Battery capacity
Battery capacity is one of the important performance indicators to measure battery performance. It represents the amount of electricity released by the battery under certain conditions (discharge rate, temperature, termination voltage, etc.) (JS-150D can be used for discharge testing), which is the capacity of the battery. It is usually measured in amperes per hour (abbreviated as A · H, 1A · h=3600C).
The battery capacity is divided into actual capacity, theoretical capacity, and rated capacity according to different conditions. The calculation formula for battery capacity C is C=Δ t0It1dt (integrating current I within t0 to t1 time), and the battery is divided into positive and negative poles.
classification
The battery capacity is divided into actual capacity, theoretical capacity, and rated capacity according to different conditions.
The minimum capacity required for discharging at a certain discharge rate of 25 ℃ to the termination voltage is the specified capacity of the battery during design and production, which is called the rated capacity of a certain discharge rate RH.
The battery capacity is generally calculated in AH (ampere hour), and another method is to calculate in watts (W) per cell (unit plate). (W/CELL)
1. Ah (ampere hour) calculation, discharge current (constant current) I × Discharge time (hours) T. For example, if the continuous discharge current of a 7AH battery is 0.35A, the time can be continuous for 20 hours.
2. The charging time is based on 15 hours, and the charging current is 1/10 of the battery capacity. Fast charging will reduce the battery life.
Battery capacity refers to the amount of electricity stored by the battery. The unit of battery capacity is 'mAh', and the Chinese name is milliampere hour (for convenience when measuring large capacity batteries such as lead-acid batteries, 'Ah' is generally used, and the Chinese name is ampere hour, with 1Ah=1000mAh). If the rated capacity of the battery is 1300mAh, that is, 130mA current is used to discharge the battery, then the battery can continue to operate for 10 hours (1300mAh/130mA=10h); If the discharge current is 1300mA, the power supply time is only about 1 hour (the actual working time may vary due to individual differences in the actual capacity of the battery). This is an ideal analysis. The actual current of a digital device during operation cannot always be constant at a certain value (taking a digital camera as an example, the working current may change significantly due to the opening or closing of components such as LCD display screens and flash lights). Therefore, the power supply time that a battery can provide to a certain device can only be an approximate value, and this value can only be estimated through practical operating experience.
Capacity unit
Usually, we say that the battery capacity is measured in ampere hours, which is based on a certain battery that has already been determined. For example, what is the battery capacity of this mobile phone; The battery capacity of this electric vehicle is specific to different batteries. The battery voltage has been determined without considering the actual voltage. Simply stating ampere hours can represent the capacity of this battery.
However, for batteries with different voltages, we cannot simply use ampere hours to represent capacity. For example, a 12V 20AH battery, a 15V20AH battery, even if they are both 20AH, can supply the same power load, and the equipment can work normally, but the duration is different. The standard capacity used should be in units of power.
For example, if a device can support both 12V and 24V, and is powered by a 12V (20AH) battery that can provide one hour, then connecting two in series will result in 24V (20AH). The ampere hour does not increase, but the duration will double. Therefore, the capacity should be considered based on the power contained by the battery at this time, rather than simply considering ampere hour.
W (work)=P (power) * T (time)=I (current) * U (voltage) * T (time)
Discussing battery capacity in this way has practical significance and must be realistic, otherwise there may be a claim that a mobile phone battery has a larger capacity than a battery car battery, which is obviously not scientific.
Testing method
Charge a battery with constant current and voltage, and then discharge it with constant current. The amount of electricity released is the capacity of the battery, such as batteries and nickel hydrogen batteries, but not lithium batteries. It has a minimum discharge voltage of 2.75V, which is usually 3.0V as the lower limit protection voltage. For example, if the capacity of a lithium battery is 1000mAh, the charging and discharging current is 1000mA. If the maximum voltage of the battery is within 4.2V and it is placed at 3.0V, the released capacity is the most true capacity of the battery.
