Analysis of the key voltage value of lead-acid battery charging and its explosion and swelling causes

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Analysis of the key voltage value of lead-acid battery charging and its explosion and swelling causes [Copy link]

1. Charging oxygen evolution voltage: When charging, oxygen will be generated on the surface of the positive plate. When the charging voltage is low, less oxygen will be generated. In the sealed battery, it will react chemically with the hydrogen generated by the negative plate, turning into water and returning to the electrolyte. This is normal. But when the charging voltage reaches 2.35V/cell, oxygen will be generated violently, just like when the water temperature reaches 100 degrees, a large amount of water vapor will be generated. The large amount of generated oxygen increases the pressure inside the battery. It will push open the safety valve of the battery and run out, causing water loss. If hydrogen has not yet been evolved at this time, the pressure inside the battery is not large enough, and the safety valve can still play a better role. The gas that escapes will not be too much, and it will not cause a large amount of water loss. The oxygen evolution voltage of 2.35V/cell is a voltage value that allows a short-term breakthrough during charging.

2. Charging hydrogen evolution voltage: When charging, hydrogen will be generated on the surface of the negative plate. When the charging voltage is low, less hydrogen will be generated. In the sealed battery, it will react chemically with the oxygen generated by the positive plate, turning into water and returning to the electrolyte. This is normal. However, when the charging voltage reaches 2.45V/cell, hydrogen will be generated violently. The oxygen that has been generated in large quantities, together with the hydrogen that is now generated in large quantities, will cause the internal pressure of the battery to increase abnormally, and the safety valve can no longer be closed. At this time, a large amount of gas will escape, causing serious water loss. The hydrogen evolution voltage of 2.45V/cell is a voltage value that is not allowed to be exceeded during charging.

3. Safe charging voltage: In order to ensure the battery charging safety, fully charge and avoid water loss, the long-term charging voltage should be slightly lower than the polar oxygen voltage. At present, most manufacturers choose 2.3V/cell.

Analysis of the causes of lead-acid battery explosion and swelling

Cause of battery explosion

If the charging voltage of the battery is too high or the charging time is too long, a large number of bubbles will be generated. At the same time, the electrolyte temperature will rise, causing a large amount of water to evaporate. This is the side reaction when the battery is charged. When the battery is charged to the end, after the two electrodes are converted into effective substances, if it continues to charge, a large amount of hydrogen and oxygen gas will be generated. H2:O2 is precipitated at a volume ratio of 2:1. According to the electrochemical equivalent of hydrogen and oxygen gases, 0.4181L hydrogen and 0.20907L oxygen are produced for every 1Ah of overcharge. When the concentration of this mixed gas accounts for 4% in the air, it has no time to escape. If the exhaust hole is blocked or there is too much gas, it will explode when it encounters an open flame, which will damage the battery at the least and injure
people and property at the worst. The prevention methods are:
1. Control the charging amount and do not overcharge to reduce the amount of gas precipitation. Fireworks are strictly prohibited in the charging room
, and the width should be kept open;
2. Use better charging equipment, such as a fast pulse charger. This charging mode
has a small amount of gas evolution, low temperature rise and fast charging time during the charging process;
3. During the charging process, each connection point must be firm to avoid sparks due to looseness;
4. The vents of the battery must always be kept unobstructed;
5. Prevent cracks in the battery shell and electrolyte leakage. Electrolyte leakage into the cable trench
will cause sparks in the line, fire and explosion;
6. The internal short circuit and electrode plate sulfation of the battery should be eliminated in time;
7. The charging voltage of the charger and the regulator on the generator that charge the battery is usually adjusted between 13.8V and 14.1V;
8. It is strictly forbidden to use metal objects such as wrenches to ignite the positive and negative poles of the battery;
9. When repairing electrical equipment, the flammable gas inside the battery should be removed first.

