As a marine surveyor, one of the first and most common problems I encounter during vessel inspections is a dead or severely depleted battery. These difficulties almost always originate from a lack of understanding of the type, size and quality of battery that should be fitted to the boat.
Equally as important, if not more misunderstood, is just what is required to properly maintain batteries. After a brief period of initial use, many batteries will show improved performance. But all batteries will begin a downward spiral that can be slowed down only by proper usage and recharging. A very expensive battery — if not designed for the application and recharged through an appropriate charging system — will fail prematurely, often more quickly than a less expensive battery.
It’s important to have a basic understanding of how batteries work and how they are built. Many consumers and marine professionals alike are unaware of the different battery technologies available to power a 12-volt DC system. There is a wide array of boat sizes and styles, each with different electrical requirements, so having a basic understanding of battery technology and its appropriate application can save time and maintenance costs while enhancing safety.
Each installation provides a unique set of circumstances, varying by the vessel in question, the charging methods to be employed, and the boat owner’s requirements — both from use and financial perspectives. Most technicians or salespeople can only advise based on typical circumstances for the average boater. Once you have a basic understanding of the principles and products involved, you will be able to make a better-informed purchasing decision.
How batteries work
A lead/acid battery doesn’t store electrical energy; it converts chemical energy into electrical energy through an electrochemical reaction. Each cell in the battery contains alternating negative and positive plates, between which are plate separators. Each plate has a grid configuration, and within each grid is the plate’s active material. When fully charged, the active material in negative plates is pure “sponge” lead; in the positive plates it is lead dioxide. The plates are immersed in a dilute solution of sulfuric acid, known as an electrolyte.
As the battery is discharged, the acid from the electrolyte combines with the active material in the battery plates, forming lead sulfate and weakening the electrolyte. When a battery is recharged, acid is returned to the solution, increasing the strength of the electrolyte. The used portion of the plate material that formed the lead sulfate is reconverted to active material. Although the same basic chemical reaction occurs across varying battery technologies, differences in battery construction result in different power delivery characteristics and different charging regimens.
For this article, let’s consider there to be three different battery technologies: traditional flooded wet cell (flat plate), sealed valve regulated (SVR) gel cell, and absorbent glass mat (AGM), both flat plate and spiral cell (Optima). The basic types of batteries are marketed as starting/automotive, marine and deep cycle. Within these groups are different technologies applied in battery construction as well.
Starting batteries are capable of providing high bursts of amperage for short periods and have a typical discharge rate of 1 to 3 percent of battery capacity. They have a large number of very thin, porous plates that provide maximum surface area to yield the high burst amperage. The active material on the plates has a low density to accelerate acid diffusion.
It isn’t uncommon for the plates to shift or flex under the constant pounding on small boats. (Starting batteries typically are installed in outboard powered, as well as many entry-level boats.)
In addition to engine cranking, starting batteries can power momentary loads, such as a small bilge pump, or low-amperage loads, such as navigation lights. They are designed for applications where they are constantly recharged (by engine alternator), which will keep their discharge rates below 5 percent. They are rated in CCA (cold cranking amps) or CA (cranking amps), though the rating often isn’t indicated on the case. This is not the view you want to see after a day on the water. Sea Tow says battery and fuel problems make up a good portion of its calls.
Unfortunately, this type of battery won’t tolerate deep discharge cycles and will quickly disintegrate internally when subjected to them. You might find starting batteries inappropriately labeled as “marine” or “auto/ marine.” They usually will be priced much lower and weigh much less than a true marine or deep-cycle battery.
The term marine battery has become very popular, as most battery manufacturers seem to have created one for their product lines.
A true marine battery is considered a hybrid and is designed for the dual purpose of engine starting and intermittent light-duty house service, such as electronics, bilge pump and lighting. They generally contain a greater number of thicker plates than starting batteries, and will be larger and heavier. A true dual-purpose marine battery will have more reserve capacity than a starting battery and more cranking power than a deep-cycle battery. These batteries work well for the one-battery boat or a lightly rigged boat designed for dual battery installation, without the required complexity of advanced charging systems.
