Marine Batteries: Types, Use Cases, Charging, Storage, and Testing

Overview

Batteries are used throughout yachts and ships for starting, emergency power, communications, navigation backup, UPS systems, service loads, alarm panels, portable equipment, tenders, toys, safety equipment and, increasingly, hybrid or electric propulsion systems. For ETOs, CTOs and AVIT officers, batteries should not be treated as simple consumables. A battery installation is an electrical energy source, a fire risk, a maintenance item, a safety-critical support system and, in larger or networked installations, part of the vessel’s operational technology environment. A failed battery can prevent a generator from starting, drop a UPS-supported server, interrupt communications, disable safety equipment, damage sensitive electronics, release hydrogen, leak acid, trigger thermal runaway, or create a fire that is difficult to contain. Correct battery selection, charging, storage and testing are therefore essential onboard maintenance skills. This article covers common marine battery types, typical yacht and ship use cases, charging methods, basic sizing calculations, storage practice and practical testing.

Reference Basis

This article is written for maritime technical use and should be read alongside the applicable vessel-specific requirements from flag state, class, builder documentation, manufacturer manuals and the vessel Safety Management System. Relevant reference areas include:

• SOLAS Chapter II-1, for machinery and electrical installations.
• SOLAS Chapter II-2, for fire protection, fire detection and fire extinction.
• IMO IMDG Code, where batteries are carried, shipped or handled as dangerous goods.
• EMSA Guidance on the Safety of Battery Energy Storage Systems on board ships.
• IEC 60092 series for shipboard electrical installations, including accumulator batteries.
• IEC 62619 for safety requirements of secondary lithium cells and batteries for industrial applications.
• ISO small craft electrical standards where applicable to smaller craft and tender installations.
• Classification society rules and notations for marine battery systems, especially for hybrid, propulsion or large energy storage systems.
• Manufacturer manuals for exact charge voltage, current limits, temperature compensation, ventilation and storage instructions.

This article is not a substitute for class-approved design, flag-state acceptance, manufacturer instructions, or a vessel-specific risk assessment.

Battery Terminology

Before selecting or testing a battery, the technical team should understand the basic terms used in datasheets and chargers. Voltage is the electrical potential of the battery. Common marine DC systems include 12 V, 24 V, 48 V and higher-voltage DC systems for propulsion or energy storage. Amp-hours, or Ah, describe capacity at a stated discharge rate. A 100 Ah battery can theoretically provide 5 A for 20 hours if rated at the 20-hour rate. This does not mean it can provide any current for any duration without loss. Watt-hours, or Wh, describe stored energy. Formula: Wh = V × Ah Example: A 12 V, 270 Ah battery has: 12 × 270 = 3240 Wh That is approximately 3.24 kWh nominal energy. Depth of discharge, or DoD, is how much of the battery has been used. If a battery is discharged from 100% state of charge to 60%, the DoD is 40%. State of charge, or SoC, is how full the battery is. C-rate describes charge or discharge current relative to battery capacity. For a 100 Ah battery:

• 0.1C = 10 A
• 0.2C = 20 A
• 0.5C = 50 A
• 1C = 100 A

Cycle life is the number of charge/discharge cycles a battery can complete before reaching a defined end-of-life capacity, often 80% of original capacity. Internal resistance affects voltage drop, heat generation and cranking ability. Internal resistance generally increases with age, sulphation, corrosion, dry-out, cell damage or poor maintenance. Peukert effect applies mainly to lead-acid batteries. The faster a lead-acid battery is discharged, the less usable capacity it provides. This is why a lead-acid battery may perform acceptably on a small steady load but collapse quickly under a heavy inverter or thruster load.

Main Battery Types Used Onboard

Flooded Lead-Acid Batteries

Flooded lead-acid batteries are traditional wet-cell batteries with liquid electrolyte. They are common for generator starting, engine starting, some service banks and older emergency systems. Advantages:

• Mature and widely understood technology.
• Relatively low purchase cost.
• Good surge current for starting applications.
• Can be tested using voltage, specific gravity and load testing.
• Tolerant of many marine environments if properly installed and maintained.

Limitations:

• Require ventilation due to hydrogen off-gassing during charging.
• Require electrolyte checks and topping up with approved water.
• Must generally remain upright.
• Can spill acid if damaged or incorrectly mounted.
• Lower usable depth of discharge than lithium.
• Poor performance if left partially discharged for long periods.
• Heavier and larger than lithium for equivalent usable energy.

