What is Thermal Runaway?

One of the largest hazards related to UPS systems, Emergency Lighting Inverters and DC Applications is thermal runaway. What is it, and how can you prevent the potential loss of millions?

What is Thermal Runaway?

Thermal runaway can be caused by a number of factors creating an elevated internal temperature. When batteries are unable to disperse this built up heat faster than it can be generated, it can cause an event known as thermal runaway. Over time, if the batteries are not able to lower their temperature, cells may dry out and the warmer containers will soften and bulge.  As the temperature continues to rise, housings may rupture and melt, causing your batteries to ignite. 

While there is no straightforward way to determine what batteries will go into a state of thermal runaway, there are several conditions that can lead to it.  Below, we have listed the common indications that your batteries could be at risk, and how to lower your facilities chance of a fire.

Common indications that your batteries could be at risk:

Float charging voltage

One common way that batteries can become “overcharged” is due to issues with the charger. Your unit may have settings that allow for different capacity batteries to be installed. If the charger is set up to charge 16-volt batteries, but your UPS has 12 volt UPS batteries – this would lead to the float voltage exceeding the recommended rating of the battery, and can cause the batteries internal temperature to increase. UPS systems that have not been calibrated in a number of years, can also create the same effect. UPS systems that have been installed with two different sized batteries, may have been set up to provide a float voltage that is incorrect for all the batteries. The best way to avoid this is to have your UPS batteries designed appropriately, and inspected regularly to identify issues before harm can be done. 

Incorrect Charging Current

Another issue with the charger, is that the batteries have specific rates at which they can be charged, and this can vary from battery to battery.  As an example, Thin Plate Pure Lead batteries can charge at a faster rate than a traditional VRLA. If your charger is set to recharge batteries at the incorrect amperage, this could cause the batteries to overheat – leading to thermal runaway. 

Age of UPS Batteries

By far the most common reason for thermal runaway is batteries that have exceeded their design life. While we often refer to years as a way to age batteries, a battery life is measured in cycles. Both the physical age of a battery, as well as the amount of charge/discharge cycles should be considered when determining end of life. As batteries age, their chemistry begins to deteriorate, and can lead to a variety of issues.

Operation outside of recommended temperature

Traditional VRLA batteries prefer to be stored in areas with a temperature of around 77 degrees Fahrenheit. As a general rule of thumb, for every 15F to 18F (8C to 10C) rise in operating temp above 77F expect your battery service life to decrease by 50%. When VRLA batteries are stored above that temperature, it makes it more difficult for the batteries to cool themselves, this leads to thermal runaway.  However not all UPS units can be placed in areas that can be cooled, in these instances it would be beneficial to look at nontraditional VRLA batteries. Both TPPL and Lithium batteries have a much wider range of temperatures that they can safely operate. 

Battery Failure

Battery failures that result in thermal runaway can be avoided by proper design and routine maintenance. The typical life expectancy of batteries range from 3-5 years and your service partner should be reaching out to you when your batteries are due to be replaced. Your UPS service technician should be determining if the batteries may have a charging issue that they are able to correct.  If you would like to read more about causes of thermal runaway, you can read more here!

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