Power management

Power Management

Power Management is one of the keys to successful ocean cruising and long distance passaging. To begin with most of us underestimated how much power we would consume especially when sailing in the tropics and also what was needed to provide this power.The need for power is the consequence of one’s life style and modern day sometimes high tech power hungry equipment. Autopilot, fridges and freezers electric cookers , microwaves, communication equipment, music and video players, computers not to mention TV, all take their toll on batteries.
Electric energy taken out needs to be replaced using wind solar water power or fossil fuel. Many cruising yachts are equipped with solar panels as well as wind and water generators. The larger yachts have diesel generators for that task.

This chapter intends to discuss what is needed to bring your yacht’s power system up to serious passage making capability and give you the tools to discuss requirements with suppliers.

The basic questions for long distance cruisers are:
– How much electric energy do we use per day?
– How do we store it?
– How often per day do we intend or need to generate?
– What sort of systems do we need to generate this energy?

Calculation of usage.

This is an imprecise science. Each and every yacht is as different as their crew. Some yachts have no refrigeration and rely on the wind for steering whilst others are packed with all the mod cons available. Let’s try as an example the usage on a passage:

Category                      amps          hours of usage          amp-hours per day

Nav. Instruments      2 amps              24 hours                     48 amp. hours
Autopilot                    5 amps              24 hours                   120 amp. hours
Masthead light          2 amps              12 hours                     24 amp. hours
Interior lights           4 amps              2-3 hours                     10 amp. hours
Refrigeration            6 amps                 8 hours                     48 amp. hours
SSB & VHF                 2 amps              4-5 hours                     10 amp. hours

Total usage in a 24-hour period:    260 amp. hours

All this is to get a general idea.
Cruisers who use an autopilot will appreciate that it pulls more amps out of the batteries in certain sailing conditions than in others. It is generally the heaviest user.
Fridges and freezers are also guilty of gulping vast amounts of power. Particularly in the tropics with the ambient air and water temperature of 24º+ Celsius you will notice the difference. An engine driven compressor for the freezer will use no amps at all but one needs to run the engine even when connected to shore power in a marina.

I have intentionally not mentioned in the calculation a watermaker which provides a certain level of luxury on extended passages. Many even smaller yachts have one providing the crew with the occasional shower and sometimes to flush the cockpit free from sprayed-in salt. Most water makers need 110 or 240 volt so a generator would be necessary to run it. Therefore one only runs the watermaker when charging with the generator which is then not a drain on the battery bank.
To be effective one should choose a high output water maker to reduce the running time of both the generator and the water maker. 12 or 24 volt units do not get a good press, they must run nearly permanently to keep up with demand and are an often overlooked drain on the battery bank.
A compressor belted to the main engine could be a solution for yachts without generator.
Yachts without a watermaker use a rain catcher made of lightweight cloth and suspended over the deck or cockpit with a plug and a hose to lead water into the tank giving a free water supply from the down pours one occasionally encounters.

Changing the standard bulb in your masthead tricolour for a LED one will reduce consumption from 2 amps to 0.15 amps. The same is true for your anchor light navigation lights and interior lighting.
If you are parsimonious with your resources, you will be able to save quite a lot of energy.


Batteries with the 160 year old lead-acid technology, are until recently the main storage medium for electric energy. We have in this category the choice between flooded or wet cell batteries and dry closed cell batteries. Within the dry closed cell battery family we have two species, the Gel cells and AGM batteries. AGM stands for Absorbed Glass Mat.
Whichever type you choose for the house bank they must be deep-cycle batteries since usage on board a yacht will inevitably cycle the batteries.
Deep cycle batteries are sturdily built and release and take up energy more slowly than the so called starter battery.
Starter batteries are built with more and thinner plates and capable of giving a short powerful burst of electric current as required by starter motors. Starter batteries do not take kindly to deep discharging. All batteries have a limited lifetime.

The life expectancy of batteries depends on:
– The type and quality of the batteries.
– How deep we discharge
– How well we recharge.
– Maintenance in case of wet cell batteries

Wet cell and closed-cell deep cycle batteries of good quality can last for many years. When properly cared for the wet-cell batteries have the longest life expectancy expressed in the number of charge cycles they can handle. As a number lead acid batteries can handle approximately 750 charge cycles which translates to 2-3 years for a full time cruiser.

A popular choice of a type of wet-cell battery is the so-called Golf cart battery. They are good value for money, which is attractive. All wet cell batteries require careful maintenance. They must be topped up with distilled water as well as, after deep discharging, need equalization or reconditioning. Equalization is a process of charging at 16 volts and at low amps for which one needs special equipment. This is important to reverse the internal sulfation that shortens battery life.
Using the Flow-rite system that fits the 6 Volt Trojan and Endurant batteries with the same footprint makes topping up with distilled water simple.

useful website: https://flow-rite.com/battery-watering

Gel-cell and AGM batteries need none of this care but have, in real life at least, a shorter life expectancy than the wet cells. Some say half of the flooded cell batteries. Again, lifetime is to be expressed in number of charge cycles rather than years.

