Analysis of UPS battery capacity and discharge rate

When the UPS manufacturer configures the battery , the selected design capacity fully meets or exceeds the power capacity and power supply time requirements of the uninterruptible power supply of the load. However, after the UPS is put into operation, the user often finds that the UPS does not power off after the mains power failure. The actual time is much smaller than the design value. The cause of this phenomenon is that in most cases, the spare capacity of the battery is not enough in the initial configuration, but the capacity of the battery is not exerted. There are many reasons for the actual capacity reduction of the battery. There are battery quality problems, but more are the problems of use and maintenance.

(1) Battery capacity

In the manufacturing process of the lead-acid battery plate, the cathode plate is charged, and the lead on the positive plate becomes lead dioxide, and the lead on the negative plate becomes spongy lead, but the manufacturer converts the plate into a plate. The time is limited, it is impossible to convert all the substances into active substances. For this reason, the national standard stipulates that the new battery reaches 90% capacity, and only in the subsequent daily use, the capacity gradually reaches the normal value. Reached 100%.

The rated capacity of the battery pack is obtained at the specified discharge rate. The discharge rate (1/H) = discharge current (A) / battery rated capacity (Ah), for example, one of the typical specifications of the small battery used in the UPS power supply. It is l2V, 6Ah/2Ohv, this specification is defined as the output DC voltage l2V, the nominal capacity is 6Ah, and the discharge rate condition is 20hr. The specific meaning is: the battery pack with the output DC voltage of l2V is discharged under the condition of 20H constant discharge rate, and the total safety time measured when the output voltage is reduced from l2V to l0.5V, the measured total hours should be 6Ah. .

China, Japan, Germany industrial batteries use a 10-hour rate (expressed as C10), the US industrial battery standard is an 8-hour rate (expressed as C8,). In actual use, the discharge rate is not equal to the discharge rate specified by the standard capacity. When the actual discharge rate is greater than the discharge rate specified by the nominal capacity, the actual output capacity is less than the nominal capacity.

China's electricity, postal and telecommunications standards stipulate that the 10-hour rate battery, when using 1 hour rate discharge, its capacity is 55% of the nominal capacity, which is 0.55C10. Japanese industrial standards stipulate 2V/10 hour rate battery, the capacity is 0.65C10, 6V, 12V, 10 hour rate battery at 1 hour rate, 1 hour rate capacity is 0.6C10. 20 hour rate battery, 10 hour rate capacity is 0.93C20, The 1-hour rate capacity is 0.56 C20.

There are two ways to express battery life: one for deep recycling and the other for "floating power" batteries. The deep-cycle battery uses a deep cycle count to indicate its service life. The battery used for charging and discharging at a depth of 0.8C10 has a life of more than 1200 times, and the battery used for floating charge can reach 10 to 20 years. The battery is considered to have an end of life when it has only 80% capacity.

The actual service life is very different from the design service life, which depends mainly on the loss of water in the battery. When used under design conditions, the design life can be reached. When external conditions such as temperature, charging voltage, and discharge depth exceed the design requirements, the actual service life will be much lower than the design life, and the actual use capacity will be lower than the design capacity.

(2) The effect of discharge rate on the actual output capacity of the battery

The battery capacity C(Ah) is equal to the product of the discharge current (A) and the discharge time (h) at which the battery voltage reaches the lower limit value, and the discharge rate (1/h) is the actual discharge current (A) and the battery nominal capacity (Ah). The ratio of ).

In the actual operation of the UPS, after the mains power is turned off, the battery inverter is required to bear all the load power. The discharge rate varies greatly depending on the backup time. For example, the standard machine is about 10 minutes, the maintenance time is short, and the discharge rate is low. Very large, long delay machine up to 4h or 8h, the discharge rate is very small. Therefore, the actual discharge rate of the battery is not the discharge rate in the battery specification definition. The discharge curve shown in Figure 5-1 reflects the effect of different discharge rates on the battery capacity.

The smaller the actual discharge current of the battery, the longer the battery's voltage can be maintained, and vice versa. For example, for the 1OOHR battery pack, when the discharge current is 5A, the discharge rate is 0.O5C, and the output voltage is maintained above 12V for more than 10h. When the battery voltage drops to the threshold voltage of 10.5V, the discharge time is Up to 2Oh, the capacity released by the battery is basically its nominal capacity. When the discharge current is increased to 100 A and the discharge rate is 1 C, the output voltage is maintained at l2 V or more for less than 10 min. When the battery voltage drops to the threshold voltage, the discharge time can be maintained for more than 30 minutes, and the actual discharge capacity is about 58.3.M, which is much lower than the nominal capacity of 1OOAh.

The allowable discharge threshold voltage value of the battery pack is closely related to the actual available capacity (the discharge current of the AM bulb battery).

The allowable discharge time of the battery is the time it takes for the battery voltage to drop from its rated value to its allowed threshold voltage when the battery is discharged at the actual discharge current. The efficiency with which the battery is available is the ratio of the actual maximum capacity that it can release at its actual discharge current to its rated capacity.

It should be noted that at different discharge rates, the critical value of the battery terminal voltage drop also changes. When the discharge rate is low, for example, 0.01 C, the actual released capacity is close to the nominal capacity, and the allowable battery terminal voltage drop is also high ( 10.5V), when the discharge rate is large, for example, 1C, the actual discharge capacity is small, but the allowable battery terminal voltage can also be lower (8V).