Most power sources deliver power immediately after generating it, with the exception of batteries, which store power and deliver it to the load only when needed. (Chemical) Batteries are electrochemical energy sources that use chemical reactions to generate direct current. The rechargeable battery can also be charged after delivering energy to the load, and the process can then be repeated. Mainly take lead-acid battery as an example to discuss the electrochemical process, battery types and their characteristics (Note: the battery mentioned refers to chemical battery).
1. The development history of batteries
In the early 19th century, Alessandro Volta used salt solutions as electrolytes to conduct chemical experiments between different metals. Volta devised a battery in which the zinc and silver sheets were separated by an insulator saturated with a salt solution. Volta stacked these pole pieces and called the battery a “voltaic stack” (see Figure 1). A battery pack consists of a set of battery cells connected together, each of which produces a certain voltage based on the material it is made of.
Figure 2 shows the symbols for battery cells and battery packs.
2. Primary batteries
During chemical reactions inside the battery, certain pole piece materials are destroyed. Figure 3 shows a primary battery using copper and zinc as pole pieces. In the chemical reaction of this kind of battery, the copper sheet cannot be recovered after being attacked by acid. So the primary battery cannot be charged.
3. Secondary batteries
Many secondary batteries are made from the materials listed in FIG. 4 . These materials are not destroyed by chemical reactions and can be restored by charging operations. Therefore, the secondary battery can be charged.
Figure 5 shows a battery pack assembled from cells wrapped in specially treated wax paper.
Figure 6 shows a lead-acid battery composed of multi-layer plates in parallel, this structure can increase the chemical reaction area and current. One set of pole plates constitutes a positive pole, and the other set of pole plates constitutes a negative pole.
4. Internal resistance
The battery has two rated voltages, corresponding to no-load and loaded states, respectively. The battery voltage at rated load is listed in the manufacturer’s specifications, printed on the battery’s label. Because the battery has internal resistance, the no-load voltage is higher than the load voltage. All batteries have internal resistance, and when the load draws current, some voltage “falls” across the battery’s internal resistance. Therefore, in order to obtain accurate voltage readings, the battery must be measured under load. Figures 7 and 8 show the different output voltages due to the internal resistance of the battery at no load and at full load, respectively. If the electrode voltage of the battery at no-load is 13.5V and the full-load voltage is 12V, then 1.5V voltage falls on the internal resistance of the battery (13.5V-12V). If the load current is 10A, the internal resistance is 1.5V/ 10A = 0.15Ω.
In a lead-acid battery cell, one plate is composed of pure lead (Pb), the other plate is composed of lead dioxide (PbO2), and the electrolyte is dilute sulfuric acid H2SO4. The plates are immersed in the electrolyte. When the battery is fully charged, the relative density of the electrolyte solution is 1.215 ~ 1.280, depending on the temperature and occasion. Relative density indicates the amount of acid in the liquid. The relative density of water is 1.000, and acid will increase the relative density. The device used to measure the relative density of the electrolyte is called a relative density meter. In practice, relative density readings can be used to accurately measure the state of charge and discharge of a battery.
6. Discharge process
A battery is discharged when current flows from the battery to the load. In Figure 9, a load is connected between the positive and negative poles of the battery. The positive plate of the battery is composed of lead dioxide, and the negative plate is composed of pure lead. When the battery discharges, the negative and positive plates are coated with lead sulfate, and water is produced to dilute the sulfuric acid. The electrolyte relative density of a fully discharged lead-acid battery is 1.000. During discharge, the voltage of a 12V lead-acid battery will drop to 10.8V, and by charging, it can return to its original state.
7. Charging process
In Figure 10, a DC power source is charging a battery. The voltage of the charging source must be higher than the battery voltage in order to “push” current into the battery. The charging voltage of a 12V battery is about 14V. The charging current reverses the discharge chemical reaction and brings the relative density of the electrolyte back to 1.215 ~ 1.280. During charging, the lead sulfate on the surface of the electrode plate falls off and is dissolved by the electrolyte. If the charging current is too large, the lead sulfate will fall off faster than the dissolution rate, so it will fall to the bottom of the battery, causing erosion and short circuit of the plate. Proper charging will reduce the plates to pure lead and lead dioxide. A charge controller can be used to control the discharge and charge of the battery.
Note: Hydrogen gas is produced inside the battery during charging, and this hydrogen gas is vented to the ambient atmosphere. Hydrogen is a highly flammable and explosive gas that can be ignited by sparks or open flames.
8. The structure of the battery
A lead-acid rechargeable battery is constructed by a group of secondary battery cells (see Figure 11). The plates of the different cells are separated by cell separators, and the entire cell has a non-conductive casing.
1) The separator separates the positive and negative plates.
2) The cell separator plate separates different battery cells.
3) Electrode posts are used to connect the load to the + and – terminals of the battery.
4) The exhaust hole is used to release the gas.
5) The shell is used to protect the plate and hold the electrolyte.
Sulfation refers to the crystallization of lead sulfate on the plates of lead-acid batteries. When the battery is charged and discharged, lead sulfate slowly turns into a strong crystalline structure that cannot dissolve during charging, preventing the plates from reducing to lead and lead dioxide. Over time, the activity of the plates will decrease and the charging capacity will decay. If the battery is in a fully or partially discharged state, the sulfation process is accelerated. The sulfation phenomenon can be partially reversed by charging with short pulses of high current for a period of time.
Layering occurs in lead-acid batteries with liquid electrolytes. If the battery is not cycled for a long time, the electrolyte will stratify, the water will rise to the upper layer, and the acid will settle to the bottom layer of the case. This will increase the erosion of the bottom of the battery cell, reducing the output power. Frequent charging and discharging minimizes stratification by allowing the acid and water to mix evenly.
