€8 per Ah

2Ah per Kg 

Approx 100% of cap.


€1.76 Per Ah

.9Ah per Kg

40-50% of capacity


And after all of this has been taken into account, the deciding factor is budget.

As with all things, often the biggest factor is price. Lithium ion batteries are great, but they’re also really expensive! We work with customers everyday to find the best solution which works for them, their boat and their budget. Make sure you contact us to discuss your project so we can point you in the right direction.