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Gas Production in Value Regulated Lead Acid Batteries

Published by Jason Koffler on 11 November 2013

Gas Production in value regulation lead acid batteries can cause critical issues as hydrogen can be released.

1. HYDROGEN PRODUCTION.

Hydrogen is produced within lead acid batteries in two separate ways:

a. As internal components of the battery corrode, hydrogen is produced. The amount is very small and is very dependent upon the mode of use. However, with a continuous float charge an approximate amount produced would be:

H = 100 millilitres per ampere-hour capacity/ cell/annum.

b. Hydrogen is also produced in lead acid batteries when charging takes place due to the electrolysis of the water constituent of the electrolyte. The amount of hydrogen produced in this manner is potentially significant and may be calculated using the following formula:

Hydrogen (H) = 0.456 x (I – %) litres per hour per cell.

(I) is the over-charge current and (%) is the recombination efficiency.

As can be seen, the fraction of hydrogen produced from electrolysis that is actually emitted depends upon the recombination efficiency of the cell. This may be determined from the graph shown below.

Critical Power Supplies - Gas Production in Value Regulated Lead Acid Batteries image 1

GAS RECOMBINATION EFFICIENCY

GAS PRODUCTION IN VALVE REGULATED LEAD ACID BATTERIES

2. EXAMPLE CALCULATIONS

a. If the recombination efficiency is 100% then any hydrogen evolved will be at a very slow rate from the corrosion of internal battery components. With a continuous float charge an approximate figure for a 24Ah-12v would be:

100 millilitres per ampere-hour capacity/ cell/annum = 14.4 litres per year.

b. If the recombination efficiency of the battery was only 85% and the overcharge current was 10 amps, the volume of hydrogen produced would be:

H = 0.456 X 10 × 15/100

H = 0.684 litres/hour/cell

Under these conditions, additional hydrogen is being evolved as per paragraph 2a above.

3. RECOMBINATION EFFICIENCY OF YUASA BATTERIES

From the graph above, it can be seen that VRLA batteries have, practically, a recombination efficiency of 100% if the charging current is kept below 0.01CA. As the maximum overcharge current under normal float conditions is less than 0.004CA, it can be seen that the only Hydrogen evolved is as described in paragraph 2a. above. There is no hydrogen evolved from overcharge current. This applies for VRLA batteries subjected to normal float conditions.