What is thermal runaway of valve-regulated lead-acid batteries and its countermeasures

In VRLA batteries, small amounts of hydrogen evolution on the negative plate and corrosion of the positive grid both result in electrolyte water loss. Water vapor permeation through the cell can be considered as a water loss source, but for practical wall thicknesses and moderate relative humidity, permeation is minimal. Water vapor also escapes from the battery with the exhaust hydrogen, but has a minimal effect.

Valve-regulated lead-acid batteries used as backup power sources face severe temperature environments, especially in high-temperature summer environments. Due to electrolyte hydrolysis during floating charge, the battery is prone to dry-out or thermal runaway, which in turn damages the battery performance.

Battery dry-up during float charge is a phenomenon in which the charge electrolysis causes electrolyte water loss and reduces the battery discharge capacity. Thermal runaway is a phenomenon in which the temperature rises and the charging current increases, which causes abnormal heat generation in the battery, and finally leads to dry-up that can damage the battery. Thermal runaway is prone to occur when the battery is used for a long time at a high temperature of not less than 60 ℃. When the battery is used at a high temperature of not less than 70 ℃, thermal runaway can even occur during short-term use.

2 Thermal runaway

Thermal runaway is defined as a critical state that occurs during constant voltage charging [1]. At this time, the battery current and temperature have a cumulative and mutually reinforcing effect, which gradually increases and causes damage to the battery.

As the temperature rises, the charging current increases. This is the result of the complementary effect of the increase in the amount of oxygen generated by the decomposition of the electrolyte on the positive plate and the increase in the oxygen absorption reaction rate on the negative plate accompanied by the improvement of the sealing reaction efficiency. With the increase in reaction heat and charging current, when the Joule heat generation rate is greater than the heat dissipation rate of the battery, the battery temperature rises above the ambient temperature. The increase in battery temperature further causes the charging current to increase, which in turn causes the battery temperature to rise. In this way, a vicious cycle occurs. Finally, thermal runaway occurs.

3. Countermeasures for battery use and maintenance

Drying out of VRLA batteries can cause thermal runaway. Drying out is most associated with very severe charging regimes, often in combination with high battery temperatures. The best way to overcome this problem is to prevent extreme conditions in the battery operation, that is, for example, to avoid high rate charging, especially after deep discharge and when the battery temperature is high. In high temperature applications, even special cooling systems may be required. Generally speaking, drying out reduces battery capacity and ultimately shortens battery life. Drying out also increases seal reaction efficiency, which can be a serious problem in UPS applications, where excessive seal reaction efficiency can dramatically increase battery temperature. This can cause thermal runaway.

The best way to avoid dry-out and thermal runaway is to avoid the heat generated by the battery during float charge. Be sure not to place the batteries too close together. The distance between batteries should be at least 10 to 20 mm. Maintaining this distance usually dissipates heat effectively.

Valve-regulated lead-acid batteries are very sensitive to operating temperature, resulting in reduced service life at high temperatures, and in extreme cases, thermal runaway. When the temperature rises, the float charge constant voltage generates a large current. This in turn causes the battery temperature to rise, and as mentioned above, thermal runaway occurs. The current can increase to the point where the battery gasses and begins to dry out; when the battery dries out, the internal resistance increases, generating more heat, and the tank softening failure may occur, or in extreme cases, the lead parts may melt. However. These phenomena can be avoided through good cooling and ventilation, temperature compensation of the float charge voltage, and limiting the effective current.

High charging voltages help reduce charging time and avoid sulfation. However, it must be noted that high currents flowing through the battery under such conditions can cause thermal runaway. Therefore, high charging voltages are only allowed for a limited time after discharge. Chargers that automatically reduce the charging voltage after a given time are a suitable way to overcome this problem.

The best way to avoid thermal runaway is to monitor the battery temperature and automatically change the charge voltage or charge current according to the measured temperature. To suppress the charge voltage or charge current, it is recommended to use the measured battery temperature instead of room temperature.

4. Countermeasures in battery manufacturing

4.1 In the case of AGM VRLA batteries, it is important to control the separator electrolyte saturation during processing, because it is known that too low saturation gives a high sealing reaction efficiency, resulting in an increase in charging current and causing thermal runaway.

4.2 A high safety valve opening pressure is beneficial in reducing water loss, but it will inevitably cause some other problems. Generally speaking, a pressure between 100 and 2 mbar seems to be a good compromise to avoid excessive water loss.

4.3 To avoid thermal runaway, there are several ways to reduce the increase in charging current of valve-regulated lead-acid batteries:

4.3.1 Increase the amount of lignin added to the negative electrode lead paste to improve the overvoltage rise effect.

4.3.2 Reduce the ratio of the negative electrode active matter mass to the positive electrode active matter mass to reduce the heat generated when the oxygen generated by the positive plate is absorbed by the negative plate.

