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At present, there are two types of electric vehicle batteries: lead-acid batteries and lithium batteries! Lead acid batteries have capacities of 12AH, 14AH, 20AH, 24Ah, but the capacity of lithium batteries is not necessarily the same, as lithium batteries can be combined freely to obtain high-capacity batteries such as 48V40Ah, 72V80Ah, etc

The capacity of an electric vehicle battery represents the amount of electrical energy stored by the battery. After being fully charged, the electric vehicle battery continuously discharges at a certain battery value, from the beginning of discharge to the end voltage dropping to the specified discharge termination voltage. The product of discharge current and time represents the capacity of the electric vehicle battery in ampere hours, usually expressed as "Ah". After fully charging the specialized electric bicycle battery, it is left to stand in an environment with a temperature of (25 ± 2) ℃ for 1-4 hours. Then, it is discharged at a constant current (A) of 1/2 of the rated capacity of the battery (2-hour rate) until the average voltage of the individual battery reaches 1.6V (4.8V for 6V batteries, 9.6V for 12 batteries), and the discharge time is recorded. The new electric vehicle battery is considered qualified if the discharge time exceeds 120 minutes in one of the three tests. The actual discharge current of an electric vehicle multiplied by the actual discharge hours is the actual capacity of the battery.

The capacity of electric vehicle batteries depends on the amount of active substances and electrolyte mass involved in electrochemical reactions. The manufacturing process, geometric shape and size of the electrode plate, as well as the temperature during battery use, also have an impact on the measured actual capacity. The more plates there are, the larger the relative area, and the more active substances participate in chemical reactions, resulting in a larger electric vehicle battery; The higher the temperature during battery use, the stronger the penetration ability of electrolyte into the electrode plate, and the deeper the active substances involved in chemical reactions, resulting in a larger electric vehicle battery.

Battery capacity is one of the important performance indicators for measuring 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 ampere hours (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.

The battery capacity is divided into actual capacity, theoretical capacity, and rated capacity according to different conditions.

The minimum capacity required to discharge at 25 ℃ to the termination voltage at a certain discharge rate is the specified capacity of the battery during design and production, which is called the rated capacity of a certain discharge rate RH.

Square lithium-ion battery

Square lithium-ion battery

The battery capacity is generally calculated in AH (ampere hours), and another method is to calculate in watts (W) per cell. (W/CELL)

1. Ah (ampere hour) calculation, discharge current (constant current) I x discharge time (hour) 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, a current of 130mA is used to discharge the battery, then the battery can operate continuously 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 will undergo significant changes due to the opening or closing of components such as LCD 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.

Usually, we talk about battery capacity in ampere hours, which is based on a specific battery that has already been determined.

For example, what is the battery capacity of this mobile phone; The capacity of this electric vehicle battery is determined by different batteries. The battery voltage has been determined without considering the actual voltage, and 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 12V20AH 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. Therefore, the standard capacity should be measured in power.

For example, if a device can support both 12V and 24V, and is powered by a 12V (20AH) battery for one hour, then using two pieces 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 capacity of the battery, rather than just ampere hours.

W (work)=P (power) * T (time)=I (current) * U (voltage) * T (time)

Discussing battery capacity in this way has practical significance and must be truthful. Otherwise, there may be a claim that a mobile phone battery has a larger capacity than a car battery, which is obviously unscientific.

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 a nickel hydrogen battery, but not a lithium battery. 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, then the charging and discharging current is 1000mA. When the maximum voltage of the battery is 4.2V and it is placed at 3.0V, the released capacity is the most true capacity of the battery.

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 x discharge current. The actual capacity of a battery depends on the amount and utilization rate of active substances in the battery. The more active substances there are, the higher the utilization rate of active substances, and the larger the battery capacity. Conversely, the smaller the capacity, the many factors that affect the battery capacity, commonly including the following:

(1) The influence 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, equivalent to 0.79 hours. Its capacity is only 10 x 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 multiple. Simply put, it is how much current is used for discharge.

(2) The influence of temperature on battery capacity

The 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 the 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. If t1 is the actual temperature and t2 is the standard temperature (usually 25 degrees Celsius), the negative electrode plate is more sensitive to low temperatures than the positive electrode plate. When the electrolyte temperature decreases, the viscosity of the electrolyte increases, ions are subject to greater resistance, diffusion ability decreases, and electrolyte resistance also increases, resulting in electrochemical resistance. Increase, some lead sulfate cannot be converted normally. The charging capacity decreases, resulting in a decrease in battery capacity

(3) The effect of termination voltage on battery capacity

When the battery is discharged to a certain voltage value, the voltage drops sharply, and in fact, the energy obtained is very small. If it is discharged deeply for a long time, the damage to the battery is considerable. Therefore, it is necessary to terminate the discharge 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 the electric motorcycle battery we repair is 1.75 volts per cell, which means that a 12 volt battery is 6 cells, and its discharge termination voltage is 6 x 1.75=10.5 volts [2]

(4) The influence of the geometric dimensions 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, leading to an increase in battery capacity. Therefore, the influence of the geometric size of the electrode plate on battery capacity cannot be ignored

① The influence of plate thickness on capacity

The amount of active substance is constant, and the battery capacity decreases with the increase of electrode thickness. The thicker the electrode, the smaller the contact surface between sulfuric acid and active substance, the lower the utilization rate of active substance, and the smaller the battery capacity

② The influence of electrode height on capacity

In batteries, there is a significant difference in the utilization efficiency of active substances between the upper and lower parts of the electrode plate. Experiments have shown that in the early stages 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 as discharge progresses, but the current density in the upper part is higher 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.

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