A Guide to Understanding Boat Batteries

boat batteries, connecting boat batteries

Knowing what you need for batteries and how to use and take care of them can extend their life.

By Evan Honore — Pacific Powerboat magazine

Batteries for marine use, whether engine start or house batteries (Deep Cycle), can mean the difference between blissful boating or an exercise in crisis management. This article takes a look at the technology and make-up of marine-battery systems. The objective is to help boat owners make informed decisions, understand what is happening to batteries on board, know when to consult a specialist marine electrician, and what makes up the various battery technologies that affect their life and performance.

What many recognize as a typical battery is called a lead-acid battery. This is a plastic box that has plates constructed into cell groups inside. These plates are generally made of lead with metal additives to promote strength and electron flow plus a coating of lead-oxide paste. The solution is surrounding the plates is called electrolyte, generally made up of 35% sulphuric acid and 65% distilled water. The chemical reaction that takes place causes electrons to be produced thus releasing some of the energy stored in the battery.

A battery only stores energy that has been put into it. It does not produce power on its own. An engine-starting battery commonly provides high current for a short discharge period resulting in approximately 1% of the capacity being discharged, easily recovered by the charging system. Conventional charging systems, such as an engine driven alternator, provide a charge for marine engine starting batteries.

marine battery, deep cycle battery, EverStart

The biggest difference between a marine battery and one for automotive use is the “deep cycle” designation.

Basic Principles — Flooded Lead-Acid Batteries

This section deals with the basic principles of the battery, the figures quoted generally apply to conventional ‘ wet’ or ‘ flooded’ batteries commonly used in marine applications. Many common problems are experienced with house batteries because  they are the ones doing most the work. In electrical systems, volts is the electrical pressure and the electrical flow is the amps. The amount of flow depends on the restrictions in the electrical circuit called resistance. The battery simply stores power. 

If the battery is not charged at the right pressure (volts) or does not receive adequate charge because of the amount of current flow or the duration of charge, it won’t be able to deliver if required. It is worth noting that small variations in charging voltages have significant effects.

A half-charged battery has a terminal voltage of 12.2 volts. If we look at the difference between a charging system supplying 13.8 volts to the battery or 14.5 volts, the difference in electrical pressure pushing the charge current into the battery is in 1.6 volts versus 2.3 volts. While the actual difference is only 0.7 volts this represents a 43% higher charging voltage that will roughly represent the increase in charge current or capacity gained by the battery in a given charging period. Batteries and electrical systems can be likened to water tanks and water flow. The battery is the storage tank. It has a positive pressure outlet and a negative pressure return and the electrical system operates on a closed piping system. The alternator is simply an electrical pump used to refill the tank.

Balmar alternator, marine alternator, inboard alternator

An alternator (white object to right) on a marine engine may look like its automotive counterpart, but it is designed to operate without creating a spark.

Charging the Battery

The alternator or battery charger is an electricity producing pump that generally has a voltage (pressure) restricting device on it called a regulator. To force electrical current back into the battery, the voltage of the alternator or battery charger needs to be higher than the voltage of the battery. If the voltage is too low (at the battery), not enough storage capacity will be achieved. If the voltage is too high, the battery may be damaged through overcharging. As with water tanks, the more full (or charged) the battery is the higher the back pressure. So with a fixed inward voltage and a growing back pressure the current flow into the tank steadily tapers downwards.


When the battery nears a full charge condition, bubbles of hydrogen and oxygen gas are produced that leak out of the battery through vent plugs. This condition is normal but does require that flooded batteries are placed in a ventilated space. Some types of batteries do not produce any gas in normal working conditions, but if overcharged even these batteries will produce gas which can be dangerous meaning that ventilation is required for all battery types.

batteries installed in a yacht, properly vented batteries, batteries in series

The cutouts for the cables also act as ventilation outlets for the batteries in this installation.


When a battery is being recharged dangerous gases (hydrogen and oxygen) are given off. More gas is produced at higher charge voltages. If ignited by a mere spark, this mixture will explode and can cause serious injury. Most at risk are the eyes so when working around or on the batteries, particularly during or after charging, always wear safety glasses.

Float Charge

Once the battery is charged, a longer life will be achieved if the charging voltage being applied by the alternator is reduced. A continued application of the normal recharge voltage results in deterioration of the battery’s internals, preventing it from holding or delivering as much capacity. Once the battery is fully charged, the reduced charging voltage is called putting it on “float”. However, even a float charge can cause corrosion of the positive plates. Some marine battery chargers completely automatically turn off, monitoring the battery sate of charge and turning back on when required. Continuous float charging of engine starting batteries is not recommended because it shortens battery life.


