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Battery management for the coastal voyager

Apr 14, 2008
The average boat’s DC electrical system is easy to use and generally reliable, however, new boat owners and many experienced sailors may be operating the system in a way that can lead to unexpected problems, especially undetected battery deterioration that suddenly leaves them with only one or perhaps no batteries capable of starting the engine.
The electrical system on a typical sailboat and many power boats is comprised of an engine-powered alternator (an automotive unit with an internal voltage regulator) two lead-acid storage batteries, a battery selector switch labeled “off,” “1,” “both” (or “all”) and “2,” and circuit breakers that protect the DC distribution wiring.

The first possible problem many owners may face is undetected deterioration of a battery. The ability to start an engine is a primary concern for the owner of almost any boat. Since few of today’s diesel engines can be hand started we must rely upon the electric starter. We may not know the actual current demand of the starting motor but usually understand that it can be substantial (more than 150 amps for a typical 20-hp diesel and as much as 900 amps for one in the 400-hp class).
Many boat owners respond to the prospect of the high starting current demand by selecting the “both” position on the battery switch to parallel the batteries so that both can contribute the energy needed to start the engine. Although this practice will usually ensure a trouble-free start, I propose that you will be better served by starting your engine on one battery, not both. Once the engine is running smoothly (usually less than a minute in most climates) the battery selector switch should be moved to the “both” position.

Some (many) boat owners will likely consider this advice to be controversial, if not totally wrong and even dangerous. After all, placing two batteries in parallel will allow the starting load to be shared, reducing the strain on each battery and perhaps ensuring a faster start. However, while it is true that starting with the battery switch in the “both” position will likely reduce the current drain from each battery, doing so may mask a battery problem, that if left undetected, may leave you unable to start the engine at a later time.
Best practical test
You want to know the real condition of each of your batteries before their depleted condition is announced by a failed engine start. Short of using a special battery tester, a single battery start is the most practical way to assess its ability to deliver a burst of energy. Many boat owners believe that measuring battery voltage (a DC voltmeter is often a part of the main electrical panel) will provide an accurate indication of the ability of the battery to deliver sufficient energy to start the engine or supply the house loads while at anchor. Unfortunately this is not the case.

While the open circuit (no loads connected) voltage of a lead acid battery does correlate rather precisely with the state of charge of the battery (>12.6 V = 100 percent charge, 12.4 V = 75 percent, 12.2 V = 50 percent, 12.0 V = 25 percent, 11.8 V = 0 percent) the measurement does not tell you anything about the condition of the active material (spongy lead in the negative plate and lead peroxide in the positive plate) in the batteries plates or about the integrity of the electrical connections between the six cells that comprise a 12-volt battery. The ability of the battery to respond to a demand for a significant amount of power — the 100+ amps required to start even a small diesel engine — is largely determined by how much of the battery’s original active plate area (the area occupied by the spongy lead and lead peroxide) remains. Active plate material is required to support the chemical reaction with the electrolyte that converts the stored chemical energy into an electric current.

Each time a lead acid battery delivers an electrical current, some of the active material in both the positive and negative plates is converted into lead sulfate. The process is reversed when the battery is recharged, however, the reverse process is not 100 percent effective. Over time the amount of lead sulfate that is not restored to its original state (lead and lead peroxide) increases, reducing the amount of active material available for the next use cycle. Ultimately, the ability of the battery to store energy decreases to a level where the battery is considered to have reached the end of its useful life. (The useful life of a deep-cycle battery is defined as the point where its energy storage capacity is half of what it was when new.)
 
Measuring the open-circuit voltage of the battery will indicate the state of charge of the active material in the battery, not how much is available to support a load. A battery whose energy storage capacity may be only 10 percent of what it was when new may deliver 12.6 volts, indicating 100 percent charge, when measured with a voltmeter (a conventional analog mechanical voltmeter will draw a current of about 1/1000 of an amp, a digital voltmeter less than 1/1000th of that small amount). This apparently fully-charged battery might power one running light for a few hours before the light becomes noticeably dim. The voltmeter measurement by itself cannot detect the seriously deteriorated condition of this battery.

