The State of Storage

Many (for some, virtually all) of our daily activities depend on batteries, especially the rechargeable batteries we use to start our cars, and power our cell phones, laptop computers, electric tools, toothbrushes, handheld radios and almost everything electrical on our boats. We need to know how to select, use and care for our batteries. For voyagers this is especially true, as they can spend long stretches away from any source of power other than their vessel’s batteries.

A wide variety of cell chemistries are used in storage batteries, including lead-acid, nickel-iron, nickel-cadmium, nickel metal-hydride and lithium-ion. The overall balance of energy density, volumetric efficiency, cycle life, maintenance requirements and cost results in our using a modern version of Gaston Plante’s 1859 lead-acid battery.

Lead-acid batteries are produced in a number of task-specific types. Starting, lighting and ignition (SLI) for vehicles, diesel starting batteries, dual-purpose starting and deep cycle batteries for light marine and RV use, and deep cycle batteries for specific applications including traction batteries for forklift trucks, stationary batteries for cable TV and cell phone systems, boats and railroad locomotives.

What is a deep cycle battery? The battery industry trade association, Battery Council International (BCI), defines a deep cycle battery as one that provides a low, but steady level of power for a longer period of time than a starting battery. The term “deep” refers to the fact that this type of battery is designed to withstand repeated removal and restoration of a significant part (usually to a recommended average limit of 50 percent) of its total stored energy. In comparison the energy removed from an SLI battery when cranking an engine is usually less than one percent of its nominal amp-hour rating. The number of discharge/charge cycles a deep cycle battery can deliver before it must be replaced is largely determined by how much of its stored energy is withdrawn during each cycle, the management of the recharge process and the lifetime it was designed to deliver.

The 50-percent depth of discharge proposed by the BCI definition is intended to maximize the total amount of energy the average deep cycle battery can deliver during its life. The less energy removed in each cycle the longer the cycle life, however, the total amount of energy provided by the battery during its life may be less than what would have been obtained by withdrawing 50 percent of the stored energy during each cycle. Repetitive withdrawal of much more than 50 percent of the stored energy reduces cycle life. Cycle life is also affected by the rate of energy withdrawal. High current loads reduce the amount of stored energy that can be recovered during discharge. It’s important to remember that a deep cycle battery is considered totally discharged (0 percent of charge) when its open circuit voltage (no load connected) is 11.8 volts.

Styles of deep cycle

Deep cycle batteries are available in three styles: open cell (or flooded liquid electrolyte batteries), and two kinds of VRLA (valve regulated lead-acid) batteries – gel cells, in which the electrolyte has been mixed with a gelling agent, usually fumed silica, and AGM (absorbed glass mat), in which virtually all the electrolyte has been absorbed in a fiberglass material similar to blotting paper. Gel cells are generally losing marine market share to the AGM style.

Boats that allow reasonable access to the battery bank can use either flooded cell or VRLA batteries. Flooded cell batteries require periodic inspection of the level of the electrolyte in each cell and addition of distilled water to make up for the amount lost when, during the last stages of the charging process, some of the water is converted into hydrogen and oxygen gas. A monthly check of electrolyte levels is generally sufficient. This interval can be extended to an annual check with the use of special catalytic cell caps that recombine the evolved gases into water; however, the cost of the special caps usually restricts their use to very large capacity, and therefore expensive, batteries. The battery box for flooded cell batteries must be ventilated to allow the hydrogen gas that evolves during the last part of the charge cycle to dissipate (a concentration in excess of 4 percent hydrogen in air can be explosive), fortunately hydrogen is easy to disperse .

Gel cells and AGMs are referred to as recombinant batteries. The hydrogen and oxygen evolved in a VRLA battery is trapped within each sealed cell, raising the pressure to the degree sufficient to assist the recombination of the gases into water. The cells of a VRLA battery are fitted with pressure relief valves that will open in the event an internal fault or overcharging creates excessive internal pressure. VRLAs require no maintenance attention other than keeping the battery cable connections tight and the top surface clean.

Use, maintenance and cost

Choice of battery style is best decided by considering how you use your boat, your willingness to maintain a battery and the cost. Flooded cell batteries are available in sizes and with cycle life ratings far in excess of any VRLA battery; however, a flooded battery requires regular maintenance. With proper care, gel cell, AGM and flooded cell batteries are available at a quality, energy rating and life expectancy suitable for most any recreational vessel. VRLAs have a significantly lower rate of self-discharge and are generally more tolerant of delayed recharging than flooded cell batteries, however, VRLAs are usually more sensitive to charging voltage differences than flooded cell batteries. The difference in charging voltage required by each style of battery makes it desirable that all the batteries on a boat be the same style unless special provisions are made to ensure that each battery is charged properly.

