The World of Marine Batteries

Club Marine Magazine | VOLUME 19, ISSUE 6
A hydrometer will give a more accurate reading than a voltage meter.
Batteries are an expensive item aboard any vessel, so every effort should be made to sustain their lifespan and optimise performance.

The battery system aboard a vessel provides the power for most of the onboard systems, including: lighting, bilge pumps, fresh water and waste water pumps, a wide array of navigation systems, anchor winch, entertainment systems and the engine starting system.

Batteries can be divided into several different types, these include: lead acid batteries; gel batteries (commonly called recycling batteries), and nickel cadmium or Nife batteries.

Lead acid batteries

These are by far the most common type of batteries used aboard boats today. Lead acid batteries contain lead plates with separators to differentiate the positive plates from the negative plates. The electrolyte is made up of 35 percent sulphuric acid and 65 percent pure water.

Many people use truck batteries aboard their boats. These batteries are generally not well suited to the marine environment because truck batteries are designed for constant use, while marine batteries are designed to survive less frequent use. Truck batteries will self-discharge at the rate of approximately two percent per week while marine batteries will self-discharge at the rate of approximately 0.5 percent per day. However truck batteries can last well aboard boats that are in constant use, such as charter vessels and fishing vessels.

Lead acid batteries are by far the most commonly used batteries today. They can be divided into two categories. These are deep cycle batteries and engine starting batteries.

Deep cycle batteries

These batteries are designed for continuous heavy use and are more robust in their construction than starting batteries. This ensures a quick recovery during use. Typically these batteries will sustain 700 deep cycles and approximately 7000 shallow cycles before the battery fails. The lead plates are approximately four millimetres thick and the separators are made of glass fibre with a mat backing to encapsulate the positive plate as it expands and contracts during the cycling process. The grid metal has twice the antimony content when compared to starting batteries.

Water loss is a marked characteristic of these batteries; thus the water level should be checked regularly. Deep cycle batteries do not recover well if subjected to water loss for a sustained period of time.

Engine starting batteries

These batteries are designed to provide high amperes output for a very short period of time. A typical scenario when starting an engine. The lead plates are approximately 1.5mm thick. These batteries do not lose water as rapidly as the deep cycle batteries. If starting batteries are subject to deep cycling, their performance will typically deteriorate rapidly.

Maintaining long battery life:

Water loss does irrevocable damage to a battery, especially if the water loss is sustained over a prolonged period. Batteries should only be filled with distilled ion-free water up to the bottom of the filler tubes. If the battery is over-filled the risk of spillage is increased. The water in a battery also acts as a cooling agent; this is another reason why it is important to maintain the correct water level at all times. If battery plates are exposed while under heavy load or while being charged, they can glow red hot. If this is ever encountered the operation should be stopped immediately and the area evacuated, as there is the serious risk of a violent explosion.

The outside case of the batteries should be kept spotlessly clean, especially around the posts. This is to ensure an electrical short is not created that can quickly flatten the battery or reduce its performance.

Lead acid batteries should be fully charged every three weeks in the wintertime and every fortnight during the summer to maintain optimal performance and lifespan. If a battery is left in a discharged state it will quickly start to grow lead sulphate crystals on the plates. This insulates the plates from the electrolyte and seriously hinders recharging. If the lead sulphate crystals are left to grow for sufficient time the battery cannot be recharged and thus becomes useless. The lead sulphate is easily identified as a white film on the lead plates. The warmer the temperature, the quicker the crystals will grow. For every five degrees (Celsius) increase in temperature, the growth of the sulphate crystals is doubled.

Batteries should never be charged with excess volts as this does permanent damage. The usual charging voltage for a 12 volt battery is 14.2 to 14.4 volts. Excess voltage during charging also promotes gassing and water loss. When a battery is being charged it should not gas (produce bubbles in the cells). If this occurs it indicates a problem with the battery cell and it should be investigated.

No two cells in a battery behave exactly the same. Especially after the battery has been subject to heavy discharge. Thus it is a good idea to keep batteries in good condition to ensure optimal serviceability.

Testing the charge of a lead acid battery:

The charge of a lead acid battery is commonly tested using a hydrometer. A quality hydrometer can be obtained from any large hardware store for approximately $25.

Table 1 (below) is an approximate guide for the state of charge (at 26.7°C). Specific gravity is the measure of a liquid when compared to fresh water at 15 degrees Celsius, which equals 1.0. When the battery is at full charge the electrolyte in the battery is 26 percent heavier than fresh water, (SG = 1.260). Note the specific gravity of the electrolyte will change with the ambient temperature, but for temperate climates this can largely be ignored.

Using a voltage meter does not always provide a reliable reading, as they are influenced by the recent battery cycles. When a battery has just been charged, the charge remains on the surface of the battery plates for several hours, this will give you a false high reading. Conversely any recent heavy load on the battery will lower the charge of the battery for several hours also giving a false reading. The hydrometer will provide a more reliable reading, however it is affected by phenomenon known as gravity bounce and stratification.

Gravity bounce:

Gravity bounce occurs as the battery is being charged. The heavier and more charged electrolyte sinks to the bottom of the battery case. The sample of electrolyte that is drawn into the hydrometer is from the top of the battery. Thus, it is not as dense or as charged as the electrolyte at the bottom of the battery. As the charging process progresses, the gasses that are trapped between the plates and the separators escape and mix the layers within the electrolyte, this gives rise to the characteristic sudden rise in charge of the battery cell. Convection currents generated by charging and heavy discharging also assist to reduce the gravity bounce phenomena.


