The main content of this article is the commonly used terms of photovoltaic batteries: 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 charging and discharging conditions. At the end of charging, the voltage is 2.5 ~ 2.7v, and then slowly drops to a steady state of about 2.05v. If the battery is used as the power source, the voltage will drop to about 2v quickly at the beginning of discharging, and then it will drop slowly and stay between 1.9 and 2.0v. When the discharge is close to the end, the voltage quickly drops to 1.7v; when the voltage is lower than 1.7v, it should not be discharged anymore, otherwise the plates will be damaged. After stopping use, the battery voltage can rise to 1.98v by itself.
(2) Capacity of 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 prescribed end voltage under certain discharge conditions is called battery capacity, which is represented by the symbol c. The common unit is ampere hour, abbreviated as ampere hour (a•h). Usually the discharge rate is indicated at the lower corner of c, and this following number indicates the time (h) for this type of battery to discharge to the set voltage with a certain intensity of current. For example, c10 indicates the discharge capacity at a discharge rate of 10h, and c120 indicates the discharge capacity at a discharge rate of 120h.
The battery capacity is divided into theoretical capacity, actual capacity and rated capacity. Theoretical capacity is the highest capacity value calculated according to Faraday’s law based on the mass of the active material; the actual capacity refers to the amount of electricity the battery can output under certain discharge conditions. When forming a battery, in addition to the main reaction of the battery, there are side reactions that occur, and for 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 country or relevant departments, the battery design requires the battery to discharge the minimum amount of electricity under certain discharge conditions (communication batteries generally require discharge at a loh discharge rate current to the final voltage at 25°C). The factors that affect the actual capacity are mainly related to the quantity and utilization of the positive and negative active materials of the battery. The utilization of active materials is mainly affected by the discharge system, electrode structure, and manufacturing process. What affects the actual capacity during use is the discharge rate, discharge system, termination voltage and temperature.
(3) Discharge rate
According to the size of battery discharge current, it is divided into time rate and current rate.
The discharge rate refers to the length of time from discharge to the end of discharge voltage under certain discharge conditions. Commonly used time rate and magnification.
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 discharge time rate is 20, 10, 5, 3, 1, 0.5 hour rate, respectively marked as 20h.
10h, 5h, 3h, 1h, o.5h, etc. The most common are 20h, 10h hour rate. Then, divide the number of hours by the capacity to get the rated discharge current. In other words, batteries with the same capacity but different discharge rates have far different nominal discharge currents. For example, a battery used in an electric bicycle has a capacity of 10a•h and a discharge rate of 2h, written as 10a•h2, and its rated discharge current is 10a•h/2h=5a; while the battery capacity for starting a car is 54a•h , The discharge rate is 20h, written as 54a•h20, its rated discharge current is only 54a•h/20h=2.7a! From another perspective, if these two batteries are discharged with currents of 5a and 2.7a, they should It can last for 2h and 20h respectively before dropping to the set voltage.
The current intensity of the battery in operation is often expressed by the rate (current rate), written as nch, which refers to the multiple of the discharge current value equivalent to the battery rated capacity (a•h). n is a multiple, c represents the ampere hour of the capacity, and h represents 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 describing a battery at a certain time rate, the rate is often written in the form of nc without writing the label. The multiple n times the capacity c equals the current (a). For example, a 20a•h battery is discharged at a rate of 0.5c, o.5×20=10a. Take another example: the starting battery capacity of a car is 54a•h, and the measured output current is 5.4a, then its discharge rate nc at this time is (5.4/54)c=0.1c.
(4) Termination voltage
The termination voltage refers to the lowest working voltage at which the battery discharge voltage drops to the point where it is no longer suitable for discharging (at least it can be recharged and used repeatedly). In order to prevent the battery from over-discharging and damaging the plates, various standards stipulate the battery’s end voltage when discharging at different discharge rates and temperatures. The termination voltage of the backup power series battery for 10h discharge rate and 3h discharge rate is 1.80v/cell, and the 1h discharge rate termination voltage is 1.75v/cell. Due to the characteristics of lead-acid batteries, even if the termination voltage of discharge continues to decrease, the battery will not release too much capacity, but the termination voltage is too low to damage the battery greatly, especially when the discharge reaches 0v and cannot be charged in time Greatly shorten the battery life. For solar batteries, for different models and uses, the end voltage value is not fixed, it decreases with the increase of the discharge current, usually, less than 10h small current discharge, the end voltage value is slightly higher; more than 10h For high current discharge, the final voltage is slightly lower.
(5) Cycle life
The battery undergoes a charge and discharge, which is called a cycle (a 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 floating charge life to measure the battery life. For example, the floating charge life of a valve-regulated sealed lead-acid battery is generally more than 10 years, but the cycle life of the battery can also be used to measure. The main factor affecting the cycle life of the battery is the performance and quality of the product, followed by the quality of maintenance work. For the back-up power supply, the cycle life of 100% discharge depth is generally 100 to 200 times, that is, the battery discharges 100% capacity, and the battery discharges to a terminal voltage of 1.8v/cell. After 100 to 200 cycles, the battery discharges The termination voltage is 1.8v, and the discharge capacity is lower than 80% of the rated capacity, at which time the battery life ends. The factors that affect the battery life are comprehensive factors, not only the internal factors of the plate, such as the composition of the active material, crystal type (high temperature curing or normal temperature curing), plate size and grid material structure, etc., but also depend on 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 over time during the charging and discharging process, because the generation of active material, electrolyte concentration and temperature are constantly changing. The internal resistance of lead-acid batteries is very small and can be ignored when discharging with a small current, but when discharging with a large current, the voltage drop loss can reach hundreds of millivolts, which must be paid attention to. The internal resistance of the battery has two parts: ohmic internal resistance and polarization internal resistance. The ohmic resistance is mainly composed of electrode material, diaphragm, electrolyte, terminal, etc. It is also related to battery size, structure and assembly factors. Polarization internal resistance is caused by electrochemical polarization and concentration polarization, and is the internal resistance generated by polarization when the two electrodes undergo a chemical reaction during battery discharge or charging. Polarization resistance is not only related to battery manufacturing process, electrode structure and activity of active materials, but also related to factors such as battery operating current and temperature. The internal resistance of the battery seriously affects the battery’s operating voltage, operating current and output energy, so the smaller the internal resistance, the better the battery performance.
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