Li-ion rechargeable batteries.
The latest superior development in electric energy storage is the Li-ion battery technology of which the rechargeable Li-phosphate cells have the safety characteristics we are looking for in marine use.
– light and small compared to the lead acid types.
– low self discharge about 1% per month.
– allows discharge of 70-80% of capacity without damage or voltage drop.
– quick charge to 100% without gassing.
– recyclable non poisonous materials.
– very long life expectancy of more than 3000 cycles.
– very expensive but prices are coming down.
– need a new monitoring system when replacing a lead acid battery bank.

Self discharge.

All batteries suffer from self-discharge caused by not well understood chemical reactions even without any connection or load between the electrodes. The rate of self discharge is different for the various types of battery and higher in higher ambient temperatures.

Battery bank capacity and infrastructure.

How much bank capacity do I need for my usage.

This depends on the answer to the following questions:
– How deep do you want to discharge your batteries in the 24-hour period?
– How often do you want to recharge them in this period?
– How completely can you recharge in reasonable time?

Discharging a lead-acid battery for 100% before recharging dramatically reduces its life expectancy. It basically kills the battery. Discharging to 50-60% of the capacity is more realistic and generally accepted as the upper limit of discharge if you want a reasonable lifetime for your batteries. Therefore for the 260 amp. hours of use we calculated we need to install a minimum of 520 amp. hours of battery capacity.
Take into account that the capacity of batteries will diminish over time installing at least 600 amp. hours looks a more realistic proposition.

Recharging lead-acid batteries at first goes fast with high amps going into the bank, the so-called bulk-charging phase. Some time later, with climbing voltage of the batteries, the charging current tapers off entering the absorption phase. The last 20% or so of the required re-charge takes more time than the first 80 %. To reach 100% level of recharge will take many hours. To reduce charging time we stop charging when the charging current tapers off to say 20 amps and the battery monitor tells us we are in the absorption phase. We assume we have charged up to 90%. Now we can take out only 30% of the rated capacity of the battery bank when we don’t want to discharge below the 60% capacity level. Our 600 amp. hours battery bank that has been charged up to 90% or 540 amp. hours capacity therefore allows us to take out 180 amp.hours, which will get us down to 360 amp. hours, before we need to recharge.
This leads in most cases to recharging twice daily.

The closed cell gel and AGM batteries are maintenance free.
AGM has some advantages over the more common gel cell batteries. The AGM battery was developed for military use and is lighter and more robust than wet cells or gel-cells.
AGM batteries have a high discharge capability and are thus good starter batteries. They endure deeper discharging than wet cell or gel-cells without damage. This makes them also deep-cycle batteries. Their internal resistance is low and they accept therefore higher charge currents resulting in shorter recharging time and better recharging levels. For calculation purposes, if you can recharge quickly to 90% and discharge to 40 %, then you will have available 50% of the battery bank capacity as opposed to 30% in case of the wet or gel-cells. Theoretically this would lead to needing a bank capacity only just more than half of that required for wet or gel-cells
AGM’s are charged at the same voltage as wet cells and, because they accept high currents, you must make sure the alternator is capable of delivering this high current without overheating.
Another advantage is that the self-discharge rate is even lower than that of gel cells. On the down side, their life expectancy seems lower compared to wet cell and gel cells particularly in tropical conditions. The Smart regulator would need an AGM setting

For Li-ion batteries the calculation differs since the permissible discharge is at least 70% of capacity without voltage drop and the recharge to 100% capacity is quick. Thus more than double the capacity of a Li-ion bank is available compared to a comparable lead-acid bank in theory allowing for a much smaller bank and/or less frequent charging.
Together with a very long life expectancy this would partly compensate for the much higher initial cost which will over time be coming down making Li-ion attractive.

Making your choice based on financial calculations is a guess. There are too many assumptions and no warranty about useful lifetime. Battery lifetime depends also on factors such as laying up for a long period of time, temperature etc. One thing is certain wet cell batteries will only add to that list of maintenance jobs to do!

The basic electric infrastructure should be:

  • One big house bank for domestic use. If you have two separate and smaller banks, you will discharge too deeply one and than the other bank with life shortening effect. This one big bank is to be charged with our choice of generator(s).
  • A dedicated starter battery separate from the house bank. The standard alternator that comes with the engine will charge this battery, but in case something goes wrong with your starter battery you would want to be able to start the engine or generator from the house bank. A heavy duty lead could do that job.
  • A battery monitor is a device that measures the amps going in or out of the bank, the number of amp. hours taken out or put back in; the temperature of the battery when charging and the voltage of the bank.
  • A separate inverter/charger when one has a generator or an external power connector for shore power.
  • A smart charger instead of the standard regulator that comes with the alternator or generator is necessary to reduce the charging time. The standard voltage regulator on an alternator is set to a fixed output voltage that is mostly too low for sufficient recharging over limited periods of time. This is to avoid overcharging when running the engine continuously. A smart charger is a regulator that keeps the charging voltage high until the batteries are sufficiently recharged and then smartly reduces the output voltage to avoid overcharging. A smart charger should also monitor the temperature of the batteries when charging and reduce the charging current before the temperature rises to a damaging level. Particularly important for closed cell batteries since one cannot replace water lost by gassing.  A smart charger with an equalization mode would be very useful when you decide to have wet-cells for your battery bank.

