Why is the constant-voltage charging current gradually decreasing

1. What is the charging terminal voltage of the battery?

Charging terminal voltage is the peak voltage value of battery charging. The voltage at the terminal stage of charging is not allowed to exceed this value, but the terminal voltage of charging is higher than the rated voltage. Terminal voltage refers to the peak voltage value allowed in the charging state.

2. What is the discharge terminal voltage of the battery?

Discharge terminal voltage is the bottom-line voltage of battery discharge. At the end of discharge, the load voltage of the battery at the end of discharge is specified, and its value is n*10.5V (the number of batteries connected in series in a lead-acid battery is indicated by 'n').

The voltage under load cannot be lower than this value (sometimes it is wrong to compare with open circuit voltage, which should be the working voltage under load). Terminal voltage is lower than rated voltage.

3. What are cell battery and cell voltages?

A positive plate and a negative plate constitute a cell battery, which is the most basic unit of the battery. For lead-acid batteries, the rated voltage of single cell is 2.0V, the charging terminal voltage of single cell is 2.3~2.35V, and the discharging terminal voltage of single cell is 1.75V V.

A single battery is composed of a cell battery, such as lead-acid batteries. A single battery can be composed of 2~6 or even more cell batteries connected in series. Its three voltage values (rated voltage, charging terminal voltage and discharging terminal voltage) are the sum of the number of cell battery. For square plate batteries, the number of negative plates in a cell battery is always one more than that of positive plates.

The voltage between the positive and negative plates is the cell voltage. The cell voltage is different with different plate materials, which is why the cell voltage of different kinds of batteries is different.

4. What is internal pressure?

Refers to the internal air pressure of the battery, which is caused by the gas generated during the charging and discharging process of the sealed battery, and is mainly influenced by the battery material, manufacturing process, battery structure and other factors. The main reason is that the water in the battery and the gas generated by decomposition of organic solution gather in the battery.

Continuous overcharge at high rate will lead to the increase of battery temperature and internal pressure. In severe containers, it will have destructive effects on battery performance and appearance, such as liquid leakage, bottom bulge, increase battery internal resistance, shorten discharge time and cycle life, etc.

4. What is the rated voltage of the battery?

Rated voltage is the voltage value that a cell battery or a single battery should have, which represents the characteristics of batteries made of different materials. For example, the rated voltage of lead-acid battery is 2V, that of nickel-hydrogen battery is 1.2V, and that of lithium battery is 3.6~3.7V. Generally, the rated voltage of a single battery is the sum of the rated voltage of a cell battery.

5. What is the open circuit voltage of the battery?

It means that when the battery is in the non-working state, that is, when there is no current flowing through the circuit under the condition that the battery is not externally connected with any load, the voltage value between the two terminals of the battery is directly measured by an electric meter. Open-circuit voltage cannot be used as a standard to measure battery voltage, but the measured open-circuit voltage of single battery can be compared with each other. It can also measure the state of charge change of the battery itself at different times for reference. Through the open circuit voltage of the battery, the state of charge of the battery can be judged.

6. What is the working voltage of the battery?

Working voltage, also known as terminal voltage, refers to the potential difference between the anode and cathode of the battery when the battery is working, that is, when there is current in the circuit. When the battery is discharged, when the current flows through the inside of the battery, there is no need to overcome the resistance caused by the internal resistance of the battery, so the working voltage is always lower than that of the open-circuit battery, and it is the opposite when charging. Any electrical equipment can be the load of the battery, however, the rated voltage of the load must be the same as that of the battery, and the power of the load should be equivalent to the capacity of the battery.

7. What is the charging limit voltage?

It refers to the voltage value when the battery changes from constant current charging to constant voltage charging according to the manufacturer's regulations.

8. What is the nominal voltage of the battery?

Used to represent the approximate value of battery voltage.

9. What is the average voltage of the battery?

It refers to the average voltage from the beginning to the end of discharge when the battery is discharged.

10. What is pressure drop?

Charge the battery to more than 80% qualitatively, and measure its no-load voltage. The 5W/2W battery is used as a load to connect the positive and negative terminal switches of the battery, and the devices of the battery circuit and path are connected in series. The voltage drop of 5 seconds after the switch is turned on is not more than 0.4V, which is qualified, mainly for testing the battery load performance.

11. What is the charging acceptance of the battery?

Under the specified charging voltage and current conditions, the charge amount accepted by the battery per unit time. This is very important for electric bicycles. If the ability to accept charging is very poor, and it takes 8~10 hours or even more, and the time ratio between discharging operation and charging exceeds 1:(4 ~ 5), the battery cannot do anything except charging time. It is required that the acceptance of battery charging be extremely strong and fast to meet the needs of electric vehicles. In the future, it is ideal that the charging speed of electric vehicle power supply should reach 1:(0.1 ~ 0.25) or even faster, and it can charge enough electric energy in the shortest time.

12. What is the self-discharge rate of batteries?

After charging the battery, the phenomenon that the capacity decreases automatically during storage is called self-discharge, also known as the charge retention capacity, which refers to the retention capacity of the battery's stored electricity under certain conditions when the battery is in an open circuit state. The percentage of battery self-discharge in total capacity measured by a certain time is called 'self-discharge rate'. Mostly calculated by monthly self-discharge rate, if the battery pack with capacity of 12Ah self-discharges 0.36Ah in one month and the remains 11.64Ah, the self-discharge rate is 3% per month. According to the industry standard, the remaining electricity of lead-acid batteries stored for 28 days should be no less than 85%.

13. Why is the constant-voltage charging current gradually decreasing?

Because at the end of the constant current process, although the electrochemical polarization inside the battery remains at the same level in the whole constant current process, the concentration polarization of internal pb is gradually eliminated under the action of constant electric field in the constant voltage process, and the migration number and velocity of ions show that the current gradually decreases.

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