Typical use cases:

• Main engine starting.
• Generator starting.
• Emergency generator starting.
• Small DC backup banks.
• Older service banks where weight and maintenance access are acceptable.

Flooded lead-acid batteries remain suitable where simplicity, availability and predictable starting performance are more important than weight or deep cycling.

AGM Batteries

AGM stands for Absorbent Glass Mat. These are valve-regulated lead-acid batteries where electrolyte is absorbed into glass mat separators. Advantages:

• Sealed construction with minimal routine maintenance.
• Good cranking performance.
• Lower off-gassing under normal charging than flooded batteries.
• Can often be installed in more orientations than flooded batteries.
• Good for UPS, starting and moderate cycling applications.
• Lower internal resistance than many flooded batteries.

Limitations:

• Sensitive to incorrect charging voltage.
• Can dry out if overcharged.
• Usually less tolerant of high temperature than expected.
• Cannot normally be recovered once badly sulphated or dried.
• More expensive than flooded lead-acid.
• Not ideal for repeated deep discharge unless specifically designed for deep cycle use.

Typical use cases:

• Generator start batteries.
• Main engine start batteries.
• UPS battery strings.
• Electronics backup.
• Emergency lighting or alarm panel backup.
• Service banks on smaller vessels.

AGM is often selected where maintenance access is poor or where the battery is installed close to sensitive equipment, but ventilation and heat management still matter.

Gel Batteries

Gel batteries are valve-regulated lead-acid batteries with gelled electrolyte. They are commonly used for deep-cycle applications where controlled charging is available. Advantages:

• Good deep-cycle capability when correctly charged.
• Low maintenance.
• Reduced risk of free electrolyte spill compared with flooded cells.
• Low self-discharge.
• Good for standby and service applications.

Limitations:

• Very sensitive to overvoltage.
• Incorrect charger settings can create gas pockets in the gel and permanently reduce capacity.
• Generally not the best choice for high-current engine starting unless designed for it.
• Requires charger profile matched to the battery manufacturer’s data.

Typical use cases:

• Service banks.
• Emergency lighting backup.
• Navigation and communication backup banks.
• UPS or standby applications.
• Low-to-moderate discharge systems where long life is valued.

Gel batteries should be charged using a charger profile specifically approved for gel batteries. They should not be equalized unless the manufacturer explicitly allows it.

OPzV and OPzS Batteries

OPzV and OPzS batteries are both tubular plate lead-acid battery types commonly used in industrial standby, telecom, emergency power, UPS and long-duration DC backup systems.

OPzV batteries are sealed, valve-regulated tubular gel batteries. They do not require flooded-cell watering or electrolyte topping up during normal operation. Maintenance is still required, but it is mainly inspection-based: checking voltage, temperature, cleanliness, terminals, torque, ventilation, charger settings and signs of damage or abnormal heating.

OPzS batteries are flooded tubular lead-acid batteries. They require flooded-cell maintenance, including electrolyte level checks, topping up with approved water where required, ventilation control, spill precautions and specific gravity testing where appropriate.

Advantages:

• Long service life when correctly installed.
• Good deep-cycle and standby performance.
• Suitable for large stationary battery banks.
• Robust construction.
• OPzV offers lower routine maintenance than OPzS because it is sealed gel VRLA.
• OPzS allows electrolyte and specific gravity checks but requires more maintenance.

Limitations:

• Heavy and space-consuming.
• Require proper racking, access, labelling and ventilation.
• OPzS requires flooded-cell maintenance.
• OPzV does not require water topping-up, but still requires routine inspection and correct charger settings.
• More specialist than common AGM or gel monobloc batteries.

Typical use cases:

• Larger UPS systems.
• Emergency power systems.
• Communications rooms.
• Navigation and control system backup banks.
• Shore-side or vessel support infrastructure.
• Long-duration DC backup systems.

For yacht and ship installations, the distinction is important: OPzV should be treated as sealed tubular gel VRLA, while OPzS should be treated as flooded tubular lead-acid.