Generating the energy.

Energy has to be harnessed from somewhere and somehow. There are the eco friendly systems that use wind, water or sun, and there are those that require fossil fuel to generate power.

  • A second, heavy-duty, alternator on the main engine. This is a must.
  • Solar panels require direct sunlight to be efficient. On a yacht they often have shadows thrown onto them. To work at their rated output they should be angled so that they face the sun at a right angle without shadows from other equipment.
Quiet but needs strong winds
Quiet but needs strong winds
  • Wind generators. Only work in a breeze! When sailing downwind the apparent wind is less and that in turn affects output of the generator accordingly. Some, in theory high capacity generators are so noisy that one switches them off to make life bearable. When at anchor the wind is rarely constant or strong enough to produce more than a few Watts a day.
  • Propeller shaft Alternator. Effective but only when underway. One needs enough room round the propeller shaft to mount a generator. Not commercially available so a trial and error procedure to get it right.
  • Water towed generator. Only works when underway and interferes with trolling a fishing line. Also gets snarled up in seaweed and other flotsam. Retrieving the propeller is difficult and potentially dangerous.
  • Small petrol portable generator. They need petrol and are noisy. One should consider one that uses the same petrol mix as the outboard motor in case of a 2-stroke motor. Anchor far out if you decide to use one.
  • A dedicated, stand-alone diesel generator.  The solution for most yachts.
  • A fuel cell. This is a green and promising new development of a 180 year old technology. Hydrogen combined with oxygen in a fuel cell produces electricity and water and a little CO2 in the case of methanol as fuel. The problem is availability of the fuel.

What to choose?

Effective but noisy and unstable
Effective but noisy and unstable

– All yachts should fit a second heavy-duty alternator on the main engine to supply a good charge when motoring but running the main engine for 3-4 hours a day without a proper load doesn’t do it any good.

– Solar, wind and water power options have their limitations in output. These “renewables” are a useful extra and as a backup, but in our experience, not a reliable and sufficient source of energy. The most efficient wind generators are also the noisiest. I also found that one of these high efficiency generators is not stable facing the wind and pivots round the base even in a marina. This produces a start and stop noise with the result that the thing will be switched off by the crew (or the neighbours) in higher wind speeds.
– The water generators work well at speeds above 5 knots. Retrieval of the propeller could be dangerous and needs to be carefully planned. One can foresee more development in this green micro-hydro energy field but research is not yet aimed at usage on yachts.
– A fuel cell for smaller yachts is worth considering. The “Direct Methanol Fuel Cells” currently available for marine use derive their hydrogen from very pure 100% methanol supplied in safe containers. These cells operate at low temperature come in 12 Volt and 24 Volt with a power output of 80, 105 and 140 Watt They are fuel efficient, lightweight, small, operate quietly and continuously and are maintenance free. The required pure 100% methanol is not as worldwide available as is diesel fuel. They also work well together with solar, wind or water generators. A Hydrogen gas powered fuel cell is also currently marketed. Again the problem is availability of the hydrogen gas cylinders.
– More power hungry yachts will certainly need a dedicated diesel generator. It is expensive, requires space and it is yet another bit of hardware needing maintenance. This generator produces 110 or 240 Volts as output and thus one needs a separate inverter/charger to charge the batteries. This charger uses also the shore power when plugged in in a marina.


Tips and considerations:
– A solar panel is useful when laying up the boat without external power supply. The batteries get a charge that can counteract their self-discharge. But beware of overcharging and make sure someone tops up the wet-cell batteries regularly.
– Never mix up new batteries with old batteries or different types of battery in your house bank. Always change the whole house bank even if only one battery fails. All batteries should ideally be of exactly the same age and brand.
– One should be able to switch the house bank to the starter battery. This could be very handy when the starter battery fails. The simplest way is, a heavy-duty set of starter cables.
– Put as much insulation around the fridge and freezer as possible to reduce running time of the compressor.
– Size Matters! A bigger house bank will gives the batteries a more than proportionately longer life expectancy.
– Choose as big a second alternator as can be fitted on the main engine. It will be less susceptible to overheating because it will work less time at full capacity in the hot engine room. This is especially important when you make a choice for AGM or Li-ion batteries.

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