What is the meaning of battery capacity (Ah)?
The rated capacity (C) of the storage battery is in ampere-hour (Ah), which is the product of the discharge current (A) and the discharge time in hours (h). Since the Ah obtained by using different discharge parameters for the same battery is different, in order to facilitate the description, measurement and comparison of battery capacity, a unified condition must be set in advance. In practice, battery capacity is defined as the amount of electricity given by discharging the battery to a set voltage with a set current. It can also be said that the battery capacity is the product of the time taken to discharge the battery to the set voltage with the set current and the current.
In order to set a unified condition, first, according to the differences in battery structure characteristics and uses, set several discharge time rates, the most common ones are 20 hours, 10 hours rate, electric vehicle special battery is 2 hours rate, written as C20 , C10 and C2, where C represents the battery capacity, and the following numbers indicate the number of hours for this type of battery to be discharged to the set voltage with a certain intensity of current. Therefore, the rated discharge current is obtained by dividing the hours by the capacity. That is to say, batteries with the same capacity but different discharge rates have very different nominal discharge currents. For example, the battery capacity of an electric bicycle is 10Ah, and the discharge rate is 2 hours. It is written as 10Ah2, and its rated discharge current is 10 (Ah) / 2 (h) = 5A; and the battery capacity of a car starting is 54Ah , The discharge rate is 20 hours, written as 54Ah20, its rated discharge current is only 54 (Ah) / 20 (h) = 2.7A! To put it another way, if these two batteries are discharged with a current of 5A and 2.7A, they should last for 2 hours and 20 hours before dropping to the set voltage.
The above-mentioned so-called set voltage refers to the termination voltage (unit: V). The termination voltage can be simply understood as: the battery voltage drops to the minimum value that will not cause damage during discharge. The end voltage value is not fixed, it decreases with the increase of discharge current, the greater the discharge current of the same battery, the lower the end voltage can be, otherwise it should be higher. That is to say, the battery voltage is allowed to drop to a lower value when discharging with a large current, but not when discharging with a small current, otherwise it will cause damage.
The current intensity of the battery in operation is also often expressed by magnification, written as NCh. N is a multiple, C represents the ampere-hour of capacity, and h represents the number of hours specified by the discharge rate. The value of h here is only used as a reminder that the relevant battery belongs to the discharge time rate, so when describing a battery with a certain time rate in detail, the rate is often written in the form of NC without a subscript. The multiple N multiplied by the capacity C is equal to the current A. For example, a 20Ah battery is discharged at a rate of 0.5C, 0.5×20=10A. Let’s take another example: a car’s starting battery capacity is 54Ah, and the measured output current is 5.4A, then its discharge rate N at this time is 5.4 / 54=0.1C. The figure below shows the relationship between the end voltage and discharge time of a 20-hour battery product at different discharge rates. These values are representative of ordinary lead-acid batteries.
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