Pluses and Minuses

[Don Casey]

	Before you head off to some exotic location, do your math and find out what you need to do to keep your batteries close to full charge.

I often hear fellow sailors lament their apparent inability to keep the house batteries at anything close to full charge. The striking thing is that the problem seems to come as a total surprise. Balancing your likely consumption with your charging ability is something you can do in front of the fireplace, long before you head off to some palm-tree republic and discover that you need to run your engine five hours a day to keep the T-bones frozen.

Consumption            A balanced 12-volt system starts with the demand side. Both battery and charging capacity follow (directly or indirectly) from electrical consumption, never the other way around, so your analysis necessarily starts with current out. The unit of measurement is the amp-hour, abbreviated Ah. An appliance that draws one amp operated continuously for two hours consumes two Ah. An appliance that draws two amps operated for one hour also consumes two Ah. This would be all you needed to know to calculate your total daily power consumption if all appliances were rated in amps. Alas, appliances are often rated in watts. Fortunately it is easy to convert watts to amps. You simply divide the wattage rating by the operating voltage. In the case of battery-operated appliances, that means divide by 12. So a 25-watt light, for example, draws 2.1 amps from your battery when it is energized.

Armed with no more than this, you can calculate the probable daily discharge level on your house bank. Let’s do this for a hypothetical boat at anchor in the tropics. Since this is just to clarify, we will limit the number of appliances to five.

 

Appliance	Draw in Amps	Hours Per Day	Daily Ah Consumption
Cabin Light	2.1	4	8.4
Fan	0.2	24	4.8
Freshwater pump	6.0	0.1	0.6
VHF (receive)	0.5	14	7.0
Refrigerator	5.5	12	66.0
		Total	86.8

 

Typical draw for VHF transmission is six amps, but unless you are a harbor magpie, it will be the receive function that consumes the most power. If you burn two cabin lights all evening, cabin light consumption becomes 16.8 Ah. Similarly, run two fans and the daily draw doubles to 9.6 Ah. If your refrigerator lacks efficient insulation so that the compressor runs for 45 minutes of every hour rather than the 30 minutes assumed here, the consumption by this single appliance increases to 99 Ah daily.

	Knowing how much power you need is as simple as making a list of every single appliance on board and calculating the daily draw of each one.

Knowing how much 12-volt power you need is as simple as making a list of every single electrical appliance aboard and calculating the daily draw of each one based on your most liberal estimate of daily use. This is nearly always an eye-opening exercise. It is not uncommon for a 40-foot sailboat equipped with a computer and VCR and configured with a factory-installed refrigerator box (translation: large and indifferently insulated) to have total daily power consumption exceeding 200 Ah. A 12-volt watermaker adds another two Ah daily per gallon of water produced.

I cannot overemphasize how essential it is to actually write down this list. Do it in your logbook so that you can later compare your estimates to reality. Entering the devices in ink but the values in pencil will allow you to adjust your estimated daily consumption to unanticipated use patterns.

Storage            Only after you know how many amp-hours you will need daily are you ready to determine the appropriate size of your house battery bank. A generous quantity of alternative charging capacity—a large solar array, for example—can offset the discharge of the batteries in real time, but there will be plenty of days when the sun fails to shine and the wind fails to blow. Unless you are prepared to run your main engine more than once on those days, you will need a house bank capable of supplying all of your electrical needs for at least 24 hours. So if your daily consumption at anchor will be 200 Ah, then you need at least 200 Ah of battery capacity. This does not, however, mean that a pair of 100 Ah batteries connected in parallel will fill the bill. Far from it.

	As a rule, says the auhor, you we can estimate useable battery capacity at approximately 40 percent of rated capacity.