Lithium-Ion Batteries

Lithium-ion is a broad family of chemistries. In marine service, lithium iron phosphate, often abbreviated LFP or LiFePO4, is common because of its favourable safety profile compared with some higher-energy lithium chemistries. Other chemistries include NMC, NCA, LTO and others. Advantages:

• High usable energy for size and weight.
• High charge acceptance.
• High cycle life when correctly managed.
• Flat discharge voltage curve.
• Low self-discharge.
• Good efficiency.
• Suitable for high-current and high-cycle applications.

Limitations:

• Requires a suitable Battery Management System.
• Requires compatible chargers, alternators, inverter-chargers and protection devices.
• Thermal runaway, off-gassing and fire propagation risks must be properly assessed.
• Faults may develop rapidly and require automatic isolation.
• Some systems require class or flag approval.
• Drop-in lithium batteries are not automatically suitable replacements for lead-acid batteries.
• Low-temperature charging can damage some lithium chemistries unless the BMS prevents charging.

Typical use cases:

• Hybrid propulsion energy storage.
• Hotel-load service banks.
• High-capacity inverter systems.
• Electric tenders and toys.
• Weight-sensitive installations.
• High-cycle applications.
• Large UPS or technical backup systems.

Lithium systems should be treated as engineered systems, not just batteries. The correct approach includes cell chemistry, BMS, containment, ventilation, fire detection, fire suppression, emergency isolation, charger compatibility, system integration, crew training and maintenance procedures.

Nickel-Cadmium Batteries

Nickel-cadmium batteries are less common in modern yacht systems but may still be found in some emergency, aviation-style, industrial or specialist applications. Advantages:

• Robust.
• Good temperature tolerance.
• Can tolerate deep discharge better than many lead-acid batteries.
• Long service life in some industrial applications.

Limitations:

• Cadmium is toxic.
• Disposal is controlled.
• Can suffer memory effect depending on use.
• Requires correct maintenance and charging practice.
• Less common in yacht inventory.

Typical use cases:

• Legacy emergency systems.
• Specialist industrial backup systems.
• Some aviation-derived or safety equipment.

Portable Lithium Batteries

Portable lithium batteries are now everywhere onboard. They are found in radios, tablets, laptops, cameras, drones, power tools, e-foils, scooters, toys, torches, guest devices and portable power banks. They create a different risk profile from fixed installations because they are often charged in cabins, crew areas, workshops, AV racks, offices or lazarettes without a formal installation design. Common controls should include:

• Use only manufacturer-approved chargers.
• Do not charge damaged or swollen batteries.
• Do not charge lithium batteries unattended in accommodation spaces.
• Avoid charging on soft furnishings or combustible surfaces.
• Keep charging areas ventilated and away from escape routes.
• Inspect chargers and leads for damage.
• Segregate damaged batteries.
• Record and safely dispose of failed packs.
• Avoid storing large numbers of spare lithium packs in random lockers.

Portable battery control is increasingly important onboard because the number of battery-powered devices has increased faster than many vessel procedures have adapted.

Matching Battery Type to Use Case

Starting Batteries Starting batteries are designed to deliver high current for a short time. They are rated using cranking amps rather than deep-cycle performance. Suitable types:

• Flooded starting battery.
• AGM starting battery.
• Specialist lithium start battery only if approved by the engine, generator and battery manufacturer.

Avoid using deep-cycle-only batteries for cranking unless the datasheet confirms sufficient cranking performance. Critical checks:

• Correct voltage.
• Correct cold cranking or marine cranking rating.
• Correct terminal type and polarity.
• Secure mounting.
• Correct cable size and protection.
• Charger or alternator compatibility.
• No loose or corroded lugs.
• Clear label showing installation date.

Service or House Batteries Service batteries support DC loads, inverters, lighting, AV, IT, refrigeration, controls, pumps or hotel loads. Suitable types:

• Deep-cycle flooded lead-acid.
• Deep-cycle AGM.
• Gel.
• OPzV.
• LFP lithium with suitable BMS and integration.

Selection depends on load profile, allowable weight, charging source, expected DoD, redundancy requirements, space, ventilation and budget.

UPS Batteries UPS batteries support servers, network switches, storage systems, AV processors, automation controllers, communications equipment and security systems. Suitable types:

• AGM VRLA, common in small UPS units.
• Lithium UPS packs where supplied and approved by the UPS manufacturer.
• External battery modules only where designed for that UPS.

Key practice:

• Do not mix old and new batteries in the same string unless the manufacturer allows it.
• Replace complete strings where required.
• Keep UPS units cool.
• Test under controlled load.
• Confirm graceful shutdown behaviour for servers and storage.
• Ensure UPS battery alarms are monitored.

