TL;DR — Battery installations are classified as large, moderate, or small based on charger output (>2 kW, 0.2–2 kW, <0.2 kW respectively), and each category has specific location, ventilation, and protection requirements; emergency battery sources must supply connected loads for at least three hours and be positioned aft of the collision bulkhead, outside the machinery space, and above the uppermost continuous deck.
What the Rule Says
Battery Categories
The Coast Guard classifies shipboard battery installations into three categories, determined by the power output of the battery charger — not the battery capacity itself. 46 CFR §111.15-3
- Large: Charger output exceeds 2 kW (computed from highest possible charging current × rated voltage).
- Moderate: Charger output is between 0.2 kW and 2 kW.
- Small: Charger output is less than 0.2 kW.
Batteries that generate less hydrogen than an equivalent lead-acid battery under normal charging and discharging conditions — for example, sealed batteries — may have their installation category reduced by one equivalent category.
Location Requirements by Category
Large battery installations must be in a room used solely for batteries, or in a box on deck. All electrical equipment within that enclosure must meet hazardous location requirements. 46 CFR §111.15-5 Under the T-boat rules (Subchapter T), a large installation must be in a locker, room, or enclosed box dedicated solely to battery storage, and electrical equipment inside must be approved for Class I, Division 1, Group B hazardous locations. 46 CFR §183.354
Moderate battery installations may be in a battery room, a box on deck, or a box or locker in another space such as an engineroom or storeroom. If the moderate installation is in a ventilated compartment like an engineroom and is protected from falling objects, a box or locker is not required. Moderate installations must not be placed in sleeping spaces. Engine cranking batteries must be located as close as practicable to the engine or engines served.
Small battery installations must not be located in poorly ventilated spaces such as closets, nor in living spaces such as staterooms. Under Subchapter T, a small installation must be in a well-ventilated space, protected from falling objects, and must not be in a closet, storeroom, or similar space.
Battery Construction Standards
A battery cell must not spill electrolyte when inclined at 40 degrees from the vertical. 46 CFR §111.15-2 Note carefully: the emergency power regulation under 46 CFR §28.870 uses a different threshold — a battery cell for emergency power must not spill electrolyte when inclined at 30 degrees from the vertical. 46 CFR §28.870 These two figures appear on exams and are frequently confused.
Fully charged lead-acid batteries must have a specific gravity meeting Section 11 of IEEE 45.1-2017. Batteries must be constructed to account for the marine environment, including temperature, vibration, and shock. Batteries must not evolve hydrogen at a rate exceeding that of a similar-size lead-acid battery under similar charging conditions.
Tray, Lining, and Physical Installation Requirements
Battery trays must be chocked with wood strips or equivalent to prevent movement, and must have non-absorbent insulating supports on the bottom and spacer blocks at the sides to provide air circulation around each tray. Trays must allow adequate access for installation, maintenance, and removal.
Battery rooms and lockers must have a watertight lining either on each shelf to a height of at least 76 mm (3 inches), or on the deck to a height of at least 152 mm (6 inches).
- For lead-acid batteries, the lining must be at least 1.6 mm thick lead or equivalent corrosion-resistant material.
- For alkaline batteries, the lining must be at least 0.8 mm thick steel or equivalent corrosion-resistant material.
Battery boxes must have a watertight lining to a height of at least 76 mm (3 inches) meeting the same material standards.
Under the general Subchapter T rules, each battery must be located as high above the bilge as practicable, secured against shifting with roll and pitch, and free from exposure to water splash or spray. 46 CFR §183.350 Batteries must be mounted in trays lined with or constructed of material resistant to damage by the electrolyte.
Labeling
Each battery must permanently display the manufacturer's name, model number, type designation, either the cold cranking amp rating or the amp-hour rating at a specific discharge rate, and — for lead-acid batteries — the fully charged specific gravity value.
Overload and Reverse Current Protection
An overload protective device must be installed in each battery conductor, with two exceptions: conductors of engine cranking batteries, and batteries with a nominal potential of 6 volts or less. For large storage battery installations, overcurrent protective devices must be located next to, but outside of, the battery room. 46 CFR §111.15-25
Except when a converter is used, charging equipment for batteries with a nominal voltage more than 20 percent of line voltage must automatically protect against reversal of current.
Battery Charger Requirements
Each charger must be suitable for the size and type of battery installation it serves. Chargers incorporating grounded autotransformers must not be used. Except for converters, chargers with a voltage exceeding 20 percent of line voltage must have automatic protection against reversal of current. 46 CFR §111.15-30
Under Subchapter T, battery chargers must have an ammeter connected in the charging circuit.