Unless your taste for toys extends to owning an expensive battery test unit, or you are willing to remove the batteries from your boat and cart them off to a place where a proper battery condition test can be performed, you will have no better way to determine the real condition of your battery than to use only one battery at a time to start the engine. Starting with the selector switch in the “both” position will allow a good battery to supply the current needed, masking the inability of the deteriorated battery to do any useful work. In most cases an engine in good condition will start within a few seconds, limiting the degree to which the test will assay the energy storage capacity of the battery but it’s the best choice available. You might hold the engine “stop” button while cranking the engine to prolong the start process but that’s not likely necessary — besides no one likes to hear the starter grinding away for long.
Use alternate batteries
In my opinion, alternating batteries when starting the engine is a good practice. Use a note pad or some other aide-memoir to keep track of which battery you used last; using No. 1 on odd days, No. 2 on even days of the month will work, unless your superstitious beliefs require that you begin all voyages on odd or even days. Once the engine is running and has stabilized, switch to the “both” position so that the alternator can begin to restore the energy removed during the start process. Powering an engine starter motor that draws 200 amps for three seconds will drain 0.166 amp-hours of energy from the battery. Allowing for charging inefficiency, this energy drain will be restored in less than one minute once the engine’s alternator is operating normally. Except in unusual circumstances, a marine diesel engine in good operating condition will start in much less than 10 seconds.

The second most common electrical problem encountered by most boat owners is less than expected battery life. A deep-cycle battery is considered to have reached the end of its useful life when its energy storage capacity has decreased to 50 percent of what it was when new. It can be difficult to determine when this degree of deterioration has been reached. Regardless of type, flooded cell, gel cell or AGM, all batteries should be promptly and, if possible, fully recharged after use. The lead sulfate that is formed as a normal consequence of the delivery of electrical energy must be converted back into lead and lead peroxide before the sulfate, which is an insulator, has a chance to harden and thereby become resistant to the recharge process.

A shore-powered battery charger will usually do an excellent job of keeping the batteries fully charged, particularly if it can provide automatic three stage charging and, for flooded-cell batteries, the option of periodic equalization charging. Some shore-powered chargers deal with each battery (up to three or four) individually, however, you will connect the batteries in parallel when recharging them from the engine-driven alternator. Don’t worry that a less discharged battery will be overcharged. Each battery will accept the charging current appropriate for its state of charge since the current flowing into the battery will be determined by the difference between its voltage and the charging voltage.
Charging problems
Battery charging problems occur most often when relying on an engine-driven alternator to recharge the batteries. The automotive alternators fitted on most production boat engines have internal voltage regulators. The voltage necessary to properly charge a battery varies inversely with the temperature of the battery — the higher the temperature the lower the required charging voltage. The regulator contains an internal temperature sensor intended to automatically adjust the charging voltage, assuming that the temperature of the battery being charged is related to the temperature of the engine and alternator.

This method can be satisfactory in a vehicle where the battery temperature is likely to be linearly related to the temperature of the alternator’s case (except in cars where the battery is located under the back seat or in the trunk — these vehicles can have boat-type problems). In a boat, especially in a sailboat where the batteries may be at a much cooler temperature than the engine compartment, the temperature sensor in the alternator will set a charging voltage that is too low to achieve a full charge in a reasonable time. An additional problem results from the fact that the internal voltage regulator assumes that the voltage it is sensing inside the alternator is the same as the voltage at the battery. The batteries in your boat may be 10 or 15 feet from the alternator and at a significantly lower voltage due to the resistance of the battery cables. The net result of relying on the alternator’s internal voltage regulator can include slow, incomplete recharging of the boat’s batteries.

Installing a remotely mounted voltage regulator that senses both temperature and voltage at one of the batteries (the system assumes that all the batteries are connected in parallel and are at the same temperature) will correct the problem. Check the alternator model number to determine the compatibility of the external regulator. If the alternator does not have an external field (F) terminal one may have to be installed by an alternator repair shop. Some external voltage regulators can also sense the temperature of the alternator and will automatically limit the output current when necessary to prevent overheating (the windings in a heavily loaded alternator can reach 400° F). If an excessive temperature problem occurs it will most likely be the result from attempting to charge a very large capacity bank of flooded cell batteries (perhaps more than 400 to 500 amp-hours) or when charging a number of deeply discharged AGM batteries (their very low internal impedance encourages a high-charging current).

When recharging batteries from the engine during a typical weekend cruise its best to assume that unless you are under power for an extended period of time the batteries will be recharged to no more than 85 percent of their nominal capacity. (The final stage of recharging, from 85 percent to maximum is a slow process and is usually accomplished by a shore-powered battery charger over a period of many hours.)

The message is simple: by starting the engine using one battery you will likely avoid discovering that an apparently good battery was in fact seriously deficient in its ability to deliver the required engine starting current, even though it may have been satisfactorily powering the radio and a light or two.     
   

Contributing editor Chuck Husick is a sailor, pilot, flight instructor, electrical engineer and rides a mean bicycle.