Today’s voyaging sailboats have extensive electrical systems. Unfortunately, we can’t directly store electrical energy. Except when an engine is turning an alternator or when a solar panel, wind generator, or towed generator is in use, we rely upon the electrochemical reactions in a lead-acid, deep-cycle storage battery to supply our electrical energy needs.
Batteries used in our boat’s electrical systems therefore must be both reliable and long lasting. The life of a deep-cycle battery depends to a significant degree on how much stored energy is withdrawn during each use cycle and how promptly and completely the energy removed is replenished. Management of the discharge/recharge cycle requires knowledge of the amount of energy remaining in a battery. The battery-management task is often complicated by a sailor’s desire to run the engine for as short a time and as infrequently as possible. We hope that the following information will assist you in achieving your goals in electrical-energy management.
A boat’s storage battery bank may consist of a single group of large cells, connected in series to produce the desired voltage. When lead-acid cells are used, each of which produces a nominal voltage of 2.1 volts, six are usually connected in series to produce a battery whose full charge voltage will be 12.6 volts. The cells may be individual units or housed in groups. The most common cell groups used on boats are single cell, groups of three cells (six-volt battery), or six cells (12-volt battery).
Three different types of lead-acid cellsflooded, gel and absorbed glass mat (AGM)are used in marine deep-cycle batteries. Flooded cells, those with removable cell caps, are the most common. The battery industry refers to the other two types of cells, gel and AGM, as Valve Regulated Lead Acid (VRLA) batteries. Each type of cell has particular virtues and liabilities that we won’t cover here. Regardless of which type of battery you choose for your boat, it is important to use only one type in the battery bank. The charging requirements of each type of cell vary enough to cause problems if you mix types.
A few basic rules apply to the use and recharging of any battery bank. Except in special circumstances, batteries in the bank should be discharged individually and recharged collectively. An effort to start a reluctant diesel engine with depleted batteries is an example of the special circumstance where paralleling all available batteries during discharge is advisable. Proper management of a battery bank consisting of a number of individual batteries requires switching between batteries, using manual selector switches, as necessary to equalize the amount of energy withdrawn from each battery prior to recharging. All batteries in the bank are connected in parallel for recharging. If desired, switches can be used to automatically parallel all batteries whenever power is available from an engine-driven alternator. When charging, the battery with the lowest state of charge will automatically accept the greatest amount of charging current.
Batteries have a finite life. The end of useful life for a deep-cycle battery is defined as the point where its energy storage capacity is reduced to 50% of what it was when new. The number of times a deep-cycle battery can be discharged and recharged before reaching the end of its useful life depends in large measure on how much of the stored energy is withdrawn during each discharge cycle.
The deeper the average discharge on each cycle, the fewer cycles before the end of useful life. For most deep-cycle batteries, limiting the average discharge to 50% of new battery capacity will yield an acceptable life-cycle energy balance.
Batteries should be recharged as soon after discharge as possible. The recharge process is only about 80% to 87% efficient; thus the amount of energy used to recharge the battery must typically exceed the amount removed during the discharge cycle by 15% to 20%.
The battery/engine balance
Voyaging sailors usually wish to achieve two goals in managing their electrical energy system: minimum engine running time and maximum battery life. Finding a suitable balance between these opposing goals can be challenging. It is important to observe a few battery management guidelines when solving the problem of the discharge/recharge cycle.
1. The average charging current supplied to a flooded, deep-cycle battery should not exceed about 20% to 30% of a battery’s amp-hour (AH) rating, with the initial charging current limited to about 30% of the AH rating. Repeated charging at higher rates can decrease battery life.
2. The charging process does not proceed linearly with time. Restoring the battery to 70% to 75% of charge occurs rapidly. Reaching 85% of full charge requires proportionally more time. Achieving 100% recharge by running the engine is often too time-consuming to be practical.
3. It’s prudent to assume from the beginning that the useful AH rating of the battery will be 85% of the manufacturers rating.
We can begin our analysis of the energy balance problem with an estimate of the average electrical load that must be supplied from the batteries when the engine is off and no other electrical energy source is available. A total average consumption of 20 amps will usually allow for the refrigeration, autopilot, instruments, radar, radio, lights, and potable water and bilge pumps. We next need to provide for the power consumed by equipment that is likely to be operated only with the engine. The 25-amp drain of a 12-volt motor-powered reverse-osmosis watermaker is a useful example. In this example, the total electrical load imposed on the alternator, before allowance for battery charging, is 45 amps.
