Failure modes for stationary power Vented Lead Acid (VLA) batteries and the cause of failure in Valve Regulated Lead Acid batteries (VRLA) have been increasingly understood as the industry has continually utilized them in Telecommunications, UPS, and Utility applications at a growing rate.
The Main Culprits
The gradual buildup of passivation layers in cells, loss of electrolyte and resultant dry out of cells due to overcharging, corrosion of positive plates as a result of improper float charging can all be slowed down when attention to the proper operation is adhered to. Adherence to the manufacturer’s guidelines for operating temperatures, float voltages, storage limits and expectations of cycle life go a long way towards meeting the life expectations of batteries.
Temperature effects lead acid battery life. The higher the ambient temperature the shorter the life expectancy. A temperature of 77 degrees F (25 degrees C) is optimum for most lead acid products. For every 10 degrees F of constant float temperature you can expect the cut the life of the battery in half.
Distributed and outside plant applications of batteries can make hitting the proper temperature target problematic. Most rectifiers today come with a temperature compensation algorithm build into them to adjust float voltages up or down depending on the ambient temperature. Such a device will lower float voltage when the temperature goes up and increase the charge voltage upon cooler temperatures.
How often have you inspected a battery bank and find that the thermistor connected to a pilot battery in the string is not connected to the cell? Best maintenance practices should include a step for visual inspection for this issue and corrective action.
Additional visual inspection should include searching for corrosion on the positive post and swelling of VRLA cases. Corrosion of the positive plate may indicate a post seal leak. Swelling of a case can indicate potential improper charging voltage of the cell for the ambient temperature of the site.
Proper float voltages are a function of the lead acid battery’s positive plate alloy, the specific gravity of the cells and the temperature expected in the site. Example: A VRLA cell of lead calcium or lead tin alloy, with a specific gravity of 1.300 acid at 77 Degrees F typically is float charged at 2.25 volts per cell +/- 1%. Voltages across the string should be fairly constant.
Verify the age of products being installed into your site. Has the battery been in storage for a long time. Has the stored battery been periodically charged in the warehouse to keep it healthy? Ask for records of this! Float charging of batteries in service slows down the corrosion rate of the positive plate. Charging of stored batteries helps in this regard.
The cycling capability of lead acid batteries, needs consideration. Vented lead acid cells have traditionally been marketed with various “flat plate” positive plate thicknesses, targeted at specific applications. Thicker plates typically for long duration discharges and heavy cycling , medium plate thickness for general purpose (substation) application and thinner plates for high rate/short duration discharge (UPS).
Tubular positive plates are also prevalent in the market. All these plate designs need to be considered for the features/benefits they bring to the specific application. VRLA batteries have improved in cycling performance over the years as well.. HOWEVER, be aware of the number of cycles your site is experiencing or may be required to perform and specify the proper technology for the application.
Essential Regular Maintenance
Lastly, there is no such thing a maintenance free. Regular testing using available ohmic testing technology is a good tool for accessing the temperature, charging voltage and relative ability of the battery to delivery current. Load testing, when logistically viable a good test of capacity. Visual inspection always needed.
All of the above will help to identify and allow for adjustment of your operating conditions to maximize life expectancy of your batteries.