Stratification is similar to gravity bounce in that the specific gravity of the electrolyte varies with the depth of the electrolyte. However, where gravity bounce occurs during charging, stratification occurs when the battery is left to sit perfectly still for a period of time. This allows the heavier electrolyte to sink and form a layer at the bottom of the battery. Stratification can cause the voltage regulator to indicate that the battery is fully charged. This is one of the reasons why some voltage regulators have a periodic equalising charge mode, to stir the electrolyte by causing the cells to gas for a short period of time to stir the electrolyte and break up the layers.

Quick battery cell test:

A quick method of testing the condition of each cell in a battery is to first open each battery cell cap. Then turn the engine over, with the engine stop engaged and carefully look inside each cell. A cell that bubbles indicates it has begun to break down. The bubbling looks much like lemonade. The battery may still produce power. However, if you test it you will find the voltage and/or amperage has dropped. The battery should be replaced. The damaged cell is producing a high rate of discharge and this is why it bubbles. When looking into a battery cell it is always wise to wear eye and face protection.

Gel batteries

Gel batteries are commonly called recycle batteries. These batteries are completely sealed and contain no antimony in the battery. The plates are separated by a glass fibre mat and have no liquid in the cells. As there is no liquid, these batteries do not suffer from the problems associated with gassing. Gel batteries are compulsory for all Category One racing yachts, to prevent the spillage of acid in the event of a capsize.

Gel batteries are more expensive to purchase than lead acid batteries. They have a life span of about six to seven years, which is similar to that of a lead acid battery. Gel batteries designed for deep cycle provide excellent service.

Nickel cadmium batteries

Nickel cadmium batteries are also called Nife batteries. The electrolyte in these batteries is an alkaline mixture. Although these batteries are expensive to purchase, they provide excellent service and typically last from 12 to 15 years in marine applications. These batteries are frequently used in heavy applications and where maintenance requirements must be minimal.

Battery Installation:

Batteries should be securely fastened into a strongly constructed acid-proof box with a strong lid attached. The box should have a vent near the top to allow the ventilation of hydrogen gas to outside of the vessel. The battery box should be placed well above the bilge, in a position where it is subject to minimal vibration.

Excessive vibration will eventually break down the internal construction of a battery, rendering it useless. Where practical, the battery box should also be placed well away from sources of heat to ensure optimal battery performance. Ideally, batteries should be above the level of the engine starter motor. This allows the batteries to continue to operate essential safety equipment such as a marine radio, should the engine become submerged.

The cables from the battery to the common power bar should be the same size as the engine starter cables. Table 2 features a guide to the correct size.

Two cables of the same cross-sectional area can be run in parallel if a single cable of sufficient size is not available. It is imperative that the cables for the positive and negative leads be of the same cross-sectional area.

Solar panels and wind generators

Solar panels and wind generators provide a good way of keeping the batteries charged while the yacht is not in use. However, it is important to ensure a voltage regulator is fitted in the charging circuit. This is to ensure the batteries are protected from over-charging. A unidirectional device should also be fitted, so the batteries do not discharge through the solar panel or wind generator when they are not providing a charge to the batteries. The unidirectional device can be as simple as a diode. But I would always recommend consultation with a specialist.

Care of battery posts

The seal between the lead posts and the top of the battery relies on the adhesion between the two disparate materials. This seal is weak. Due care should be taken when connecting or disconnecting the battery terminals. Methods such as hammering the terminal, levering the terminal off and excessive tightening or loosening without sufficient support should be avoided. When this seal has been broken, acid will tend to work its way up the battery post and start to corrode the terminal. Once the seal has been broken there is no effective way of repairing it.

Battery terminals

To ensure efficient battery connections, the terminals should be soldered to the heavy battery cables and covered with a plastic shrink wrap to prevent corrosion. The bare metal terminals should be covered with Vaseline or light-grade grease to prevent the build up of corrosion.

Battery banks

Batteries can be joined together in two different ways - in series or in parallel. Batteries connected in series are connected positive to negative; this increases the voltage of the battery bank. Batteries connected in parallel are connected positive to positive and negative to negative. This increases the amperage of the battery bank.

More than one battery bank can be connected together to increase both amperage and voltage. See diagram 1 below.

Battery maintenance

Some simple battery maintenance will keep your batteries working at optimal levels.
Maintenance can be divided into two categories, regular checks and annual tasks.

Regular checks

Regular battery checks are mandatory if the operation and life of the battery are to be maintained. Regular checks include:
• Keep the battery case clean.
• Regular checks of the electrolyte level. Always use ion-free distilled water to top up the level of each cell.
• Use a hydrometer to check the state of the battery charge for each battery cell.
• Ensure the battery terminals are free of any corrosion.
• Ensure the bare metal terminals are covered with Vaseline or light-grade grease to prevent corrosion.

Annual tasks

Annual checks should be carried out during the winter maintenance period. These include:
• Ensuring the battery tie-downs are in good order and have not been damaged by spilt electrolyte.
• Remove the batteries from their box and clean the inside of the box.
• Ensure the vent at the top of the battery box is clear.

How to-Safety