The main content of this paper is the common term of photovoltaic battery: battery voltage, battery capacity, discharge rate, termination voltage, cycle life, battery internal resistance.
(1) Battery voltage
The nominal voltage of each cell of the battery is 2v, and the actual voltage varies with the charge and discharge. At the end of charging, the voltage is 2.5 to 2.7 volts, and then slowly drops to a stable state of about 2.05 volts. If the battery is used as the power source, the voltage will drop to about 2v at the beginning of discharge, and then slowly decrease, and keep it between 1.9 and 2.0v. When the discharge is nearing the end, the voltage drops quickly to 1.7v; when the voltage is lower than 1.7v, it should not be discharged again, otherwise the plate will be damaged. After stopping use, the battery voltage can rise back to 1.98v.
(2) Capacity of the battery
The concept of battery capacity (c): The amount of electricity that a lead-acid battery in a fully charged state can give when discharged to a specified termination voltage under a certain discharge condition is called the battery capacity, indicated by the symbol c. The commonly used unit is ampere hour, referred to as Anshi (a?h). The discharge rate is usually indicated at the lower corner of c. This following number indicates the time (h) at which the battery is discharged to the set voltage with a current of a certain intensity. If c10 indicates the discharge capacity at a discharge rate of 10 h, c120 indicates the discharge capacity at a discharge rate of 120 h.
Battery capacity is divided into theoretical capacity, actual capacity and rated capacity. The theoretical capacity is the highest capacity value calculated according to Faraday's law according to the mass of the active material; the actual capacity refers to the amount of electricity that the battery can output under certain discharge conditions. Since the battery is composed, in addition to the main reaction of the battery, there is a side reaction, and for various other reasons, the utilization rate of the active material cannot be 100%, so it is far lower than the theoretical capacity; the rated capacity is also called the nominal capacity abroad. According to the standards promulgated by the state or relevant departments, the battery should be designed with a minimum amount of electricity that should be discharged under certain discharge conditions (communication batteries are generally specified to discharge at a loh discharge rate to a termination voltage at 25 ° C).
The factors affecting the actual capacity are mainly related to the quantity and utilization rate of the positive and negative active materials of the battery. The utilization rate of active materials is mainly affected by the discharge system, electrode structure and manufacturing process. The actual capacity is affected during use, such as discharge rate, discharge regime, termination voltage, and temperature.
(3) Discharge rate
According to the discharge current of the battery, it is divided into time rate and current rate.
The discharge rate refers to the length of time from discharge to discharge end voltage under a certain discharge condition. Common time rate and magnification are expressed.
The time rate (hour rate) refers to the time required to discharge the rated capacity of the battery at a certain current value.
According to the iec standard, the time rate of discharge has a rate of 20, 10, 5, 3, 1, and 5. hours, respectively, which is identified as 20h.
10h, 5h, 3h, 1h, o.5h, etc. The most common are 20h, 10h hourly rate. Thus, the rated discharge current is obtained by dividing the number of hours by the capacity. That is to say, batteries with the same capacity and different discharge rates have a very different nominal discharge current. For example, an electric bicycle has a battery capacity of 10a?h, a discharge rate of 2h, and is written as 10a?h2, its rated discharge current is 10a?h/2h=5a; and a car starts with a battery capacity of 54a?h The discharge rate is 20h, written as 54a?h20, its rated discharge current is only 54a?h/20h=2.7a! From another angle, if the two batteries are discharged with the current of 5a and 2.7a respectively, then It can last for 2h and 20h respectively and then drop to the set voltage.
The current intensity of the battery during operation is also often expressed in terms of magnification (current rate), written as nch, which means that the discharge current value is a multiple of the rated capacity (a?h) of the battery. n is a multiple, c is the number of hours of capacity, and h is the time (h) specified by the discharge rate. Here, the value of h is only used as a reminder of which discharge rate the relevant battery belongs to. Therefore, when a battery of a certain time rate is specifically described, the magnification is often written in the form of nc without writing down the mark. Multiplying the multiple n by the capacity c is equal to the current (a). For example, the 20a?h battery is discharged at a rate of 0.5c, o.5×20=10a. An example of changing the angle: a car starts the battery capacity 54a?h, and the measured output current is 5.4a, then its discharge rate nc is (5.4/54)c=0.1c.
(4) Termination voltage
The termination voltage refers to the minimum operating voltage at which the voltage drops when the battery is discharged to a level that is unsuitable for re-discharging (at least for repeated charging). In order to prevent the battery from being damaged even if it is damaged, the termination voltage of the battery is specified in various standards when discharging at different discharge rates and temperatures. The discharge voltage of the backup power series battery and the termination voltage of the discharge discharge of 3h discharge rate were 1.80v/cell, and the discharge voltage of 1h was 1.75v/cell. Due to the characteristics of the lead-acid battery itself, even if the termination voltage of the discharge continues to decrease, the battery will not emit too much capacity, but the termination voltage is too low, which is extremely harmful to the battery, especially when the discharge reaches 0v and cannot be charged in time. Greatly shorten the battery life.
For solar energy storage batteries, the termination voltage value is not fixed for different models and applications. It decreases with the increase of discharge current. Usually, the small current discharge is less than 10h, and the termination voltage is slightly higher; greater than 10h. Large current discharge, the termination voltage value is slightly lower.
(5) Cycle life
The battery undergoes a charge and discharge, which is called a cycle (one cycle). Under certain discharge conditions, the number of cycles the battery can withstand before the battery is used to a certain capacity is called the cycle life. The backup power supply generally uses the life of the float to measure the service life of the battery. For example, the float life of a valve-regulated sealed lead-acid battery is generally more than 10 years, but it can also be measured by the cycle life of the battery. The main factors affecting the cycle life of the battery are the performance and quality of the product, followed by the quality of the maintenance work. For the backup power supply, 100% discharge deep discharge, its cycle life is generally 100 ~ 200 times, that is, the battery is 100% capacity discharge, the battery discharge to the termination voltage is 1.8v / monomer, after cycling 100 ~ 200 times, the battery is discharged The termination voltage is 1.8v, and the discharge capacity is less than 80% of the rated capacity, at which time the battery life is terminated. Factors affecting battery life are a combination of factors, not only the internal factors of the plate, such as the composition of the active material, crystal form (high temperature curing or room temperature curing), plate size and grid material structure, but also external factors. Such as discharge rate and depth, working conditions (temperature and pressure, etc.) and maintenance conditions.
(6) Battery internal resistance
The internal resistance of the battery is not constant and changes continuously with time during charging and discharging because the formation of active substances, electrolyte concentration and temperature are constantly changing. The internal resistance of lead-acid batteries is very small, which can be neglected when discharging small currents. However, when discharging large currents, the voltage drop loss can reach hundreds of millivolts, which must be taken seriously. The internal resistance of the battery has two parts: ohmic internal resistance and polarization internal resistance. The ohmic internal resistance is mainly composed of an electrode material, a separator, an electrolyte, a terminal, and the like, and is also related to the size, structure, and assembly factors of the battery. The polarization internal resistance is caused by electrochemical polarization and concentration polarization, and is the internal resistance generated by polarization when the two electrodes are chemically reacted during battery discharge or charging. In addition to the battery manufacturing process, electrode structure and active material activity, the polarization resistance is also related to the battery operating current magnitude and temperature. The internal resistance of the battery seriously affects the battery operating voltage, operating current and output energy, so the better the internal resistance, the better the battery performance.
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