If the battery is left flat or partially charged, it develops a condition called sulphation that inhibits current flow into the battery during recharge. This condition can be likened to a buildup of “sludge” in a water tank that effectively increases the back pressure, resulting in less inward flow. Severe sulphation can render a battery useless.

Guest battery charger, triple bank battery charger

A quality battery charger monitors the voltage and adjusts accordingly.


The action of charging also causes the loss of water in most batteries, so it needs to be topped off with clean distilled water. If a sealed battery is over-charged, the water loss still takes place and permanently damages. Terminals should be kept clean and dry.

Maintaining Capacity

The action of charging and discharging the battery causes a change in the consistency of the electrolyte. When the battery is charged, acid that is more dense or heavier than the electrolyte is produced. When the battery is discharged water is produced that is lighter than the electrolyte. As a result the water floats on the top of the heavier acid. This is called “acid stratification.” When the battery is recharged to near full charge, the resulting higher voltage causes production of hydrogen and oxygen bubbles that gradually move up through the electrolyte and out through the vent plugs in the top of each cell.

The movement of these bubbles up through the electrolyte has the effect of mixing the acid. This mixing process is vital to achieve long battery life because acid stratification increases the incidence of sulphation. When house batteries are subjected to normal charge/discharge use, unless subjected to long engine running times or shore power charging, effective DC-DC charging (or similar), the battery is rarely fully charged. The lead sulphate found in the plates (a normal condition) will “harden” over time and is difficult if not impossible to remove. This results in a loss of capacity. This loss of capacity is progressive and cumulative and will result in reduced performance and life.

Optima battery, Optima marine battery, marine battery

Optima makes a popular line of AGM batteries. The blue top indicates marine use.

Depth of Discharge

If the battery is repeatedly drained to low levels of capacity the life will be less than if it is only partially discharged. This is called the depth of discharge or DoD. The greater the DoD, the shorter the life.

Temperature Compensation

When a battery is heated it requires lower charging voltage to receive the correct charging current. Accordingly a lower charge voltage should be applied as the temperature increases. This is called temperature compensation.

Battery Environment

Because a battery is a chemical device, the activity inside is affected by temperature. A common cause of battery failure is grid corrosion. This is the gradual deterioration of the lead based grid/plate that is corroded by the electro-chemical action. The rate of this corrosion is increased with the temperature surrounding the battery when the battery is being recharged. House batteries in particular will last longer if installed in a cool place as opposed to common engine room installations, even the temperature compensated charging is applied.

batter in a boat, battery bracket

A battery installed in a ventilated, cooler space will last longer.

The Difference Between Automotive and Marine Batteries

Batteries for cars and trucks are required to provide a high cranking current to start the engine for a short time. After this is achieved, the battery simply receives charge from the alternator. Only rarely is the battery required to deliver power for any duration of time. To achieve high cranking current output, the manufacturer uses thin plates designed so the acid has good access to the active material that is producing the current. Place this type of battery into a marine environment and it will likely have a short life.

Marine batteries fall into two categories — engine starting and house bank — of Deep Cycle batteries. The engine start type has similar construction to an automotive battery. Quality marine-specific batteries are manufactured to be highly resistant to damage caused by shock and vibration. In general terms, a marine battery will have a more robust construction than automotive batteries that tend to be built to a price rather than a standard. When a battery is regularly charged and discharged, this causes deterioration of the positive plate. Deep-cycle batteries have a thicker plate and a more dense active material that can withstand the pressures of this type of use.

Plates/Capacity — a Popular Misconception

As described above, plate thickness varies and should not be used as an indication of capacity. To give an example, a high performance engine starting battery with 22 plates per cell is available in New Zealand and is approximately 65 ampere hours in capacity, while another battery of 350 ampere hours has only 15 plates per cell. When specifying a battery, the application must be considered. For example, when marine engine starting is required, the MCA (Marine Cold Cranking Amps) rating is an indication of the battery’ s ability to start an engine. Conversely, for auxiliary loads and deep cycle applications ampere-hour capacity must be considered.

Sealed marine battery, maintenance-free battery

A sealed marine battery is designed to be maintenance-free and doesn’t require adding distilled water.

Alternative Lead-Acid Battery Types

Marine batteries are broadly broken into two categories. The most common is the “wet” or “flooded” technology. This is the conventional type which in principle is largely unchanged from the original design going back over the last 100 years. However significant gains in efficiency, charge acceptance, reduced maintenance requirements and increased energy output have been achieved due to technology. Of more recent times the Valve Regulated Lead Acid (VRLA) type, often called “sealed” has been introduced. Some confusion may exist over this type of product as often the battery is a flooded type but maintenance free, not requiring a top up of electrolyte. There may also be some confusion over “Gelled Electrolyte” and the alternative VRLA “Absorbed Electrolyte” or “Absorbed Glass Mat” (AGM) technologies. The following passages will hopefully provide some clarity.