If you are satisfied with the service provided by the present batteries you won’t likely go wrong by just buying a new set. If your boat has more than one “house�VbCrLf battery it’s usually best to replace all at the same time. However, if you have encountered problems with the existing batteries or if you plan to change the way in which you use your boat you’ll need to look deeper into the battery selection process. The primary questions you will need to answer are: 1. How much energy storage capacity do you need/want? 2. How often do you plan to use your boat (number of cycles per year)? 3. What style of batteries do you want/have to use? 4. How much are you prepared to spend and?

5. How critical would it be if a battery failed?

The energy storage capacity you need in your boat’s battery bank will depend on the average power demand and the length of time the load must be sustained without resorting to an alternate energy source. First, list all of the electrically powered equipment you plan to operate from the battery bank. Enter the current drain (amps) for each 12-volt consumer. Convert the wattage of AC loads powered from an inverter into equivalent DC current drain by multiplying watts by 1.15 (to allow for average inverter inefficiency) and then divide by 12 to obtain equivalent DC amps (for example, a 1,000-watt microwave cooker will impose a 96-amp load on the battery bank). Multiply the amp load of each consumer by the length of time in minutes it will be powered during each operating cycle (the period of time when no other source of electrical power will be available). Divide the sum of the amp-minutes by 60 to obtain the number of amp-hours of energy your planned use of electrical power consumers will require. Double this number (to limit the average per cycle discharge of the battery bank to 50 percent) to provide the starting point for your battery bank design. If you customarily limit operation of the engine or genset to the minimum needed to recharge the battery bank consider making the battery bank capacity three times the calculated cycle load since it’s likely the batteries will be recharged to no more than about 85 percent of capacity.

Next, decide the number of battery use cycles you require before the batteries are recycled to become someone else’s new battery. The required cycle life will depend on how frequently the boat is used and the battery charging method. Many boats are used only on weekends and during an annual two-or-three-week vacation cruise. They may accumulate only 70 to 90 days of use and, therefore, battery cycles a year. Batteries in boats that don’t have shore-powered battery chargers and are not equipped with solar or wind energy sources will accumulate the equivalent of an additional 20 to 30 use cycles since the batteries will rarely be fully recharged. Almost any good quality deep cycle battery will be suitable for a boat used in this way and with proper care can be expected to provide three or more years of service. All batteries deteriorate over time; therefore, buying a battery capable of providing thousands of operating cycles will be a poor investment for a boat used in this way.

Cell life

A 112-volt battery is comprised of six cells connected in series. The failure of any one cell will render the battery useless. Voyagers planning a long offshore cruise may wish to install a battery comprised of individual, high capacity two-volt cells and carry a spare, dry, charged cell that can be used to replace a weak or defective cell. Using flooded cells makes it possible to use a battery hydrometer to measure the specific gravity of the electrolyte, a direct measure of the cell charge state. Knowing the condition of each cell makes it possible to spot a weakening cell and with use of an “equalization�VbCrLf charge (a higher than normal charging voltage at low current) to restore the battery to optimum condition. (Many AC-powered battery chargers automatically perform the equalization charge when the boat is not in use.) Equalization charging is not recommended for VRLA batteries that would be damaged or destroyed by the higher charge voltage .

A battery bank is usually comprised of a number of six-cell batteries, although you can assemble a truly massive single battery. If you want a really substantial amount of energy storage consider the two-volt Rolls 2 YS 31PM cell that has a 20-hour rating of 2,430 (121.5A for 20 hours) and a reserve rating of 7,772 (more than 3,200 AH). Used conservatively to a 50-percent depth of discharge this battery would support a 60-amp load for 20 hours. It weighs 285 pounds and has a list price of just over $2,000.

The 50-percent depth of discharge design life of the cell is 3,200 cycles. A six-cell battery would also provide about 1,800 pounds of very useful ballast. However, a 230 amp-hour, size 8D deep cycle battery that weighs approximately 160 pounds is about the heaviest single battery most boat owners are likely to want to deal with. Therefore, most battery banks will be comprised of a number of six cell batteries. All the batteries in a bank should be of the same style. Flooded cell, gel cell and AGMs require somewhat different charging voltages. For best results the batteries should be identical in size (capacity), manufacture and age. Equalization charging, a desirable process for flooded cell, is generally disallowed for AGMs and can ruin a gel cell.

Separately or in parallel?