Emergency Batteries Emergency batteries may support lighting, GMDSS-related equipment, alarm panels, fire detection panels, escape lighting, emergency communications or other critical systems. These batteries should be managed as safety-critical items. Key practice:

• Maintain records.
• Test at the required interval.
• Confirm duration under load.
• Replace before failure.
• Do not repurpose emergency batteries for convenience loads.
• Confirm class, flag and manufacturer requirements before modifying.

Propulsion and Hybrid Battery Systems Large battery energy storage systems require a formal engineering approach. They may affect propulsion, power management, redundancy, fire safety, ventilation, automation, classification and emergency response. Required considerations usually include:

• Class or flag approval.
• Battery room or enclosure design.
• Fire detection.
• Gas detection where required.
• Cooling.
• Ventilation and exhaust route.
• Water-based or approved fire suppression strategy.
• Emergency stop and isolation.
• BMS alarms and trips.
• Integration with power management.
• Failure mode analysis.
• Crew training.
• Periodic safety testing.
• Documentation and maintenance records.

These systems should not be modified casually by onboard crew without manufacturer, integrator, class or flag involvement.

Charging Methods

Three-Stage Charging for Lead-Acid Batteries

Most lead-acid charging uses a multi-stage profile. Bulk stage The charger supplies constant current up to a target voltage. This replaces most of the removed capacity. Absorption stage The charger holds a controlled voltage while current gradually tapers down. This completes the charge. Float stage The charger reduces voltage to a safe maintenance level to compensate for self-discharge without excessive gassing or heating. For flooded, AGM and gel batteries, the correct absorption and float voltages must come from the manufacturer’s data. The wrong voltage can shorten life, cause gassing, dry out VRLA batteries, damage gel cells or leave the bank undercharged.

Equalization Charging

Equalization is a controlled overcharge used on some flooded lead-acid batteries to reduce cell imbalance and sulphation. Important rules:

• Equalization is normally for flooded batteries only.
• Do not equalize AGM or gel batteries unless the manufacturer specifically permits it.
• Disconnect or protect sensitive DC loads before equalization.
• Provide ventilation.
• Monitor temperature.
• Check electrolyte level before and after.
• Record the equalization event.

Equalization should be a planned maintenance task, not a casual charger setting.

Lithium Charging

Lithium batteries are normally charged using a constant-current / constant-voltage method controlled by the charger and BMS. Critical points:

• The charger must be compatible with the lithium battery and BMS.
• The BMS must be able to stop charge or discharge under unsafe conditions.
• Alternators may need external regulation or DC-DC charging to avoid overheating or load-dump issues.
• Lithium batteries may not need the same float behaviour as lead-acid.
• Cell balancing may require periodic full charge depending on the battery design.
• Charging below the allowed temperature may damage the cells unless the BMS prevents it.

Lithium charging should be verified against the exact battery model, BMS, alternator, charger, inverter-charger, solar controller and DC distribution design.

Temperature Compensation

Lead-acid charging voltage should normally be temperature compensated. A cold battery generally requires a higher charge voltage, while a hot battery requires a lower charge voltage. Without temperature compensation:

• Hot batteries can be overcharged.
• Cold batteries can be undercharged.
• Battery life is reduced.
• Fault diagnosis becomes misleading.

Temperature sensors should be installed correctly on the battery bank, not left coiled inside a charger cabinet. Lithium charging temperature limits are handled differently. Lithium systems usually rely on BMS temperature sensors and charge inhibit functions.

Basic Battery Calculations

Nominal Energy

Formula: Energy Wh = Voltage × Capacity Ah Example: A 12 V, 270 Ah gel battery: 12 × 270 = 3240 Wh This is 3.24 kWh nominal energy. For a 24 V bank made from two 12 V, 270 Ah batteries in series: 24 × 270 = 6480 Wh This is 6.48 kWh nominal energy. When batteries are connected in series, voltage increases but Ah stays the same. When batteries are connected in parallel, Ah increases but voltage stays the same.