For the purpose of this example, we will assume that the engine of our voyaging sailboat is equipped with an alternator or alternators that can deliver a total of 100 amps (the alternator’s hot rating) when the engine is run at 60% of maximum rpm or higher. With 45 amps required to power the boat’s ongoing consumption, a total of 55 amps will be available for battery charging. Most efficient use of the engine’s electrical energy output will occur when all of the current available for battery charging can be absorbed by the battery bank.
Using the rule of thumb that the charging current delivered to a flooded-cell deep-cycle battery should be between 20% and 30% of the amp-hour rating of the battery, a battery bank that can absorb 55 amps would have a capacity of 275 amps (55A = 20% of 275AH). Recognizing the difficulty of achieving a 100% recharge and wishing to be conservative in our design approach, we will assume that the actual useful capacity of the bank is 85% of the total installed capacity: 85% of 275 AH = 234 AH. Using a goal of limiting energy withdrawal to 50% of capacity, we will plan to consume up to 117 AH before restarting the engine and recharging the bank (in practice we will round up to 120 AH consumption). When the engine is restarted we will have to restore the 120 AH consumed, plus approximately 20% to account for inefficiency in the charge process and other minor losses. Therefore, a total of 120 AH x 120% = 144 AH that must be delivered to the battery bank. With 55 amps available from the alternator the recharge will take a little more than 2.5 hours. Once again displaying our conservatism, we will elect to run the engine for a total of three hours per charge cycle.
Continuing this example, the assumed sailing electrical load of 20 amps can be supported for about six hours before the total amount of energy withdrawn from the battery reaches the 120 AH limit. Therefore, the discharge/recharge cycle would total nine hourssix hours of energy withdrawal followed by three hours of battery charging. Any reduction in the energy required by other electrical loads during periods of engine operation may contribute to more rapid battery charging; however, the suggested maximum level of charge current delivered to the batteries should be observed. We may want to modify the theoretical nine-hour cycle since it does not fit our 24-hour-clock-based lives. A modified cycle might begin with an engine-run period commencing at 0600, ending at 0900, followed by a two-hour run from 1400 to 1600. The day’s third engine-run period would be from 2100 to 2400. It is likely that the boat’s energy demand will be somewhat less in daylight hours than at night, thereby allowing more energy for battery charging and justifying the two-hour engine operating period from 1400 to 1600. In any event, the loads are approximate, and ultimately your decision to restart the engine will be based on the actual state of charge of the battery bank. Since it is unwise to operate a diesel engine with a very low load and at a speed high enough to provide full alternator output, we will be running the engine with the transmission in gear. For a typical 50-hp diesel, operating at about half power (60% of maximum rpm for most boats), the fuel consumption will be on the order of 0.8 to 1.0 gallons per hour, 6.4 to eight gallons per day.
Designing different cycle times
It is possible to design the electrical system to provide whatever time cycle you require. Using a higher-capacity alternator, or two alternators (preferable from a reliability standpoint) will allow recharging of a higher-capacity battery bank in the same amount of engine running time. A larger bank offers two opportunities: a longer period between recharge events or by reducing the depth of discharge of the batteries from 50% to perhaps 30%, an increase in battery life and a decrease in engine running time per recharge cycle. The trade-off opportunities are endless, limited only by the ability of the boat to carry the equipment (alternators and batteries) and the cost of the system. Working the problem can consume many a winter night. (Perhaps this issue might be made into a voyager’s board game.) Keep in mind that fitting of large-capacity alternators to engines cannot be done by simply bolting them in place. There are limits on the amount of mechanical load that may safely be applied to the front bearing of an engine. Whatever energy is consumed by the alternator(s) must be deducted from the energy available to propel the boat. The horsepower required to power an alternator can be approximated by converting the electrical energy output into horsepower (746 electrical watts = one hp) and doubling the result to account for alternator and drive inefficiency. For example, a 150-amp alternator delivering 14.6 volts is producing 2,190 watts, 2.94 hp, and will therefore consume about six hp from the engine. Consult the engine manufacturer’s installation manual before changing to or adding a high-power alternator or any other belt-driven load to an engine. All of the foregoing assumes that you know when to stop withdrawing energy from the batteries and begin the charging process. To do this you must know how much energy remains in the battery or batteries in the bankthe state of charge.