Today’s engines use increasing levels of electrical power to operate their computer-controlled ignition and fuel injection systems, so it’s very important to have a starting battery that is rated well above the minimum recommended by the engine manufacturer.
The term deep cycle refers to batteries that are capable of repeated deep discharging. A battery undergoes a deep cycle whenever more than 20 percent of its capacity is consumed before it is recharged.
Over the last several years, the term deep cycle has been attached to many hybrid batteries that aren’t truly deep cycle, but a cross between starting and deep cycle. They will typically contain slightly thicker plates than starting batteries, but not nearly enough to warrant the deep-cycle designation.
Deep-cycle batteries have thick plates, denser active plate material, and a specially formulated grid alloy. High-quality deep-cycle batteries will have solid lead plates as opposed to a lead powder composite. They do not have as much cranking power as a starting battery, as they are designed to provide power over longer periods of time. However, large deep-cycle batteries can handle engine starting in addition to providing reserve power for other loads, even without charging.
Deep-cycle batteries should typically be discharged more than 25 percent of their capacity to be used effectively. If used correctly, deep-cycle batteries can provide 200 to more than 2,000 discharge/charge cycles, depending on the depth of discharge.
They are best suited for use aboard boats with a multitude of powerconsuming accessories, or such highcycling applications as advanced fishfinders, live wells, trolling motors, continuous radio communications and bilge pumps. If your boat has equipment that consumes power when the engine is off — refrigerator, microwave, trolling motor — or on larger cruisers with an abundance of house accessories, you should use a marine deep-cycle battery.
They are easily identified by an amperehour rating of 20 hours, and cost two to three times more than other batteries.
“Golf cart” batteries
These are quasi deep-cycle batteries similar to marine batteries. Commonly installed in banks of six-volt batteries wired in series to provide 12 volts, they have a greater number of plates than a starting battery, and provide longer periods of constant use in deep discharging. They can be discharged up to 80 percent without damage.
Flooded wet-cell batteries have high cranking amperage and are excellent for starting applications, though certain designs are good for deep cycle use. They accept higher recharge voltages, and water can be added for periodic maintenance. The need to add water can be reduced by the use of such products as Hydrocaps, which facilitate the recombination of oxygen and hydrogen during the charging process. This type of battery is generally low in price, and replacements are readily available in standard sizes and terminal configurations.
There are some drawbacks to flooded cell batteries. Due to shedding of the active material from the plates, a continually overdischarged wet cell battery will fail. Although most common in starting batteries, plate shedding also occurs in some deep-cycle wet-cell batteries. (Alloys are added to the lead in flooded batteries to maintain flat plate integrity, which can cause an increased rate of corrosion and shortened battery life.)
Since the container is not sealed, great care has to be taken to ensure that the electrolyte doesn’t come into contact with people or sea water. The electrolyte can burn skin and clothing on contact, and when mixed with sea water it will create deadly chlorine gas.
Other disadvantages are:
• They can only be installed in an upright position.
• They lose capacity and become permanently damaged if left discharged, due to sulfation.
• They shouldn’t be installed near sensitive electronic equipment due to gassing.
• They must be accessible for required periodic maintenance.
• The open-cell design is susceptible to failure in high-vibration applications.
SVR gelled electrolyte lead-acid batteries are pressurized and sealed using special valves. The SVR (sealed valve regulated) nomenclature comes from the fact that the sealing vent is critical to the performance of the gel cell. The valve must safely release any excess pressure that may be produced during overcharging, preventing the cell from being irreparably damaged.
They use a gelled electrolyte and a recombination process, in which the oxygen typically produced on a positive plate in lead-acid batteries recombines with the hydrogen given off by the negative plate. The recombination of hydrogen and oxygen produces water, which replaces the moisture in the battery. It is important to note that a gel cell mustn’t be opened once it leaves the factory. If opened, the cell loses its pressure and causes an imbalance that destroys the recombination chemistry.