Usable Energy

Not all nominal energy should be used. Formula: Usable Wh = V × Ah × allowable DoD × system efficiency Example: 12 V, 270 Ah gel battery with 50% allowable DoD and 90% inverter efficiency: 12 × 270 × 0.50 × 0.90 = 1458 Wh So the practical usable energy is approximately 1.46 kWh. For a lithium battery where 80% DoD is allowed: 12 × 270 × 0.80 × 0.95 = 2462 Wh So the practical usable energy is approximately 2.46 kWh. This is why lithium often appears to provide more usable energy even when the nominal Ah rating looks similar.

Runtime

Formula: Runtime hours = Usable Wh ÷ Load W Example: A 24 V, 270 Ah bank has: 24 × 270 = 6480 Wh If used to 50% DoD through an inverter at 90% efficiency: 6480 × 0.50 × 0.90 = 2916 Wh If the load is 300 W: 2916 ÷ 300 = 9.72 hours Estimated runtime is about 9.7 hours. This is an estimate. Real runtime will vary with temperature, age, discharge rate, inverter efficiency and battery condition.

Charge Current

A simple method is to use C-rate. Formula: Charge current A = Battery capacity Ah × C-rate Example: 270 Ah battery at 0.1C: 270 × 0.1 = 27 A 270 Ah battery at 0.2C: 270 × 0.2 = 54 A Manufacturer limits should always take priority. As a broad practical guide:

• Flooded lead-acid often uses moderate charge current.
• AGM often accepts higher current than flooded lead-acid.
• Gel can accept useful current but is voltage-sensitive.
• Lithium can often accept high current, but only within BMS and manufacturer limits.

A charger should also support connected loads. If a bank needs 50 A charging current and the connected DC load is 10 A, the charger may need to supply around 60 A to charge the battery while also supporting the load.

Charge Time

A simplified formula: Charge time hours = Ah to replace ÷ effective charge current × charge factor For lead-acid batteries, use a charge factor such as 1.15 to 1.25 because charging is not 100% efficient and absorption takes time. Example: A 270 Ah lead-acid battery discharged by 50% has used: 270 × 0.50 = 135 Ah Charging at 30 A: 135 ÷ 30 × 1.2 = 5.4 hours Approximate charge time is 5.4 hours, but the final absorption stage may extend this. For lithium, the charge factor may be lower, but the BMS and charger profile still control the final charge time.

Inverter Load Current

A common onboard mistake is underestimating DC current feeding an inverter. Formula: DC current A = AC load W ÷ battery voltage ÷ inverter efficiency Example: A 1000 W AC load from a 12 V battery through a 90% efficient inverter: 1000 ÷ 12 ÷ 0.90 = 92.6 A At 24 V: 1000 ÷ 24 ÷ 0.90 = 46.3 A At 48 V: 1000 ÷ 48 ÷ 0.90 = 23.1 A This shows why higher-voltage DC systems are preferred for larger inverter loads.

Series and Parallel Banks

Series connection

• Increases voltage.
• Ah remains the same.

Example: Two 12 V, 200 Ah batteries in series: 24 V, 200 Ah Parallel connection

• Voltage remains the same.
• Ah increases.

Example: Two 12 V, 200 Ah batteries in parallel: 12 V, 400 Ah Series-parallel connection Used to create larger banks. Example: Four 12 V, 200 Ah batteries arranged as two series strings in parallel: 24 V, 400 Ah Rules:

• Use identical batteries where possible.
• Same chemistry.
• Same capacity.
• Same age.
• Same manufacturer and model.
• Same state of charge before connection.
• Correct fusing and isolation.
• Equal cable lengths for parallel strings where practical.
• Good labelling.
• Correct torque on terminals.

Do not mix battery types in the same bank.

Storage Practice

General Storage Principles

Battery storage should be controlled, labelled and recorded. Batteries left in random lockers are easily forgotten until they leak, self-discharge, swell, corrode or fail when needed. Good storage practice:

• Store in a cool, dry, ventilated location.
• Keep away from direct sunlight and heat sources.
• Protect terminals from short circuit.
• Keep batteries upright where required.
• Do not store directly on damp deck plating.
• Use containment where acid leakage is possible.
• Segregate damaged or suspect batteries.
• Label with date received, date charged and next inspection date.
• Keep manufacturer manuals or datasheets accessible.
• Do not store batteries in escape routes or accommodation lockers.
• Keep incompatible chemicals and combustibles away.

