Guide 05 Naval electrical power
Float vs Boost vs Equalising Charge, Explained
Float charge holds a fully charged lead acid battery at a steady voltage that just replaces self-discharge. Boost charge raises that voltage for a limited period to recharge a discharged battery faster. Equalising charge is an extended boost applied to the whole string to level every cell's voltage and specific gravity and break up acid stratification.
- Topic
- Naval electrical power
- Checked
- 23 September 2026
- Sources
- 3
- Questions
- 5
01
Float charge: the resting state
For flooded lead acid cells, float voltage is typically 2.25 to 2.27 V per cell at 25 degrees Celsius, so a 55-cell 110 V bank floats near 124 V. The correct figure depends on electrolyte specific gravity and grid alloy, so the battery maker's stated float voltage takes priority over any rule of thumb, and it is adjusted for temperature.
A quick check comes from specific gravity: cell open circuit voltage is roughly equal to electrolyte specific gravity plus 0.845, and float voltage sits about 0.12 V above that. A flooded cell at specific gravity 1.215 has an open circuit voltage near 2.06 V and floats around 2.18 V; a valve regulated cell at specific gravity 1.300 floats around 2.265 V.
02
Boost charge: faster recovery after a discharge
Boost, or absorption, voltage for lead acid cells typically lies between about 2.30 and 2.45 V per cell, set by the battery maker. Higher voltage recharges faster but increases gassing and water loss. For valve regulated cells, gassing can start as low as about 2.4 V per cell, so the boost setting has to follow the data sheet rather than a generic figure.
A float cum boost charger supplies the DC load continuously while keeping the battery on float, and switches, automatically on a timer or threshold, or manually, to boost after a discharge. Charging current on boost is capped by the maker's limit, often a fraction of rated capacity such as one fifth of the ampere hour figure at the eight hour rate, so a 100 Ah cell would not be recharged above about 20 A.
03
Equalising charge: levelling the whole string
An equalising charge is an extended boost above normal float voltage, applied to the whole string rather than one weak cell, to bring every cell to the same voltage and specific gravity, reduce sulphation and, in flooded cells, break up acid stratification that settles during long periods on float. Because every cell is being overcharged on purpose, it is limited in time, typically not beyond about 72 hours, and electrolyte levels are checked first.
Gassing increases during boost and equalising charging, and hydrogen becomes flammable in air at around 4 percent concentration, so battery compartments need enough ventilation to stay well below that during and after an equalising charge, with no flames or spark-producing work nearby until the space has cleared.
04
Setting float voltage correctly for temperature
Lead acid charge voltage has to fall as temperature rises. A common coefficient is about minus 3 millivolts per cell per degree Celsius from a 25 degree Celsius reference. A temperature compensated charger reads a probe on the battery and adjusts float and boost voltage automatically; without it, a battery that runs hot overcharges and dries out, while one that runs cold stays chronically undercharged.
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Float vs Boost vs Equalising Charge, Explained
How is float voltage worked out from electrolyte specific gravity?
A rule of thumb gives open circuit voltage as electrolyte specific gravity plus 0.845, with float voltage about 0.12 V above that figure. A flooded cell at specific gravity 1.215 works out to roughly 2.18 V float, and a valve regulated cell at specific gravity 1.300 works out to roughly 2.265 V. The battery maker's own figure always takes priority over the estimate.
What is a float cum boost battery charger?
A float cum boost charger is a single rectifier that supplies the connected DC load continuously while keeping the battery on float, then switches to a higher boost voltage, automatically or manually, to recharge the battery after it has been discharged. It replaces the older arrangement of separate float and boost chargers on one battery bank.
How long can an equalising charge safely run?
An equalising charge is typically limited to no more than about 72 hours, because every cell in the string is being deliberately overcharged for the duration. Electrolyte levels on a flooded bank are checked before starting, and ventilation needs to be adequate throughout, since gassing and hydrogen release increase during the extended boost.
What happens if a battery is boost charged for too long or at too high a voltage?
Excess boost voltage or duration increases gassing and water loss, heats the plates, and can push a valve regulated battery toward thermal runaway, a self-feeding cycle where a hot battery draws more current, heats further and draws still more. Current limiting and correct temperature compensated voltage settings are what prevent it.
Does float voltage need to change with temperature?
Yes. Lead acid charge voltage falls as temperature rises, by roughly 3 millivolts per cell per degree Celsius from a 25 degree Celsius reference. A temperature compensated charger adjusts automatically from a probe on the battery; without compensation a battery running hot is overcharged and one running cold is left undercharged.
Checked against
Sources
- 01The proper charging of stationary lead-acid batteries, Vertiv, BATTCON 2010
- 02BU-403: Charging Lead Acid, Battery University, Cadex Electronics
- 03SOLAS 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
- The maintenance contract record
- Special electrical equipment work
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