Lead acid batteries and also VRLA types emit hydrogen under all operating conditions. The hydrogen can produce explosive atmospheres in the concentration range of 4 to 94 vol %. The ventilation requirements are described in local standards and code of practice. They should be adhered strictly so to ensure a safe operation and avoid loss of life and property.


Never install any type of lead acid battery in hermetically closed environments!

VRLA AGM cells and monoblocs emit approximately 30 cm³ hydrogen (NPT) per cell and Ah rated C₁₀ capacity in 30 days under benign float conditions. In abusive conditons (higher voltages and temperatures) this value jumps to 300 cm³/cell/Ah/30 days.

The Standards for battery ventilations assume a more pessimistic gas evolution scenario and require ventilation dimensioning based on hydrogen emission data as shown in the original text of the European Standard EN 50091:1:1993 below.

Annex N (normative) of 50091:1:1993

Ventilation of battery compartments

N. 1 Notes for guidance

Batteries develop a certain amount of hydrogen gas by electrolysis. Gassing occurs particularly during overcharging and at the end of charging, and also during float charge of the battery.

Battery gases escape via vent plugs or, for valve regulated lead-acid batteries, via pressure valves. Mixtures of hydrogen and oxygen are explosive where the hydrogen component exceeds
4 % in a hydrogen/air composition.

Ventilation of battery rooms and/or battery compartments shall ensure a sufficient dilution of hydrogen to avoid any danger of a hydrogen explosion. An excess of dilution is necessary for a corresponding degree of safety. Safety factors between 4-5 shall be applied generally; but on ships or for particular application, even larger safety factors (e.g. 10) may be prescribed.

Gas generation in a vented flooded cell can be calculated from the overcharge current, since: 1 amp per hour of overcharge current will generate 0,45 1 of hydrogen at NT.

q= 0,45 x 10^-3 m³/(Ah)

N.2 Application to lead acid batteries

Flooded cell batteries

A conventional flooded lead-acid battery cell of 100 Ah nominal capacity will require a charge retention current in the range of 20 mA to 100 mA, depending on temperature, quality of electrolyte and ageing at a floating voltage of circa 2,23 V/cell. But where unusual conditions, e.g. overheating or overvoltage, exist the level of hydrogen generating current will be much higher.

In the interests of safety, one should assume a maximum current flow of

I = 1 A/100 A·h during floating at 2,23 V/cell and twice this value during equalizing charging close to the overvoltage level of 2,4 V/cell.

With batteries where the electrode lead alloy reduces gassing, one may assume the half of the above values, for battery cells with catalytic recombination of hydrogen and oxygen (recombination plugs) one may calculate with a quarter of the above values.

Valve regulated batteries (gel or absorbed electrolyte)

Valve regulated lead-acid batteries (maintenance free lead-acid batteries with oxygen recombination in the cells) still will generate hydrogen, escaping from the cell pressure valves. Here one may calculate with a hydrogen generating current of 0,025 A/l00 A·h at a floating voltage of circa 2,27 V/cell and 0,2 A/100 A·h at an overvoltage of circa 2,40 V/cell.

N.3 Ventilation requirements (normative)

To allow for equalization (boost charging) and in the case of valve regulated batteries, operation over a wider range of ambient temperatures the factors of " I " shall use the 2,4 V/cell figures.

The necessary ventilation air flow for a battery compartment shall be calculated by the following formula:

Q = v x q x s x n x I x C

where:

Q is the ventilation air flow, in m³/h;

v is the necessary dilution of hydrogen (100 - 4) / 4 = 24;

q= 0,45·10^-3 m³/A·h generated hydrogen;

s is the factor of safety, e.g. s = 5;

n is the number of battery cells;

I = 2 A/100 A·h - Conventional flooded cell batteries;

I =1 A/100 A·h - Flooded battery cells with low antimony alloy;

I =0,5 A/100 A·h - Flooded battery cells with recombination plugs;

I = 0,2 A/100 A·h- Valve regulated lead-acid batteries;

C is the battery nominal capacity, in A·h.

It is permitted to simplify the formula for Q by introducing the resultant value of v x q x s = 0,054 m³/A·h.

Q =0,054 x n x I x C

Q is the air flow, m³/h

This amount of ventilation air flow shall preferably be ensured by natural air flow, otherwise by enforced ventilation.

Inlet and outlet apertures shall allow for a free access of air flow. The mean speed of air shall be in the region of 0,1 m/s.

With this amount of natural air flow, the battery compartment shall contain air inlet and air outlet apertures with a free area of K₁= 28 h x cm²/m³

A > = K₁ x Q

A is the aperture, in cm²; K₁: 28 h x cm²/m³

or

A > = K₂ x n x I x C

K₂ = 1,51 cm²/A

NOTE 1. Natural ventilation is applicable where the electrical power for hydrogen generation keeps below certain limits. Otherwise the ventilation air outlets would exceed acceptable dimensions. The limits for natural ventilation depend on the battery capacity and the number of cells, and also on the battery technology (vented cells, valve regulated cells), and the battery charging voltage applied.

The above calculation method will result in a sufficient degree of safety against explosion, assuming hot (>300 °C) or sparking components are kept at adequate distance from battery vent plugs or gas pressure outlets. In battery rooms, a distance of 500 mm may be regarded as ensuring sufficient safety. In battery compartments or cabinets, it is permitted to reduce this distance depending on the level of ventilation.

NOTE 2. These requirements are under consideration by CENELEC/TC 21 X.

An example of calculation of the ventilation as required by the European Standard EN50091:1:1993 is shown below.

Example

A battery with 108 cells of a VRLA type with 650 Ah (C₁₀) requires an air flow of:

Q=24·0.00045m³/Ah·5·108·0.2A/
100Ah·650Ah=7.5 m³/h

Battery ventilation has also the function of avoiding heat stagnation within the cells and monoblocs which can lead, if unchecked, to thermal runaways. The amount of heat produced by a VRLA battery is approximately 0.09W per 100Ah C₁₀ and cell. For dimensioning purposes and to take in account conditions of excessive float current a value of 3 to 5x 0.09W/100Ah/cell should be considered.
Accu Oerlikon has foreseen air gaps between unit and is fixing the cell to cell or monobloc to monobloc distance with appropriate lengths of rigid intercell connectors. In cabinet installations or with the use of flexible connectors a gap of at least 10 mm between adjacent units and walls
shall be present.