Storing Lead-Acid Batteries

Lead-acid batteries should generally be stored fully charged. Leaving them partially discharged encourages sulphation, reduces capacity and can permanently damage the battery. Storage checks:

• Verify open-circuit voltage.
• Recharge at manufacturer intervals.
• Keep clean and dry.
• Check for case swelling, cracks, leaks or corrosion.
• For flooded cells, check electrolyte level where applicable.
• Avoid high-temperature storage.
• Record maintenance charging.

For flooded batteries, storage areas must consider acid spill risk and hydrogen ventilation during charging.

Storing AGM and Gel Batteries

AGM and gel batteries have low self-discharge compared with flooded batteries, but they still need periodic inspection and recharge. Good practice:

• Store fully charged unless manufacturer instructions state otherwise.
• Use the correct charger profile.
• Avoid high temperature.
• Avoid long periods in a discharged state.
• Do not equalize unless manufacturer-approved.
• Inspect terminals and cases.
• Record voltage and recharge dates.

A cool technical store is much better than a hot machinery-space locker.

Storing Lithium Batteries

Lithium batteries are usually best stored at partial state of charge, not full and not empty, unless the manufacturer specifies otherwise. Good practice:

• Follow the exact manufacturer storage SoC.
• Disconnect loads.
• Keep within approved temperature range.
• Protect terminals.
• Store away from combustibles.
• Do not store damaged or swollen packs with serviceable batteries.
• Inspect for swelling, odour, heat, leakage, corrosion or case damage.
• Use fire-resistant storage arrangements where appropriate.
• Keep high-energy lithium packs out of accommodation spaces where practical.
• Record inspection and charge intervals.

For portable lithium packs, a vessel should have a clear damaged-battery quarantine process.

Battery Testing

Visual Inspection

Visual inspection should be part of every battery maintenance round. Check for:

• Loose terminals.
• Corrosion.
• Overheated lugs.
• Discoloured insulation.
• Swollen cases.
• Cracked cases.
• Leaks.
• Salt contamination.
• Poor cable support.
• Missing terminal covers.
• Incorrect labels.
• Missing ventilation.
• Blocked battery box vents.
• Mixed battery types.
• Unsecured batteries.
• Damaged chargers.
• Unusual smell.
• Evidence of previous overheating.

Many battery failures are installation failures before they are cell failures.

Open-Circuit Voltage Testing

Open-circuit voltage can indicate state of charge, but it must be interpreted carefully. Rules:

• Allow the battery to rest after charging or discharging before measuring.
• Measure at the battery terminals.
• Compare with manufacturer voltage tables.
• Temperature affects readings.
• Surface charge can mislead.
• Lithium voltage curves are flatter than lead-acid, so voltage alone is not a reliable SoC method.

Open-circuit voltage is useful, but it is not a complete health test.

Specific Gravity Testing

Specific gravity testing applies to accessible flooded lead-acid cells. It can identify:

• Low state of charge.
• Cell imbalance.
• Stratification.
• Weak or failing cells.
• Need for equalization where allowed.

Rules:

• Use appropriate PPE.
• Avoid acid contact.
• Correct for temperature if required.
• Compare cells against each other.
• Record readings.
• Do not perform this test on sealed AGM or gel batteries.

A single low cell compared with the rest of the bank is a warning sign.

Load Testing

Load testing checks whether a battery can support a specified load without excessive voltage collapse. For starting batteries, a load tester or conductance tester is often used. For service banks, a controlled discharge test is more meaningful. Rules:

• Use a suitable test load.
• Monitor voltage and temperature.
• Do not exceed safe discharge limits.
• Stop if the battery overheats, swells, smells, vents or drops voltage abnormally.
• Record test current, duration, voltage and temperature.
• Recharge immediately after testing.

Load testing should be planned so it does not compromise operational readiness.

Conductance and Internal Resistance Testing

Battery testers often estimate condition by measuring conductance or internal resistance. Useful for:

• Starting batteries.
• Comparing batteries in a string.
• Finding weak blocks.
• Trending deterioration over time.

Limitations:

• Tester results vary by battery type and algorithm.
• A battery can pass conductance testing but still have poor capacity.
• Lithium batteries require manufacturer-approved diagnostics.
• Temperature and state of charge affect readings.

The best value comes from trending results over time, not relying on a single test.