Influence factor
The capacity of a battery is an important indicator to measure its performance. It is generally expressed in ampere hours. The total term for discharge time (hours) and discharge current (amperes) is capacity=discharge time × Discharge current. The actual capacity of a battery depends on the amount of active substance in the battery and the utilization rate of the active substance. The more active substance there is, the higher the utilization rate of the active substance, and the larger the battery capacity. Conversely, the smaller the capacity, there are many factors that affect the battery capacity, including the following:
(1) The effect of discharge rate on battery capacity
The capacity of lead-acid batteries decreases with the increase of discharge rate, which means that the larger the discharge current, the smaller the calculated capacity of the battery. For example, a 10Ah battery can be discharged for 2 hours with 5A discharge, which is 5 × 2=10; So using 10A discharge can only release 47.4 minutes of electricity, which is 0.79 hours. Its capacity is only 10 × 0.79=7.9 ampere hours. Therefore, for a given battery to discharge at different time rates, there will be different capacities. When talking about capacity, we must know the discharge rate or multiplication rate. Simply put, it is how much current is used to discharge.
(2) The influence of temperature on battery capacity
Temperature has a significant impact on the capacity of lead-acid batteries. Generally, as the temperature decreases, the capacity decreases. The relationship between capacity and temperature is as follows: Ct1=Ct2/1+k (t1-t2). t1t2 is the temperature of the electrolyte, k is the temperature coefficient of capacity, Ct1 is the capacity at t1 (Ah), and Ct2 is the capacity at t2 (Ah). In battery production standards, a temperature is generally specified as the rated standard temperature, such as t1 being the actual temperature and t2 being the standard temperature, The negative electrode plate (usually 25 degrees Celsius) is more sensitive to the influence of low temperature than the positive electrode plate. When the electrolyte temperature decreases, the viscosity of the electrolyte increases, ions are subjected to greater resistance, diffusion ability decreases, and electrolyte resistance also increases, resulting in an increase in electrochemical reaction resistance. Some lead sulfate cannot be converted normally, and the charging acceptance ability decreases, resulting in a decrease in battery capacity.
(3) The effect of termination voltage on battery capacity
After the battery is discharged to a certain voltage value, the voltage drops sharply, and the energy obtained is actually very small. If the battery is discharged deeply for a long time, the damage to the battery is considerable. Therefore, the discharge must be terminated at a certain voltage value, which is called the discharge termination voltage. Setting the discharge termination voltage is of great significance for extending the service life of the battery. Generally, the discharge termination voltage of electric vehicle batteries that we repair is 1.75 volts per grid, That is to say, a 12 volt battery has 6 cells and its discharge termination voltage is 6 × 1.75=10.5 volts.
(4) The influence of the geometric size of the electrode plate on the battery capacity
When the amount of active substance is constant, the geometric area of the electrode plate in direct contact with the electrolyte increases, resulting in an increase in battery capacity. Therefore, the influence of the geometric size of the electrode plate on battery capacity cannot be ignored.
① The Effect of Plate Thickness on Capacity
The amount of active substance is constant, and the battery capacity decreases with the increase of electrode plate thickness. The thicker the electrode plate, the smaller the contact surface between sulfuric acid and active substance. The lower the utilization rate of active substance, the smaller the battery capacity.
② The Influence of Plate Height on Capacity
In a battery, there is a significant difference in the utilization rate of active substances between the upper and lower parts of the electrode plate. Experiments have confirmed that in the early stage of discharge, the current density in the upper part of the electrode plate is about 2 to 2.5 times higher than that in the lower part. This difference gradually decreases with the passage of discharge, but the current density in the upper part is greater than that in the lower part.
③ The Influence of Plate Area on Capacity
When the amount of active substance is constant, the larger the geometric area of the electrode plate, the higher the utilization rate of the active substance, and the larger the capacity of the battery. When the battery shell is the same and the quality of the active substance remains unchanged, using a thin electrode plate to increase the number of electrode plates increases the effective reaction area of the electrode plate, thereby improving the utilization rate of the active substance and increasing the capacity of the battery.