There are a number of means for determining the state of charge of a lead-acid battery. The most direct means is to measure the percentage of acid present in the electrolyte of each cell in the battery. The electrochemical reaction responsible for the flow of electrical current can be directly related to the acid concentration. With a flooded-cell battery (the ones with cell caps that can be opened) you can use a battery hydrometer to directly measure the specific gravity of the electrolyte (the mixture of distilled water and sulfuric acid in the cell). Batteries are usually supplied with a data sheet that states the specific-gravity measurement when the battery is fully charged. Batteries built for service in hot climates may be built with electrolytes having a slightly lower specific gravity than those intended for use in cold areas. This measurement is always referenced to a liquid temperature of 80° F. Battery hydrometers have built-in thermometers to permit a simple correction to be made for electrolyte temperatures that may be below or above the 80° F reference. As the battery is discharged, the amount of acid in the electrolyte decreases. The electrolyte in a fully discharged battery is only slightly acidic. The relation between the state of charge of the battery and the ratio of acid to water, the specific gravity, is linearthe less acid, the more fully discharged the battery.
Using specific gravity measurements to determine a battery’s state of charge conveys an important additional advantage. This check will disclose differences in cell condition. All cells of a battery may not be at an equal state of charge. A significant difference in specific gravity readings between cells in a battery may indicate the need for an equalization charge (a special type of recharging process), or it may indicate that one or more cells are defective or nearing end of life. Since the internal wiring of a battery is in series, the failure of a single cell in a battery will cause the entire battery to fail. The purpose of the equalization charge is to restore, insofar as possible, all cells to the highest state of charge. It is accomplished by applying a higher-than-normal charging voltage to the battery while limiting the charging current to a relatively low level, perhaps two to three amps for a 100-AH battery. The battery undergoing the equalization charge should be isolated from the remainder of the batteries in the bank. Incandescent lights should never be powered from a battery receiving an equalization charge; the higher voltage will seriously shorten a bulb’s operating life.
Equalizing when underway
It may be possible to perform an equalization charge of a battery while underway. All loads will have to be supplied from the battery bank, with the battery to be equalized switched out of the circuit and connected to the engine-driven alternator. The alternator, assisted by a manual voltage regulator or an automatic regulator equipped to perform equalization charging, then delivers the necessary controlled current to the battery being equalized.
Since many shore-powered battery chargers provide for this type of charging routine, it may be more expedient to wait until next in port to perform the equalization charge or, if the boat has a generator set, power the charger from its output. Under no circumstance should any VRLA battery, either gel cell or AGM, be subjected to an equalization charge.
The voltage of a lead-acid battery can be an accurate indicator of the state of charge. The open circuit (no connected load) of the battery is linked to the percentage of charge remaining. The voltage must be measured with a sensitive meter. The differences of interest are only tenths of volts. The measurement must be made with all loads disconnected from the battery. If the battery has recently been charged, the immediate effects of the charging processthe surface chargemust be eliminated before the measurement is made. This is easily accomplished by using the freshly charged battery to power a low-power consumption load, such as a cabin light or two, for five to 10 minutes. The inaccessibility of the electrolyte in gel-cell and AGM batteries makes a voltage check the only means to measure the charge of these units.
It is possible to use an amp-hour meter, which acts as a form of fuel gage. The battery’s rated energy storage capacity, in amp-hours, is used as the 100% charged estimate. The meter can be set to indicate that number of amp-hours at the beginning of the use cycle. The number of amp-hours delivered to the load is subtracted from this number, providing a continuous reading of the theoretical number of amp-hours remaining in the battery. For example, for a nominal 100-AH battery, we may choose to end the use cycle and recharge the battery when the AH meter indicates that 50 AH have been withdrawn from the battery. Although convenient, this approach may fail to account for the loss of energy storage capacity that occurs as the battery ages. A check with a digital voltmeter can always be made to check the actual state of charge of the battery. In fact, given the very low cost of accurate digital voltmeters, you may want to permanently connect a voltmeter to each battery in the bank or install a multi-position switch so that voltage checks can be made easily. n
Contributing editor Chuck Husick is a sailor, pilot, and Ocean Navigator staff instructor.