Reliability of gel-cell (VRLA) batteries

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Our insurance inspector voiced concerns over our recent installation of VRLA batteries, and suggested a capacity test even though the batteries are less than one year old. This is our first VRLA set on the larger systems (2000 Amp-hour) - capacity tests are currently scheduled for our Lead-Acid sets every five years. The research I did prior to specifying VRLA did not reveal any serious problems with this style battery - assuming proper maintenance and operation is done - and the pricing and delivery made them especially attractive. I would like to hear of any negative experiences or known problems with this type of battery that would support our inspector’s concerns.


I have mixed views on VRLA. We use VRLA batteries extensively in our company. In 1989 we began using 20 year batteries. To date, they have been less than satisfactory - some failing between 7 and 11 years. The kicker was that when they failed they had a tendancy to fail in an “open circuit” state … that is to say, zero current flow. Not the ideal failure mode for a UPS system. I believe this is the history your insurance underwriter is concerned about.

I would speculate that many of the VRLA batteries were not maintained as well as flooded batteries they were intended to replace. Many of the first generation batteries were marketed as “Maintenance Free” in which many users took it literally … just like your car battery. Clearly they were not Maintenance free but more like “reduced maintenance”. Since the mid 90’s, many manufacturers and vendors have avoided the “free” aspect and improved their literature around proper maintenance.

As I understand it, the battery technology of today is considered “third” generation, in which manufacturing process have been perfected and cell design improved which virtually eliminates the old failure modes. Most of our original “First” generation VRLAs have been replaced. Batteries installed since the late 1990’s have performed more predictably with no open failures.

To mitigate this risk to our business, we have switched to:

  • 10 year batteries,
  • parallel strings
  • annual testing (pending the number of cycles the batteries are put through per year)

This established the right balanced of maintenance cost with capital cost and allows us to leverage newer and improved VRLA technology (or it’s future replacement) as it is perfected. If you take the right approach to managing risk, you should be able to address the concerns with your underwriter.

I’d like to refine a couple of points here.

  1. VRLA refers to “Valve-Regulated Lead Acid” batteries, which typically means they are installed in a sealed case and a valve to relieve pressure in case of excessive gassing. The term does not refer to the fabrication or construction of the plates inside the battery. They could be “gel-cells” or they could be absorbed glass mat plates. There are a lot of sub-varieties now, if the manufacturers are to be believed.

  2. The failure mode varies based on the plate style, therefore you should know this before deciding which type of failure is the most likely.

  3. The probability of failure due any particular cause depends on how they are installed. Details matter. Parallel strings increase the risk of failures due to string imbalance, but reduce the chance of total system failure by redundancy.

  4. Open-circuit is not a failure cause, it’s the result of a failure, which may be overheating, sulfation, or gas release due to over-charging. Each of these are the result of other failures, such as improper installation, overload, incorrect UPS settings, or manufacturing defects, etc.

  5. Disconnecting battery terminals should always be a safe thing to do. If a battery string in my system goes open-circuit, my inverters will just shut down, or use the other string until it’s exhausted. If your battery installation does unsafe things when a terminal is disconnected, or internally breaks the circuit, then I’m concerned about something in your system design.

  6. I am much more frightened of a short-circuit failure myself. A short-circuit failure in a UPS leaves your building without power AND SETS IT ON FIRE.