Reasons for the decrease of battery capacity
The capacity of battery is directly related to the normal starting and running of vehicle and tractor. If the maintenance is not good, even if the newly installed battery will also have a rapid decline in capacity, shorten the service life. The main reasons for capacity decline are as follows:
1. Plate vulcanization. That is to say, the white coarse grain lead sulfate gradually formed on the surface of the electrode plate is hard and difficult to dissolve in the electrolyte, and it is not easy to reduce with the electrolyte when charging, thus reducing the active substances. In addition, the coarse-grained lead sulfate blocks the pores of the plate, which makes it difficult for the electrolyte to penetrate into the plate, increases the internal resistance, reduces the active substances in the plate and reduces the capacity of the battery. The causes of plate vulcanization are as follows
(1) The battery is in the state of full discharge or half discharge for a long time. Due to the change of air temperature, such as high temperature T, a part of lead sulfate on the electrode plate dissolves into the electrolyte. When the temperature decreases, the lead sulfate dissolved in the electrolyte will re precipitate, produce recrystallization, and form coarse grains attached to the electrode plate.
(2) If the liquid level of the battery decreases, the upper edge of the plate will be exposed and oxidized with the air, then the oxidized part will also produce coarse grain lead sulfate in contact with the electrolyte during the bumpy driving of the machine, which will vulcanize the upper part of the plate.
(3) Too high specific gravity of electrolyte, too high discharge current and too high temperature aggravate the chemical reaction, resulting in the deposition of lead sulfate on the electrode plate, which also promotes sulfidation. In order to prevent the plate from vulcanization, the battery should always be kept in a fully charged state, the electrolyte should submerge the upper edge of the plate, and the specific gravity of the electrolyte should be correctly selected according to different regions and seasons.
2. Self discharge. The phenomenon that the fully charged battery gradually loses its power when it is not used is called self discharge. Under normal conditions, the daily discharge rate should not exceed 0.35% - 0.5%. The main causes of self discharge are as follows
(1) There are impurities in the electrode plate or electrolyte. The potential difference between the impurities and the electrode plate or between different impurities turns into a local battery, which forms a circuit through the electrolyte to generate local current and discharge the battery.
(2) The diaphragm was broken, resulting in a short circuit between the positive and negative plates.
(3) There is electrolyte or water on the surface of battery shell, which becomes conductor between poles, causing battery discharge.
(4) The active material falls off too much and deposits on the bottom of the battery, which shortens the plate and causes discharge. In order to reduce self discharge, in addition to the battery manufacturing materials should be as pure as possible, the shell surface and pile head must be kept clean, and the electrolyte must be chemically pure sulfuric acid and distilled water.
3. During the normal use and charging and discharging process of battery, the volume of active material on the plate expands and contracts continuously, which will cause the active material to fall off slowly. If it is not used properly, the active material will fall off rapidly and cause the plate short circuit. The main reason for the active material falling off is that the current is too high or the temperature is too high when charging, often overcharging and so on. When discharging, if the current is too large (for example, it takes too long to connect to the starting motor), the plate will be arched (because the active substances of the plate participate in the chemical reaction differently, resulting in the inconsistent volume change of the plate), and the active substances will fall off.
4. The shell is cracked or the sealing glue is broken.
The crack will make the electrolyte leak out and the liquid level decrease. If there is a slight crack in the internal wall, the two single cells are connected, so that the voltage drops. The main reasons for the cracks are that the screw of the battery base is too tight, the knock is too fierce, the vent is blocked, the gas can not be released, so that the pressure of the single cell is too high, the frost crack in winter, and the vibration in driving is too large.