Manufacturers tout gel batteries as maintenance-free, as they never need watering. However, connections must be retorqued, and the batteries should be cleaned periodically.
Heavier and more expensive than flooded cells, gel batteries are generally leak- and spill-proof, unless the case is damaged, and under normal circumstances there should be no gassing. They have good deep-cycle life and can be installed upright or on their side with only minor loss of output. Since they are sealed, however, there is no practical way to determine their condition, as the battery cannot be checked with a hydrometer.
A gel cell can be employed as a starting battery, but most alternators and regulators are set higher than 14.1 volts, therefore the charging system must be adjusted for the battery to recharge properly and achieve the best performance and longest life (see companion story).
Overcharging is especially harmful to gel cells because of their sealed design. Overcharging dries out the electrolyte by driving the oxygen and hydrogen out of the battery through the safety valves. If a gel battery is continually undercharged, a power-robbing layer of sulfate will build up on the positive plate, which acts as a barrier to electron flow. Premature plate shedding also can occur, reducing performance and life span.
It is critical that the charging system employed for gel batteries limits the voltage to no more than 14.1 volts and no less than 13.8 volts at 68 degrees F. Batteries used in float service should be charged at 13.8 volts. For deep-cycle service, a maximum voltage of 14.1 should be used. The charger must have automatic temperature compensation to prevent under- or overcharging caused by ambient temperature changes and the recharging process itself. Use of a constant- potential, temperature-corrected, voltage-regulated charger is essential. Constant-current chargers should never be used on gel cell batteries.
The electrolyte in gel batteries consists of a mixture of finely divided silica or sand mixed with a sulfuric acid solution. The gelled electrolyte is highly viscous, and during charge and discharge often develops voids or cracks. This impedes acid flow and results in loss of battery capacity. As these voids continue to increase, more plate area is left dry, unable to provide a path for ionic flow, thus progressively reducing the capacity of the gelled electrolyte battery.
The gel only serves to hold the battery electrolyte captive, and in deep-cycle service it results in lower capacity for the battery rating. Gel batteries usually are acid-starved without sufficient electrolyte to fully activate the plates. The result is lower capacity size for size.
Also, the gelled mixture can liquefy upon charging, due to the shearing action of gassing. After charging, it can take an hour to gel again. During this time, liquid is moving and the battery can leak if an opening has developed.
AGM batteries are the most recent step in the evolution of lead-acid batteries and use a microfibrous silica glass mat, sandwiched between the plates, to hold the electrolyte in place. The electrolyte remains a liquid for the entire battery life. Since the glass mat is only 90 percent saturated with acid electrolyte, the oxygen produced during charge can readily migrate to the negative plate and recombine into water. The AGM material has an extremely low electrical resistance, so the batteries deliver much higher power and efficiency than gel or flooded cell technology.
AGMs are hermetically sealed and operate under pressure to recombine the oxygen and hydrogen produced during the charge process. Like gels, they are considered SVR batteries. The physical bond between the separator fibers, the lead plates and the container make AGM batteries spill-proof and the most vibration- and impact-resistant lead-acid batteries available.
This type of battery can operate in any orientation, and even if a container is broken the electrolyte won’t leak. This enables the batteries, which can be submerged without damage, to be installed deep in the bilges.
AGM batteries use almost identical voltage set points for charging as flooded cells and as such can be used as dropin replacements for flooded cells. Their low internal resistance allows them to accept a high current charge more efficiently than conventional batteries.
Spiral cell AGM technology, like flat mat AGMs, also constrains all of the acid in an absorbent glass mat. The mat is spiral wound, with lead grids, and placed in one of the six cylinders within the battery case to create the lead acid reaction. Each cylinder is compressed and locked tightly into place, creating a durable and vibration resistant battery, able to withstand the roughest conditions.
A battery’s life is being drained even as it sits in the off-season. AGM technology typically limits self discharge to less than 3 percent a month at 77 degrees F. Spiral cell technology costs about twice as much as a traditional flooded marine battery. The added value of low self-discharge and up to twice the life expectancy will pay off in the long run for boaters and anglers with serious battery requirements.