Flooded Batteries

Most of the information provided in the previous paragraphs pertains to flooded batteries. However within this range two types of constructions exist. The most common is “flat plate” construction. This is available in maintainable and “maintenance free” configurations. In simple terms the battery manufacturer alters the alloys used to make the battery so that only small amounts of Hydrogen and Oxygen gas are produced during charging. This results in minimal water consumption over the life of the battery. It should be noted that if tipped over this battery will sometimes leak. 

In larger deep cycle installations where high levels of storage capacity are required, batteries come in the form of 2 volt cells connected in series to make up the required voltage. The reason for this is simply for ease of handling. In the two volt cell form “tubular positive” batteries are available. This involves a different method of construction of the positive plate of the cell, which is robust and resistant to deterioration caused by deep and regular cycling. This product is generally manufactured in Europe and carries a high price point, however over the life, dependent upon application and care and maintenance programs, can represent an excellent value.

Victron battery, sealed battery, VRLA battery

One advantage of a valve regulated battery is that its electrolyte won’t spill.

Valve Regulated Batteries

Years ago it was discovered that by changing the internal construction of the battery and maintaining a positive pressure (3 to 5 psi) inside, it was possible to get any Hydrogen or Oxygen gas that may be produced during charging to recombine internally. This resulted in completely sealed battery designs, allowing the battery to be installed without fear of acid spillage, emission of corrosive gas and almost zero maintenance. In some cases this type of product can be installed on its side and will work whilst submerged without the production of any chlorine gas which is often produced when a flooded battery comes in contact with salt water. 

As briefly mentioned above, within the VRLA group two different types of technology exist. The most commonly used in a marine environment is the Gelled Electrolyte. Provided it is well manufactured, this type of product provides excellent cycling characteristics with all the benefits of a valve regulated product. The electrolyte inside is in a “jelly” form so no spillage is possible. The second type of VRLA uses a material like blotting paper made from glass fiber to retain the electrolyte. This method also removes any loose electrolyte from sloshing around which may spill or leak. Independent tests tend to demonstrate that Gelled Electrolyte type batteries are better suited to a cycling (charge flat) application like marine house batteries than the AGM type, although premium quality products from AGM manufacturers such as the Lifeline brand provide highly competitive and proven technologies.

In reality the VRLA battery is not everything to everyone and have had some reliability problems in the past. Some of these problems can be blamed on factors not provided by the battery itself but in truth the VRLA battery is more “sensitive” than the tried and proven flooded battery.

Hella Endurant battery, deep-cycle battery

Hella’s Endurant is a highly regarded VRLA battery for boats.

This delicacy means that unless the battery is manufactured to perfection and well cared for through an accurate charging program it could provide a shorter life than a cheaper alternative flooded product. To provide an example: If a vessel suffered alternator regulator failure during a passage and the batteries were over charged, in the case of flooded batteries, the lost water would be topped up and would suffer little damage (provided the regulator was replaced within a reasonable time). In the case of VRLA batteries (depending upon running time) it is possible, if not likely that the batteries would require replacement along with the regulator. 

This is not to say that VRLA batteries do not provide a good or even the best option in some instances but these options need to be explored, ideally with a respected marine electrician who can discuss all of the options rather than just one technology. Quality AGM batteries are robust and due to the compacted internal construction perform well under in harsh vibration where traditional flooded batteries may shed their active material through vibration off the plates, gradually losing capacity.

AGM brands such as Optima, Lifeline and Hella Endurant are specified for many commercial applications both engine-starting and deep-cycle applications where they consistently outlast their traditional flooded equivalents. As with any choice of equipment and systems onboard, not all gelled or AGM products are created equally. The old adage of “you get what you pay for” rings true with battery technology. Nationwide service, support, company reputation, the country of origin and company manufacturing the product helps build a complete picture of the quality and reliability.

Battery Tips

  1. A battery stores charge it does not “Manufacture” it.
  2. Insufficient charging voltage will cause poor battery performance.
  3. Insufficient charging voltage will cause short battery life.
  4. Excessive charging voltage will cause short battery life.
  5. Leaving a battery partially charged will reduce life and performance.
  6. Periodically batteries need to be fully charged to maximize life and performance.
  7. Engine Starting Batteries are required to deliver high current for a short time. If this type of battery is subjected to many cycles of charge/discharge a short life is likely.
  8. Deep Cycle Batteries are designed for a charge/discharge usage but can start engines in emergencies.
  9. Life is reduced by a warm temperature environment.
  10. Not all Valve Regulated Batteries are the same.
  11. Valve Regulated Batteries have good features and benefits but are more “Delicate” than flooded batteries.

Report courtesy of Pacific Powerboat magazine.