Analysis of the causes of battery swelling

1. The main reasons for battery swelling are as follows:

a) The vent hole on the battery filling cap is blocked or not smooth. During the charging process, especially at the end of charging, a large amount of explosive gas will be generated inside the battery. If the vent hole on the battery filling cap is blocked or not smooth, these
gases cannot be discharged in time, and accumulate in the battery shell, the pressure is getting bigger and bigger, and finally the battery will swell.
b) The battery charging current is too large or the charging time is too long.
When the battery charging current is too large or the charging time is too long, the electrolyte temperature will rise rapidly and a large amount of gas will be generated, causing the active materials on the battery plates to loosen and fall off, causing the battery to swell.
c) The battery plates are sulfided. During the high-current charging process of batteries with sulfided plates, the single-cell voltage and electrolyte temperature will rise rapidly, and bubbles will be generated early and violently, which can easily cause the battery to swell.
d) Continuous starting of the motor for too long When starting the electric vehicle motor, the battery needs to provide a large current (usually 20-40A) to the motor in a very short time. Such a large starting current will inevitably cause a violent chemical reaction inside the battery. If the battery plate is accompanied by a slight sulfidation phenomenon, it will inevitably cause the electrolyte temperature to rise sharply and produce a large amount of gas. Once these gases cannot be discharged in time, they are prone to explosion. If the starter is used continuously for too long, it will aggravate the generation of gas and
increase the possibility of battery expansion and cracking.
e) The battery plate ears and poles are not welded firmly to the bus bar. When welding the battery plate ears and poles to the bus bar, they must be welded firmly and integrated to meet the needs of the battery when discharging with a large current. Otherwise, when discharging with a large current
, the welding point will cause ignition and ablation due to the thin contact point or poor contact, which will cause sparks and ignite the explosive gas generated by the battery, causing the battery to explode.
f) Excessive viscosity of electrolyte When the temperature is too low, the viscosity of electrolyte is high, and the speed of infiltration into the pores of the plate is slow, the internal resistance increases, and the voltage drop consumed on the internal resistance during discharge is also large, which will cause the electrolyte temperature to rise rapidly, produce a large amount of gas, and increase the gas pressure inside the battery. If the battery is over-discharged at this time, the electrolyte temperature will rise faster, and more gas will be produced, making the gas pressure inside the battery even greater, which will easily lead to battery expansion and cracking. In addition, the explosive gas generated during the battery charging process will immediately cause an explosion if it encounters an open flame, causing the battery to expand and crack. Therefore, the charging room must be well ventilated and fireworks are strictly prohibited.
g) Electrolyte drying up After long-term use, the battery will lose water, resulting in electrolyte drying up. At this time, overcharging will cause battery swelling, and severe cases will cause explosion. If the battery loses water, distilled water can be added to the battery appropriately. The amount of addition and operation method can be carried out according to the battery instruction manual.

2. Preventive measures for battery expansion and rupture

From the above mentioned reasons for battery rupture, in order to avoid battery rupture accidents,

First of all, to avoid sparks during the use of the battery, the battery must be installed firmly during use, the connection between the wire connector and the electric pile must be tightened, and the welding quality of the plate group must be guaranteed during overhaul.

Secondly, in order to allow the gas generated by the battery during operation to overflow from the vent hole of the filling port in time and prevent the internal air pressure of the battery from being too high, the battery filling cap must be tightened and its vent hole must be unblocked frequently.

Third, to avoid excessive discharge of the battery, when using the starter to start the vehicle, especially when starting the vehicle under low temperature conditions, the starter cannot be used continuously. When starting a cold vehicle, the vehicle must be preheated, the starter engagement time must not exceed 5 to 10 seconds, and it must be started every 10 to 15 seconds.
Fourth, when charging the battery, it is necessary to avoid excessive current or overcharging. For this reason, for batteries installed on the vehicle, the rated voltage of the generator must be adjusted; for batteries charged in the charging room, the charging current and charging time must be controlled.