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Care and Maintenance of VRLA Batteries

Structure and Operation of Lead Acid Batteries


When the battery is discharged water is produced, diluting the acid and reducing its specific gravity. On charging sulphuric acid is produced and the specific gravity of the electrolyte increases.
The chemical reactions that occur during charging and discharging are summarised in figures 1 and 2.

SLA Chemistry


If lead-acid batteries are over discharged or left standing in the discharged state for prolonged periods hardened lead sulphate coats the electrodes and will not be removed during recharging. Such build-ups reduce the efficiency and life of batteries.

Sulphation Damage due to Self or Over Discharge

Causing the battery to discharge below the 0% State of Charge causes the battery to “sulphate”, a phenomenon which damages the battery irreversibly.

Here is a picture of a sulphated battery that we have cut open:

Sulphated 3AH DCC CN

Therefore, it is important that the battery never enters that state, by taking the correct maintenance measures.
Even if a battery is not in service, the phenomenon of self discharge can cause the battery to drop into the sulphation state.

Battery Self Discharge Phenomenon


SLA Batteries exhibit self discharge. Even a fully charged battery will self discharge, and if the chemistry is not stabilised before it reaches the 0% state, sulphation will start to take effect.

Between 15 Mar 2018 and 14 May 1018, the Engineering Dept of Applied Systems carried out SLA Battery self discharge evaluations. A set of 4 batteries housed in a metal casing was left out in the open, and the temperature and terminal voltage were captured using a Fluke datalogger. Here are some results:

Self Discharge Results

Estimation of the State of Charge (SOC)


For VRLA batteries, we can get an indication of its state of charge from its terminal voltage

A typical table of the state of charge for a 12V VRLA battery is given in the figure below.

VRLA SOC

Noting that the table gives the SOC for a 12V VRLA, we can infer that for a 24V battery bank, such as used in the BBU of the DCC and DCC-CN Projects, the 0% State occurs at a voltage of 10.5 x 2 = 21V.

The recommended replacement interval of VRLA Batteries used in the BBU Battery Modules is 2 years. This is in line with industry norms. If the batteries have been subject to over discharge it may be necessary to replace the batteries even sooner.
Here are some links to web resources on the matter:

Lead Acid Lifetime Study : See pages 1 to 3

Hitachi Chemical VRLA Battery Characteristics - Service Life : See Fig 1 and Fig 2

Service Life Factors for VRLA Batteries : See Pg 4 and Fig 3