When matching battery capacity to demand, we are talking about useable battery capacity. As a rule, we can estimate useable capacity at approximately 40 percent of rated capacity. Why?  Because the life of lead-acid batteries is severely shortened by discharging them more than 50 percent, so we only have half of the rated capacity available. Additionally, batteries get increasingly reluctant to accept charging current the closer to full charge they get, so replacing that last 10 percent is impractical with engine-driven charging unless the boat is underway. So we typically operate the batteries between 50 percent charged and 90 percent charged. (Good alternative power sources can keep charging for long hours, effectively increasing the useable capacity to 50 percent when alternative energy is available.) The easy way to remember this relationship is that battery capacity in amp-hours should be at least 2 1/2 times daily consumption.

So back to our example, 200 Ah of daily demand will necessitate a battery bank with at least a 500 Ah rated capacity. More capacity is better, since the greater the capacity, the more charging current the bank will accept. This reduces charging times.

Charging             The maximum size of the alternator is determined by the size of the battery bank, not by the daily discharge rate. Deep-cycle wet-cell batteries should not be charged at a rate exceeding about 25 percent of bank capacity.  So if you have a 500Ah house bank, alternator capacity beyond 125 amps is wasted. Gell cell and AGM batteries will accept a somewhat higher rate of charge without damage, but rapidly rising voltage very quickly reduces the amount of current even these types of batteries will accept. So as a practical matter, there is nothing to be gained by fitting an alternator rated at much more than 25 percent of your house bank capacity. Note that this should be “hot” capacity.

	If you do not want to run your main engine three hours a day, you need to reduce your daily consumption, increase the size of your battery bank, or supplement your alternator with alternative energy.

In truth, an alternator that is too large for your battery configuration doesn’t really hurt anything except your bank account, but it will not recharge your batteries any quicker. Here is the harsh reality. Discharge your batteries to 50 percent and you can expect that no alternator/regulator configuration will take the charge level back up to the 90 percent level in less than three hours.

Alternative power sources            As a by-product of propulsion, charging the batteries with an engine driven alternator is essentially free power. But running the main engine at anchor is a costly and inefficient method of keeping the batteries charged. The longer a boat will be away from the dock, the greater the justification for equipping it with an alternative power source, either solar panels or a wind generator, or both.

The sizing of alternative power sources follows directly from daily consumption. If you consume 200Ah per day, that plus about 20 percent for battery inefficiencies is what you need to put back daily. Looking at solar panels first, how much panel capacity would it take to put 240 Ah into the battery bank? Solar panels are rated for the sun directly overhead, but since the sun actual scribes an arc over the boat, you should anticipate no more than the equivalent of five hours of rated output daily.Working backward, we can divide our target output of 240 Ah by five hours to get the amp output required to keep up with the daily discharge. So we need a solar array rated at 48 amps. But panels are typically rated in watts.We can multiply amps by panel voltage—typically around 16—to convert to watts. In this case, we need a solar panel array totaling 768 watts to keep up with the daily load.

	Wind generators are typically rated in amps according to wind speed.

If your alternative charging source will be a wind generator instead, your equilibrium target is still 240 Ah of daily output. Wind generators are typically rated in amps according to wind speed. It is nearly irresistible to use a relative optimistic wind value when selecting a wind generator. If you are going to a trade wind area where the winds average 25 knots, what could possibly be wrong with using 15 knots as the average wind speed in your calculations?  Plenty.  First, most anchorages are protected—the reason they have become anchorages—so at anchor wind speeds will be much lower than those outside the anchorage. And anchorages are near land, which is likely to have its own wind patterns that can and often do cancel the trade winds at night. When estimating the daily output of a wind generator, I recommend that you use the 10-knot amp rating. For example, if a generator is rated at 1.5 amps at 10 knots, expect not more than 36 Ah from this generator daily. So working backward from 240 Ah, if we divide by 24 hours, we find we need a wind generator capable of a 10-amp output in 10 knots of wind to keep up with the load. Typical large-blade (five feet) wind generator output is less than half that.

Bottom line         There is no free lunch. If you do not want to run your main engine three hours a day, you need to either reduce your daily consumption, increase the size of your battery bank, or supplement your alternator with as much alternative energy as you can afford and/or find room for. Of these choices, reducing consumption will be the least costly. It is a point to keep in mind every time you contemplate the purchase of a new or replacement appliance.