4.3.3 Add amine organic matter to the electrolyte.

4.3.4 Adding surfactant to negative electrode active material

4.4 Adding lignin treated with sulfuric acid to the negative electrode active material [2]

This is a valve-regulated lead-acid battery with improved negative plate charging characteristics and less prone to thermal runaway. The method is to place lignin in a heated sulfuric acid solution for a certain period of time and add the treated lignin to the negative active material. The treatment temperature in dilute sulfuric acid can be between 30 and 100°C, and the density of dilute sulfuric acid is below 1.6. For example, 10 g of lignin is added to 100 mL of dilute sulfuric acid with a density of 1.3 heated to 70°C and placed for 60 minutes, followed by filtration and drying. When preparing the negative lead paste, the treated lignin (mass fraction 0.2%) is added relative to the mass of the lead powder (converted to the mass before treatment). The valve-regulated lead-acid battery made of this negative lead paste has a floating charge current of 1 at an ambient temperature of 25°C, and even at an ambient temperature of 70°C, the floating charge current ratio does not exceed 10 times. For ordinary valve-regulated lead-acid batteries that have not been treated, the floating charge current increases significantly below 40°C, which then leads to thermal runaway, and the floating charge current ratio is about 50 times at an ambient temperature of 70°C.

Normally, lignin added to the negative electrode active material will decompose during the use of the battery and dissolve from the negative electrode plate. However, after being placed in a heated sulfuric acid solution for a certain period of time, the treated lignin can inhibit the decomposition of lignin. The mechanism of action is not yet clear, but adding sulfuric acid-treated lignin to the negative electrode active material can increase the negative electrode overvoltage during overcharging and reduce the floating charge current, resulting in less thermal runaway.

4.5 Addition of lignin with a mass average molecular weight of less than 10,000 as the negative electrode active material[3]

This is a method that can increase the hydrogen overvoltage of the negative plate and prevent thermal runaway.

Lignin is added to the negative lead paste, and the lignin has a mass average molecular weight of less than 10,000. In other words, do not add lignin with a mass average molecular weight exceeding 10,000. The lignin mentioned here is a general term for compounds whose basic skeleton contains a phenylpropane (phcnyIpropane) structure. The molecular weight of phenylpropane, the basic skeleton of lignin, is 120. Use a semipermeable membrane filter, an ultrafiltration paper filter, to sort the lignin, and select lignin with a molecular weight range of 120 to 10,000. In the negative lead paste, the mass percentage of lignin with a molecular weight of 120 to 10,000 relative to the lead powder is 0.05% to 1.0%. Batteries using lignin with a mass average molecular weight of less than 10,000 can suppress the increase of constant voltage charging current, thereby preventing thermal runaway.

4.6 Negative electrode active material contains oil [4,6]

A valve-regulated lead-acid battery that hardly experiences thermal runaway even when used at high temperatures. Its negative electrode active material contains an oil, such as animal oils and vegetable oils, preferably paraffinic oils, naphthenic oils, olefinic oils, aromatic oils and silicon-based oils, with an oil content of 0.05% to 1%. Commercially available oils containing ordinary lubricants and corrosion inhibitors may also be used in appropriate amounts.

Add appropriate amounts of barium sulfate, lignin sulfonate and carbon black to the lead powder. Then dry mix the mixture. Next, add a predetermined amount of water and dilute sulfuric acid with a density of 1.4 to the mixture in sequence. Then, make a paste. Then, add 0.05% to 1% oil by mass to the lead paste to make a negative electrode lead paste resistant to thermal runaway.

The valve-regulated lead-acid battery made with this oil-containing negative lead paste shows a thermal runaway temperature of not less than 77.5 ℃. Here, thermal runaway is judged to occur when the battery temperature is 10℃ higher than the ambient temperature due to heat generation during constant voltage charging, as the charging current is unstable and gradually increases. The thermal runaway temperature of ordinary valve-regulated lead-acid batteries is not less than 67.5 ℃.

The reason why the negative electrode active material containing oil can improve the ability to resist thermal runaway may be that an oil film is formed on the active material, making it difficult for the active material to undergo redox reaction, thereby inhibiting the increase of high-temperature charging current.

4.7 The negative electrode active material contains stearic acid or its salt[5]

When lignin or its derivatives are added, 0.05% to 1% of stearic acid or its salt is added to the negative electrode paste. The valve-regulated lead-acid battery manufactured with this negative electrode paste also shows a thermal runaway temperature of not less than 77.5°C.

Adding stearic acid to the negative electrode active material improves the thermal runaway resistance, which may be due to the formation of a stearic acid film on the surface of the active material, which makes it difficult for redox reactions to occur, thereby inhibiting the increase in charging current at high temperatures.