Thermal Runaway

Today's cell sites increasingly are being equipped with VRLA batteries. Although the VRLA design provides improvements over its predecessors, it can be prone to a condition known as thermal runaway. Under circumstances perfect for thermal runaway, elevated operating temperatures and significant overcharging cause the battery to generate heat faster than it can dissipate it. This can cause the housings that hold your batteries to melt, rupture or worse. Aside from the hardware damage, the overheating also can produce toxic and dangerous amounts of hydrogen sulfide and sulfur dioxide.

In older lead acid batteries, water from the electrolyte burns off during overcharging, and oxygen and hydrogen gases are given off from the positive and negative plates, respectively. The benefit of this battery design is that the gases then pass through the liquid electrolyte and out an open vent. However, because water also is dissipated with the gases, these batteries need periodic replacement of water.

In the VRLA battery, the generated oxygen at the positive plate migrates through a gel or absorbent glass mat (AGM) separator before reaching the negative plate. Once it gets to the negative plate, the oxygen goes through a recombination process in which it reverts to water. According to Phil Shumard, Hawker Energy Products applications support manager, this recombination can be nearly 100% with proper battery selection and care, making it less maintenance-intensive.

Even though the VRLA battery doesn't require water replacement, the water level is critical. Jim McDowall, Saft America, pointed out that there is a fine balance in the amount of water in a VRLA cell.

"Too much water, and the cell behaves like a vented unit, with little recombination of charge gases and liberation of those gases to the atmosphere," he said. "Too little water, and the electrolyte is unable to sustain the cell reaction, causing discharge capacity to be lost."

During float charging, water is broken down at the positive plate producing oxygen gas. If that oxygen reaches the negative plate, the recombination reaction occurs and returns water to the system. According to McDowall, if the glass mat separator is nearly saturated, it will not be able to transport much oxygen to the negative plates, so only a small amount of charge gas recombination occurs. Without recombination, hydrogen and oxygen are freed from the cell, and water is lost.

For a new cell, this situation actually could be good, McDowall said, because the loss of liquid will leave some void spaces in the AGM, allowing more oxygen to pass through and therefore more recombination. A new cell is more likely to improve its recombination efficiency in the first weeks and months of its life on float. As the cell ages, recombination efficiency continues, but at a slower rate.

The benefit of VRLA batteries over vented cells is twofold: a lower generation of hydrogen and lower water loss. According to Thermal Runaway -- Its Causes and Prevention, a white paper published by Johnson Controls, the VRLA battery may emit only 1% to 10% of the gas of its vented-cell brethren, which is important from a cell-site-safety standpoint. And because of the impracticality and inconvenience of replenishing the batteries' water at remotely located cell sites, the wireless industry gradually is moving away from vented or flooded batteries.

Over the Top In the wireless environment, shorted cells are the prime suspects for causing thermal runaway. Cells short out mostly because of gross failure of the charger. However, sometimes improper installation or, less frequently, a defective unit from the manufacturer will yield a shorted cell, Shumard said.

Most batteries in wireless sites are float charged, meaning there is a continuous voltage applied across the battery string. This voltage causes some heat. Generally, the float current will increase as the floating voltage and ambient temperature rise. At higher temperatures, the amount of deposited heat increases. The ideal is that the battery will be able to dissipate any unnecessary heat.

However, Bob Wittemann, C&D Technologies senior product manager, explained that thermal runaway generally occurs as a result of "a vicious circle" created by heat caused when the battery and charger react to each other. In a typical battery installation, as the temperature goes up, the voltage goes down. When the voltage goes down, the charger acknowledges it by adding more current. As more current is added, the temperature goes up even more, causing the voltage to remain at the decreased level or to decrease further. The charger continues to pump higher amounts of current into the battery, creating more and more heat.

Wittemann explained this is why temperature compensation is so important. Once this high temperature occurs, the compensator will not allow any more current to get pumped into the battery.

Consider what happens in a 24-cell system when it is overcharged. According to Tom Ruhlmann, Johnson Controls Specialty Battery Division technical services manager, you normally would charge each cell 2.3V. In a string of 24, that is 55.2V as the charging voltage. However, if one of the 24 cells shorts out, you have that same 55.2V on the string charging only 23 cells. This raises the individual cell voltage to 2.4V. Over a period of time, this overcharging results in drying and gassing of the cell. Ruhlmann said if two cells short out in a string, you can guarantee thermal runaway.

As the battery ages, its internal grids grow and change dimension, changing the clearance with other components. When there is insufficient spacing between grids, the battery tends to heat up. The aging process accelerates with high temperatures in general operating environments. Environmental factors, ac ripples in UPS and overcharging all contribute to high temperatures in wireless sites.

Hawker's Shumard said wireless carriers can stave off thermal runaway by operating with a low float current, using a charge voltage that is temperature-compensated and following the manufacturers' charging curves.

Ruhlmann agreed and strongly encouraged charging with temperature-compensation. He recommended that carriers reduce voltage and trickle charge at higher temperatures and regularly monitor battery temperatures.

* High charging voltage

* Unlimited or too-high charging current limit

* Elevated float current

* High temperature operating environment

* Unventilated battery enclosures

* Battery or system failures resulting in the above conditions

* Unrealistic life expectancy

As dramatic as thermal runaway sounds, is it really that common? According to the vendors interviewed for this story, it isn't a regular occurrence by any means. In fact, most of them had never witnessed batteries that have gone through the phenomenon. However, Bob Wittemann, C&D Technologies senior product manager, pointed out that it is a function of the application more than the battery. He said most sites have good design and ample ventilation. However, newer sites sometimes are housed in cabinets on the sides of roads and on rooftops. He also said there are cabinets that are sitting in 140 degrees to 160 degrees temperatures without ventilation, which are thermal runaways just waiting to take off.

Tom Ruhlman, Johnson Controls Specialty Battery Division technical services manager, recommended six guidelines for reducing the potential for thermal runaway. They are:

1. Correct recommended charging voltages as related to the expected operating temperatures. This includes temperature-compensated charging voltages if you expect large temperature swings.

2. Initial charging current limitation consistent with required recharge time and anticipated depth of discharge.

3. Reasonable battery operating environment temperature with a minimum of 0.5-inch spacing between units and adequate ventilation.

4. Charging current disconnect feature based on battery elevated temperature.

5. Inclusion of a ground-fault detection circuit and installation of the units on an insulated surface.

6. Gas detection and alarm capability that are sensitive to both hydrogen and hydrogen sulfide gas, which could be emitted during extended thermal runaway.