Focus on explaining constant voltage charging and floating charging

Focus on explaining constant voltage charging and floating charging

  1. Constant voltage charging

During the charging process, the charging in which the voltage of the charging power supply always maintains a constant value is called constant voltage charging.
If the charging power supply voltage is 2.3V/section, the water consumption during the charging process can be reduced to a very small extent. If the power output terminal is directly connected to the battery pole, the terminal voltage of the battery is 2.3V/cell. At this time, the charging current is:

In the formula, I is the charging current; is the battery voltage; U is the charging voltage; r is the internal resistance of the battery.
At the beginning of charging, since it is the smallest, I is the largest. With the progress of the charging process, due to the reasons of concentration polarization and electrochemical polarization, the charging current 1 gradually decreases. Connect 48 N462 batteries for locomotives in series and charge them under constant voltage conditions. When the voltage values ​​are slightly different, the curve shown in Figure 1 will be obtained. It can be seen from the figure: when the output voltage of the charger is 110V, 108.5V, 107.1V, 105V, the battery can enter 312Ah, 254Ah, 146Ah, 43Ah within 18h, respectively, which are the nominal capacity in turn. 67.7%, 55%, 35.5%, 9.4%. It can be seen that a 5% drop in the charging voltage can cause a large deficit in battery capacity. click here to open to learn more about charging.

Figure 1 - Constant voltage charging curve of battery
Figure 1 – Constant voltage charging curve of battery

The advantages of this charging method are as follows.
① Less electrolyzed water, high charging efficiency, and low battery temperature rise.
②It can avoid overcharging in the later stage of charging.
The disadvantages are as follows.
①At the beginning of charging, the current is very large. If the charging equipment does not consider this working condition, it will cause damage to the equipment.
② The current in the later stage of charging is too small, so that PbSO4 in the depth of the plate is not easy to participate in the reaction, and it is easy to produce vulcanization.

In order to prevent the charging starting current from being too large, a small resistance resistor is often connected in series between the power supply and the battery. We call this charging method “modified constant voltage charging”. As shown in Figure 2, at this time, R plays the role of current negative feedback, and the charging current is:

Figure 2 - Improved constant voltage charging
Figure 2 – Improved constant voltage charging

After the vehicle is started, the alternator begins to charge the battery. The current is very high at the beginning, and soon the current value drops. This stage is the constant voltage charging process. If the mains is used as the power supply, the constant voltage charging is supplied by rectification, and the control accuracy of the charging voltage should reach 1%. Since the voltage fluctuation of the mains is much greater than 5%, this will inevitably cause the battery to be overcharged or undercharged. In order to ensure the normal operation of the battery, when using the constant voltage charging system, a voltage regulator should be added to the rectifier to ensure that the rectifier can output a stable supply voltage.

The precise setting of the voltage of the generator used for charging of railway locomotives was once neglected, resulting in a “power deficit” failure of the locomotive battery, which affected the use of the locomotive. In order to improve this work, the original (110 ± 2) V in the process has been changed to (110 ± 1) V, which is higher than the electrical requirements of many other industries.

  1. Float charge

Floating charging means that the battery and the electrical appliance are connected in parallel on the charger, and the charging state of the car is basically floating charging after constant voltage charging. In the backup power system, this method is usually used, which is the most widely used charging method. The floating charging working system requires high accuracy and stability of the charging voltage, and the fluctuation value of the charging voltage should be less than 1%, which is a necessary condition for the normal operation of the floating charging.

If the output voltage of the charger is unstable, it will have a great impact on the retention capacity and life of the battery. If the voltage rises, since r+R is only 0.02~0.03Ω, as long as V- increases by a few volts, the charging current will increase by tens of amps. The increased current increases the temperature of the battery, which increases the ionization degree of H2SO4 and further reduces the internal resistance, which further increases the current. This vicious circle increases the evaporation loss of the electrolyte and also aggravates the corrosion of the electrode plate. Under the action of the driving vibration of the motor vehicle, the active material of the battery will be accelerated to fall off, which is very harmful.

In order to avoid this situation, on the one hand, the charging voltage of the battery should be strictly controlled; on the other hand, the batteries should always be connected in series to charge, so as to maximize the value of R+r and the two items, and the fluctuation of the charging voltage will also affect the current. reduced to a minimum. If the charging voltage decreases, it will cause overdischarge. This is because the power supplemented by charging cannot compensate for the loss caused by starting the car, stopping the power consumption and self-discharge. The reduction of the charging voltage and the replenishment of the power are not simple linear relationships. According to statistical analysis, 50% of the faults of the battery on the motor vehicle are caused by the charging, and the charging is mainly caused by the voltage regulator.

When the generator output voltage drops, the charging current is near zero on the car cab ammeter. Since the ammeter used in the car only has three scales of -30, 0, and +30, the driver often cannot correctly judge the real charging current while driving, causing the battery to “deplete”.

The correct selection of the float voltage depends on the thickness of the battery plates, the distance between the plates, the alloy composition of the positive and negative grids, the ratio of positive and negative active materials, the material and structure of the separator, the active materials and the types of additives in the electrolyte. and quantity. Ordinary car batteries, when the positive active material is charged to 70% of its structural stock, begins to have oxygen evolution. In order to reduce water consumption and prolong battery life, the charging voltage value of the critical gas outlet point is usually set as the floating charging voltage value. For sealed batteries, because the grid alloy is a lead-calcium alloy, the critical gas point voltage value during charging is 2.45V, so the floating charge voltage value can be appropriately increased to keep the capacity at 80%~85%. should.

For stationary batteries used in the communications sector, the manufacturer specifies the float voltage. Figure 3 shows the specified value of battery float voltage for switches in various countries.

Figure 3 - The specified value of battery float voltage of switches in various countries
Figure 3 – The specified value of battery float voltage of switches in various countries

As can be seen from Figure 3, there are some differences in the specified values ​​in various countries, which are caused by manufacturing differences. The principle of selecting the correct float voltage for fixed batteries is that the charged power can make up for the loss of self-discharge, and the battery will generate trace bubbles under the action of long-term charging. At the same time, the density of the electrolyte shows that its retention capacity is about 90%.

Under the correct float charging conditions, the battery will have a long lifespan. This is because, under float charging conditions, the energy throughput of the battery is the lowest in quantity. The power consumed by the electrical appliances is basically supplied by the charging circuit. Only when a short-term accident occurs, there is a large current supply, and the power released by the battery is quickly replenished. The number of active materials in the charge-discharge reaction on the battery plate is usually less than 1%, and 99% of the active materials usually do not participate in the charge-discharge reaction, but only play a “backup” role.

There is often only one full-capacity deep charge and discharge for many years, and of course there is no common aging and shedding problems of motor vehicle batteries. In important departments such as power plants, substations, communications, and TV satellite ground stations, large-capacity batteries are fixedly installed in the power supply room, and the floating charging conditions are strictly controlled. Fixed batteries produced in China have an actual lifespan of more than 20 years.

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