Battery Systems for Mission Critical Infrastructure - Design, Maintenance and Testing (2)

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Inspection


The maintenance of the batteries in mission critical facilities is especially important. When maintained regularly, batteries will perform according to design to support the critical systems, ensuring the continuity of power. Maintaining the batteries also elongates their life, resulting in lower operation costs. But the first step of maintenance is to inspect. By first inspecting and then assessing, we can develop a plan for remediation.

Let’s look at each battery type separately. IEEE has developed separate standards for just this reason.


Vented Lead-acid Batteries


The most routine inspection for this type of battery is a visual inspection. IEEE has developed a standard, 450-2010, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications (https://standards.ieee.org/findstds/standard/450-2010.html). This standard is a very good guide for the operations staff to create a maintenance procedure.

The monthly inspection, as recommended in the IEEE 450-2010 standard, includes the following:

A visual inspection of:

1. The general condition of the area. The area (room) should be as clean as possible so that dirt or excessive dust does not cover the battery cells. Unclean equipment is harder to assess during a visual-only inspection.
2. The battery cells for cracks. This step is important because a crack in the cell’s outer shell could allow the electrolyte to leak and the battery to discharge.
3. The electrolyte levels. A significant drop in the electrolyte level means that the specific gravity of the electrolyte has increased. A greater specific gravity would impact the life of the battery.
4. The battery terminals for corrosion. Corrosion of the battery terminals will increase the resistance of the connection, thereby decreasing the amount of current supplied by the battery system.

A measurement of:

1. The float voltage at the battery terminals. If the battery system performs at a float voltage outside the manufacturer’s recommended range, the battery’s life expectancy would be adversely affected.
2. Room or area temperature and ventilation. The temperature is important because it impacts battery-life expectancy. Ventilation is important because proper movement of the air mitigates high concentrations of hydrogen, which is a byproduct of chemical processes in the battery cells. High concentrations of hydrogen in the air significantly increase the risk for explosions.

The recommended quarterly inspection includes the following:

A measurement of:

1. The voltage of each cell. Lower-than-recommended (by the manufacturer) voltage levels could have an adverse affect on the life expectancy of the battery.
2. The specific gravity of the cells’ electrolyte. One needs to keep in mind that specific gravity of the electrolyte increases in a full charge. It would be best if three measurements are taken: one on top, one in the middle, and one on the bottom of the cell. The average of the three values is the value to be used. If taking three measurements is not possible, taking a reading as close to the middle as possible is best. Note that it’s not necessary to measure the specific gravity of each cell in the battery string, rather only about 10% of the cells.
3. The electrolyte temperature of a few cells. If we have a 125 V battery system, we’d have 60 cells. It would suffice to check the temperature of six of them. The desired temperature is the one recommended by the manufacturer. If the electrolyte is at a higher temperature, a higher float current is required to maintain the cell voltage. Too high of a charging current could adversely affect the electrolyte composition, as more of the hydrogen and oxygen is being gassed. On the other hand, the lower temperature causes a smaller floating current, which in turn, slows the charging process.

Corrective actions are shown for some abnormalities in vented lead-acid and nickel-cadmium (NiCd) batteries.

There is also a yearly inspection that applies the quarterly inspection to all the battery cells, which is, therefore, much more involved. If abnormalities are observed during these inspections, there are corrective actions that can be taken. See Table 1 for some common corrective actions.

Of the abnormalities mentioned in Table 1, the room temperature and ventilation usually are set during the design. Total cost of ownership analysis is usually done by taking into consideration the optimal conditions so that the battery life is longest. The rule-of-thumb correlation between ambient temperature and lead-acid battery life, be it vented or VRLA, is as follows: battery life decreases by 50% for every 8°C above the normal temperature, which is 25°C (77°F).


Battery Life


Determining the battery life is important, especially in mission critical facilities. With proper maintenance, battery life could be predicted accurately, thereby avoiding any downtime. The most important factors affecting battery life are:

Ambient Temperature:
Keeping the temperature at 25°C (77°F) is optimal.

Maintenance:
Keeping the cell temperature in check; large temperature differences among the cells will affect battery life.
Keeping the cell voltage in check. The following should be corrected:

- Voltage below open cell voltage + 0.06 V
- Voltage above open cell voltage + 0.1 V (or 0.05 V for lead antimony).

Cycling:
The more discharges occur, the shorter the battery life.

Chemical Components:
Vented batteries can be lead-calcium, pure-lead, lead-selenium, or lead-antimony. All the outside factors affect the battery life differently, depending on the chemical composition of the battery.



Next Article:
Battery Systems for Mission Critical Infrastructure - Design, Maintenance and Testing (3)



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