There are two schools of thought about the way in which the individual batteries in the bank should be employed, separately or connected in parallel and treated as a single, large battery. Paralleling identical batteries eliminates the need to switch from one battery to another to equalize the amount of energy withdrawn from each battery. However, battery manufacturers generally agree that batteries should not be connected in parallel during discharge unless they are identical in type, size, age, condition and, if possible, manufacture. In addition, paralleling batteries can create a “silent�VbCrLf problem if one of the batteries degrades faster than the others or if one cell in a battery fails and the resulting decrease in total available energy storage is not initially noted. Batteries of differing size or state of charge should be paralleled when powering a short-term, high-current load, such as starting an engine, and then switched back to the normal bank configuration.

Regardless of how the battery bank is managed, used as individual batteries or paralleled, it’s necessary to know the state of charge of either each battery or the paralleled group. A digital voltmeter able to measure to 1/10th of a volt (12.6 for example) will provide the needed information (even the least expensive digital voltmeters usually measure to 1/100th of a volt (12.60) and are sufficiently accurate. The open circuit (no load connected) voltage of a lead-acid battery is a reliable indicator of its state of charge. Aside from some minor differences between flooded cell, gel cell and AGM batteries a six-cell battery whose voltage is 12.6V or higher can be considered to be 100 percent charged. (Significantly higher voltages will exist for a time following charging.) Each decrease of 0.2 volts corresponds to a 25 percent decrease in the batteries’ state of charge.

A voltage of 12.2 volts equals the 50 percent of charge level generally recommended as the point when a deep cycle battery should be withdrawn from service for recharging. It can be inconvenient to have to remove all loads from a battery in service to check the no-load voltage (and the battery would have to “rest�VbCrLf for a period of time before a precise measurement can be made). Therefore, some boat owners allow the battery voltage to decrease to 12.1 volts before switching to another battery or beginning a charging cycle for the entire bank. A deep cycle battery is considered to be at a zero state of charge at 11.8 volts, even though it will continue to deliver energy at much lower voltages. Some of the manufacturers of VRLA batteries who boast the ability of the battery to recover from being “completely�VbCrLf discharged also warn that a small, continuous “parasitic�VbCrLf load as small as a few milliamperes may discharge the battery to the point where the voltage is close to zero, very likely resulting in significant damage to or failure of the battery.

Cost and value

The cost of VRLA batteries is typically about twice that of equal size and capacity flooded cell deep cycle batteries of the quality level sold in marine supply stores. Premium quality flooded cell batteries can be as costly as the VRLAs and are available in larger sizes than VRLAs. The “you get what you pay for – if you are careful and informed�VbCrLf rule applies to the purchase of batteries. The battery you are buying is, like a tire, opaque, generally black (although batteries now come in numerous colors) and impenetrable to visual inspection. You don’t normally cut a tire apart or X-ray it to check its construction quality. The same with batteries; you rely on the reputation of the maker and his guarantee and warranty.

The way in which deep cycle batteries are charged can have a major impact on their useful life. The charging current supplied to flooded cell batteries should normally not exceed 20 percent of their amp-hour rating (with higher current, perhaps up to 40 percent of rating permissible during the first hour for a deeply discharged battery). Some VRLA batteries can safely accept much higher charge currents, however, it is important to conform to the manufacturer’s recommended limit, many of which are the same 20 percent specified for flooded cells. Regardless of which type of battery is used and contrary to any statements made about the superior ability of VRLAs to withstand being left in a partially charged or discharged state, all secondary batteries should be recharged as promptly as possible after use. Recharging to 100 percent of capacity can take many hours and is generally practical only when a continuous supply of power is available, therefore, many users de-rate their battery bank’s capacity, assuming that on average it will contain only about 85 percent of its rated value.

In all of the foregoing comments we have assumed a six-cell, 12-volt DC electrical system. A number of boats use a 12-cell, 24-volt system, especially when providing power to large loads, such as anchor windlasses and bow thrusters. The primary advantage of the 24-volt system results from the reduction in power loss in the connecting wiring. Doubling the voltage halves the current and since the voltage loss due to wire resistance is the product of the resistance and the square of the current the loss can be reduced by a factor of four or, alternatively, a smaller cross section wire can be used. The higher voltage/lower current also makes it possible to provide more power from the motor in the windlass or thruster.

As with most of the things we buy, the service and degree of satisfaction we obtain from the batteries on our boats will be best assured by buying the right size, style and type of product from a reputable manufacturer. The lead-acid battery has been undergoing refinement for a century and a half. There are no “magic�VbCrLf batteries, only well-perfected designs, constantly being improved in small increments. n

By Ocean Navigator