Terminal Connections and Fusing
Connections to battery terminals must be made with permanent-type connectors. Spring clips and other temporary-type clamps are prohibited. If batteries are not adjacent to a distribution panel or switchboard, the battery lead must have a fuse in series, placed as close as practicable to the battery.
Ventilation
Where provisions are made for charging batteries, natural or induced ventilation must be sufficient to dissipate the gases generated. Deck boxes used for battery storage must be weathertight and have holes near the top to allow gas to escape.
Emergency Power Battery Requirements
The emergency source of electrical power must supply the following loads continuously for at least three hours: navigation lights, fire protection and detection systems, communications equipment, the general alarm system, and emergency lighting.
The emergency power source must be positioned aft of the collision bulkhead, outside of the machinery space, and above the uppermost continuous deck.
When the emergency source is supplied solely by storage battery, the batteries must be lead-acid or alkaline type, capable of withstanding pitch, vibration, roll, and salt water atmosphere exposure. Each battery bank must be fitted with its own drip-proof charging system.
Why It Matters on the Exam
Exam questions on this topic test three things above all else:
1. The three-hour emergency power duration and the required loads (navigation lights, fire detection, comms, general alarm, emergency lighting). 46 CFR §28.870 2. The category thresholds — large (>2 kW), moderate (0.2–2 kW), small (<0.2 kW) — and which location rules apply to each. 46 CFR §111.15-3 3. The electrolyte spill angles — 30 degrees under §28.870 for emergency batteries, 40 degrees under §111.15-2 for general construction. 46 CFR §111.15-2
Questions may also test the prohibition on spring clips at terminals, the ammeter requirement on chargers, the prohibition on grounded autotransformer chargers, and the specific lining thickness requirements for lead-acid versus alkaline installations.
Common Pitfalls
Confusing the two spill angles. The 30-degree figure applies only to emergency power batteries under §28.870. The 40-degree figure is the general construction standard under §111.15-2. Exam distractors will swap these numbers. 46 CFR §28.870 46 CFR §111.15-2
Misidentifying the category boundary. The cutoff between moderate and large is 2 kW; between small and moderate is 0.2 kW. These are computed from the highest possible charging current multiplied by rated voltage — not from battery capacity. 46 CFR §111.15-3
Placing a moderate installation in a sleeping space. This is explicitly prohibited. Candidates sometimes assume that any ventilated space is acceptable for a moderate installation. 46 CFR §111.15-5
Forgetting the overcurrent device exceptions. Engine cranking battery conductors and conductors of batteries rated 6 volts or less are exempt from the overload protective device requirement. 46 CFR §111.15-25
Assuming the emergency source can be in the machinery space. It must be outside the machinery space, aft of the collision bulkhead, and above the uppermost continuous deck.
Using spring clips on battery terminals. Permanent-type connectors are required; spring clips and temporary clamps are prohibited. 46 CFR §183.350
Omitting the ammeter on the charger. Under Subchapter T, every battery charger must have an ammeter in the charging circuit.
Quick Check
Q1: What is the minimum duration an emergency battery source must supply all connected loads?
Three hours continuously. 46 CFR §28.870
Q2: Where must the emergency power source be located relative to the collision bulkhead, machinery space, and continuous deck?
Aft of the collision bulkhead, outside the machinery space, and above the uppermost continuous deck.
Q3: A battery charger has an output of 1.5 kW. What category is this installation?
Moderate — charger output is between 0.2 kW and 2 kW. 46 CFR §111.15-3
Q4: At what angle of inclination must a battery cell (general construction standard) not spill electrolyte?
40 degrees from the vertical. 46 CFR §111.15-2
Q5: At what angle of inclination must an emergency power battery cell not spill electrolyte?
30 degrees from the vertical.
Q6: What type of terminal connector is prohibited on shipboard batteries?
Spring clips and other temporary-type clamps are prohibited; permanent-type connectors are required. 46 CFR §183.350
Q7: Which battery conductors are exempt from the overload protective device requirement?
Conductors of engine cranking batteries, and conductors of batteries with a nominal potential of 6 volts or less. 46 CFR §111.15-25
Q8: What instrument must be connected in the charging circuit of a battery charger under Subchapter T?
An ammeter.
Q9: What type of charger is prohibited regardless of installation category?
Chargers incorporating grounded autotransformers must not be used. 46 CFR §111.15-30
Q10: A large battery installation is in a dedicated battery room. Where must the overcurrent protective devices be located?
Next to, but outside of, the battery room.