Guide 05 Naval electrical power
How to Size a Battery Charger for a Ship's Battery Bank
Sizing a shipboard battery charger starts with the battery, not the charger: cell count and chemistry set the DC bus voltage, rated capacity and the maker's maximum recharge current set the charging current, and the connected DC load sets the minimum continuous rating the rectifier must carry even with the battery disconnected.
- Topic
- Naval electrical power
- Checked
- 23 September 2026
- Sources
- 4
- Questions
- 5
01
Start from the battery, not the charger
The number of cells in the bank, multiplied by the float voltage per cell, sets the DC bus voltage the charger has to hold. For flooded lead acid cells that float voltage is typically 2.25 to 2.27 V per cell at 25 degrees Celsius, so a 55-cell bank settles near 124 V; a different cell count or a valve regulated chemistry moves that figure and the battery maker's own value should always be used over an estimate.
The charging current rating comes from the battery, not from convention. Battery makers state a maximum recharge current, often as a fraction of rated capacity, for example one fifth of the ampere hour figure at the eight hour rate, so a 100 Ah cell should not see more than about 20 A. The charger's current limiting is set at or below that figure, whichever is lower once the connected load is added in.
02
Size for the connected load, not just the battery
A float cum boost charger commonly carries the ship's DC load continuously while it keeps the battery on float, rather than the battery supplying the load directly. That means the charger's continuous rating has to cover the load current plus the small float current the battery draws, typically around 1 milliamp per ampere hour of capacity, and its boost rating has to cover the load plus the higher recharge current at the same time.
Once commissioned, float current well above that baseline, more than about 3 milliamps per ampere hour, is worth investigating rather than treated as a sizing error: it usually signals high temperature, a shorted cell or, in valve regulated batteries, the early stage of thermal runaway rather than an undersized charger.
03
Regulation, ripple and harmonics on the input side
A charger's own regulation matters as much as its current rating: a good unit holds output within about plus or minus 0.5 to 1 percent from no load to full load, and specifications for valve regulated batteries commonly hold ripple within about plus or minus 0.5 percent of float voltage and under roughly 5 A rms per 100 Ah of capacity.
The charger also has to live within the ship's own supply tolerance. A six-pulse rectifier draws current rich in the fifth and seventh harmonics, with input current distortion around 30 percent; a twelve-pulse arrangement, combining two six-pulse bridges through transformer windings phase-shifted by 30 degrees, cancels those and cuts distortion to roughly 10 percent. Where a ship's own electrical specification sets a harmonic limit, such as a total harmonic content ceiling on its main supply, the charger's pulse number is chosen to sit inside it rather than exceed it.
04
Confirm the battery's real capacity before finalising the sizing
Nameplate ampere hours describe a new battery, not necessarily the one on the ship. Capacity fades with age and duty, and a widely used criterion for stationary batteries treats 80 percent of rated capacity as the point to plan replacement, since capacity typically falls away quickly after that. A periodic capacity test, discharging the bank under controlled conditions against its rated duty and timing the result, is how ageing capacity gets confirmed before a charger and battery bank are sized together for a refit.
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How to Size a Battery Charger for a Ship's Battery Bank
How is the DC bus voltage for a battery charger determined?
It is the number of cells in the bank multiplied by the float voltage per cell, typically 2.25 to 2.27 V per cell for flooded lead acid at 25 degrees Celsius. A 55-cell bank works out to roughly 124 V. Valve regulated chemistry and other cell counts shift the figure, and the battery maker's stated float voltage always takes priority over an estimate.
What charging current should a battery charger be rated for?
It follows the battery maker's maximum recharge current, commonly specified as a fraction of rated capacity such as one fifth of the ampere hour figure at the eight hour rate. A 100 Ah cell, for example, should not be recharged above about 20 A. The charger's current limiting is set at or below that figure once the connected DC load is accounted for.
Why does the connected DC load matter when sizing a charger, not just the battery?
A float cum boost charger usually carries the ship's DC load continuously while floating the battery, rather than the battery supplying the load itself. Its continuous rating has to cover load current plus float current, and its boost rating has to cover load current plus the higher recharge current at the same time, so undersizing for the load leaves no margin during a recharge.
Does the rectifier's pulse number matter when sizing a shipboard charger?
Yes, where the ship's own electrical specification limits harmonic distortion on the main supply. A six-pulse rectifier's input current carries roughly 30 percent distortion, dominated by the fifth and seventh harmonics; a twelve-pulse design, combining two six-pulse bridges through transformer windings phase-shifted by 30 degrees, cuts that to around 10 percent, which matters more as charger size grows relative to the ship's generator capacity.
Should charger sizing use the battery's nameplate capacity or its tested capacity?
Tested capacity, once the battery has some age on it. A commonly used criterion treats 80 percent of rated capacity as the point to plan replacement, because capacity typically falls away quickly beyond that. Sizing a replacement charger against a nameplate figure on an aged bank overstates the real backup time available, so a capacity discharge test comes first.
Checked against
Sources
- 01The proper charging of stationary lead-acid batteries, Vertiv, BATTCON 2010
- 02EED-Q-071 (R4), Specifications of Motors and Starters for Naval Ships, Directorate of Electrical Engineering, Integrated Headquarters MoD (Navy)
- 03Recommended practice basis for stationary battery replacement, the IEEE 450 criterion, U.S. Nuclear Regulatory Commission
- 04SOLAS Chapter II-1, electrical installations on merchant ships (warships are exempt from SOLAS), International Maritime Organization
This guide explains how the system works in general, as the documents above describe it, and was last checked on 23 September 2026. Rules and thresholds change: the governing document is the authority, not this page.
On this site
Where this meets the work
- The DQA(N) type approved battery charging rectifier
- Special electrical equipment work
- The two type approvals on record
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