Capacity Testing

Capacity testing is the most meaningful way to confirm the actual usable capacity of a service or standby battery. Basic method:

1. Fully charge the battery.
2. Allow it to rest if required.
3. Apply a known load.
4. Discharge to the manufacturer’s cutoff voltage or approved DoD.
5. Record current, voltage, time and temperature.
6. Calculate delivered Ah or Wh.
7. Recharge immediately.

Formula: Delivered Ah = Load current × Time Formula: Delivered Wh = Average voltage × Ah delivered Capacity testing can be time-consuming, but it is valuable for emergency power systems, UPS banks, service banks and suspect batteries.

Common Onboard Battery Faults

Loose or Poorly Crimped Terminals Loose or poorly crimped terminals create heat under load. This is especially dangerous on high-current systems such as inverters, thrusters, windlasses, lithium banks and engine starting circuits. Signs:

• Warm terminal.
• Discoloured lug.
• Melted insulation.
• Voltage drop.
• Intermittent operation.
• Burn smell.
• Charger or inverter alarms.

Corrective action:

• Isolate safely.
• Replace damaged lugs.
• Re-crimp with correct tooling.
• Apply correct torque.
• Support cables properly.
• Inspect adjacent equipment.

Sulphation Sulphation occurs when lead-acid batteries remain discharged or undercharged. It reduces capacity and increases internal resistance. Prevention:

• Keep charged.
• Avoid prolonged partial state of charge.
• Use correct charger profile.
• Perform manufacturer-approved equalization on flooded cells where applicable.
• Test and trend capacity.

Thermal Stress Heat shortens battery life. Batteries installed in hot machinery spaces or poorly ventilated cabinets may fail early. Controls:

• Improve ventilation.
• Move batteries where possible.
• Avoid mounting near exhausts, chargers or heat-producing equipment.
• Install temperature sensing.
• Use chargers with temperature compensation.
• Record battery room temperature.

Charger Misconfiguration Wrong charger settings are a common cause of battery failure. Examples:

• AGM charged on flooded setting.
• Gel charged at excessive voltage.
• Lithium charged with lead-acid float profile without BMS approval.
• Temperature compensation missing.
• Equalization accidentally enabled.
• Charge current too high.
• Alternator connected directly to lithium bank without suitable regulation.

Every charger should be labelled with the battery type and programmed profile.

Mixed Battery Banks Mixing old and new batteries, different capacities or different chemistries in the same bank causes imbalance and early failure. Avoid:

• Mixing AGM and gel.
• Mixing flooded and AGM.
• Mixing old and new blocks.
• Mixing different Ah ratings.
• Replacing one failed battery in an aged series string without assessing the whole string.

For critical systems, replace full strings unless the manufacturer and test results support partial replacement.

Maintenance and Records

Battery maintenance should be documented like any other safety or technical system. Recommended records:

• Battery type.
• Manufacturer and model.
• Serial number.
• Installation date.
• Location.
• System served.
• Charger type and settings.
• Capacity.
• Test dates.
• Open-circuit voltage.
• Load test results.
• Specific gravity where applicable.
• Internal resistance or conductance readings.
• Visual condition.
• Terminal torque check.
• Cleaning and corrosion treatment.
• Ventilation check.
• Replacement date.
• Defects and corrective actions.

For larger lithium systems, records should also include BMS alarms, firmware versions, event logs, isolation tests, emergency stop tests, cooling system checks, gas or fire detection checks and safety drill records where applicable.

Practical Example: Gel Battery Charging Calculation

Battery:

• Type: Gel
• Capacity: 270 Ah
• Nominal voltage: 12 V

Nominal energy: 12 × 270 = 3240 Wh Recommended charge current must come from the manufacturer. If using a conservative 0.1C to 0.2C planning range: 0.1C: 270 × 0.1 = 27 A 0.2C: 270 × 0.2 = 54 A If the battery has been discharged to 50% SoC: Ah to replace: 270 × 0.50 = 135 Ah Approximate charge time at 30 A using a 1.2 lead-acid charge factor: 135 ÷ 30 × 1.2 = 5.4 hours Approximate charge time at 50 A: 135 ÷ 50 × 1.2 = 3.24 hours In practice, the absorption stage may extend the total time. Gel batteries are voltage-sensitive, so increasing charge voltage to speed up charging is not acceptable unless the manufacturer permits it. For storage, a gel battery should normally be stored fully charged, cool, clean and disconnected from loads, with periodic voltage checks and recharge according to manufacturer guidance.