11. State of charge
State of charge (SOC) refers to the voltage remaining in the battery as a percentage of the fully charged voltage. Voltage is affected by remaining charge, discharge rate, and temperature. When the current flows to the load, the voltage will change to a certain extent due to the internal resistance. The greater the discharge current, the greater the voltage drop across the internal resistance, and the faster the energy (Ah, A h) is consumed. Therefore, the high current can transfer less energy (A h) from the battery (less energy transferred to the external load) before the battery reaches the cut-off voltage. A lower discharge rate allows more energy to be released (more energy transferred to an external load) before the battery reaches the cutoff voltage.
Figure 12 shows the relationship between voltage drop and energy release (A h) at different discharge rates. At lower discharge rates, the battery can deliver current to the load at a higher voltage before reaching the cutoff voltage. If the discharge rate is high, the voltage cut-off occurs very quickly. The cut-off voltage is the manufacturer’s recommended minimum discharge voltage.
12. Discharge rate
Discharge rate refers to the ratio of current amperage to rated capacity. For example, if the rated capacity of a battery is 100A·h and the discharge current is 5A, the ratio is 5/100=1/20, and the discharge rate is C/20. This means that the discharge rate of 1/20 of the rated capacity (1/20 × 100A h) is 5A/h, that is, the battery can be discharged for 20h at this rate, and the discharge rate of C/10 is 10/100= 1/10 , which means that the discharge rate of 1/10 of the rated capacity is 10A/h, that is, the battery can be discharged for 10h at this rate. In Figure 12, curve (a) represents the C/20 discharge rate, and curve (b) represents the C/5 discharge rate.
In an ideal state, a battery rated at 180A·h and 9A can discharge continuously for 20h at a current of 9A.
The unit of capacity of a battery is ampere-hour (A·h) or watt-hour (W·h). A battery can be discharged for 20 hours at a current of 10A, and its discharge capacity is 200A·h (10A×20h). If its average discharge voltage is 12V, its discharged power is 2400W·h (12 V×200A·h).
The capacity of a battery is not a fixed value and changes depending on temperature and discharge rate. The lead-acid chemical reaction is very sensitive to temperature. When the temperature of the battery increases, the internal resistance will decrease, the pressure drop across the internal resistance will decrease, and the battery capacity will increase. The capacity of the battery (the amount of current it can sustain) decreases when the temperature decreases, and increases again as the temperature increases. A hotter battery can store more power than a colder battery. Although batteries have higher capacity at higher temperatures, their charge-discharge cycle life is shortened.
Figure 13 illustrates capacity as a function of temperature and discharge rate. When curve (c) is at 25°C, the battery has 85% capacity, while at -20°C, the battery has only 20% capacity. When curve (a) is at 25°C, the battery has 100% capacity, while at -20°C, the battery has only 80% capacity. Temperature also has an effect on charging voltage: at -40°F, a 12V lead-acid battery charges 15.6V, and at 77°F, the charging voltage drops to about 13.3V. Battery ratings are measured at 77°F (25°C).
14. Self-sustaining time
Self-sustaining time refers to the time that a fully charged battery can continuously supply power to the system load without halfway charging, usually measured in days. For example, in a photovoltaic (PV) system, the self-sustaining time is calculated based on the average number of days without sunlight over a period of time. (Translator’s Note: The self-sustaining time of a photovoltaic system refers to the time that the backup battery of the system is used to supply power to the load, and the photovoltaic battery does not need to work at this time.) In the southern United States, the self-sustaining time of the system is usually two consecutive dark days. In the north, there may be three or four consecutive dark days. Therefore, the capacity and self-sustaining time of the battery should meet these conditions. The hold-up time is also a function of the application, if the load is controllable, or if there is a generator for charging, the hold-up time will change. In some applications, the allowable depth of discharge of the battery can be increased to obtain longer self-sustaining time. For example, the rated depth of discharge of the battery is 25% in summer and 50% in winter, so the time between charging in winter is extended.
15. depth of discharge
When designing a battery backup system for a photovoltaic or wind power system, both self-sustaining time and depth of discharge (DOD) should be considered. DOD is the ratio of battery energy consumed to energy when fully charged. The battery’s specification states the allowable DOD. Deep cycle batteries are typically allowed to discharge to 80%, while car batteries are only rated for 20% DOD. Allowable DOD values depend on battery type, temperature and other factors. When designing the size of the backup system, DOD can have a significant impact on self-sustaining time.
16. Battery charging method
There are many ways to charge the battery to full capacity. Charge rates are identified in the same way as discharge rates. If a 200A·h battery is charged at a rate of C/20, it can be charged at 10A until it is fully charged. Fast charging refers to rapidly charging the battery to 80% to 90% of its full capacity using a high charging rate.
When the battery is quickly charged to 80%~90% of the full capacity, the charging current becomes smaller and the battery is slowly charged. This type of charging is called absorption charging. When the battery is charged to 100%, switch to trickle charge or float charge to keep the charge near 100%. Batteries lose some of their charge after a period of time, even when no load is connected. Float charging can be used to keep the battery in a fully charged state. Most charge controllers use these three charging sequences.
Different cells in a battery usually have different charge rates and different discharge rates. Therefore, batteries using liquid electrolytes and vents must use an equalization charging method. That is, the applied voltage is higher than the normal charge value to keep the battery at a level that can generate venting. Equalization charging should usually be implemented when the battery capacity deteriorates. Regular equalization charging reduces sulfation and stratification. Please refer to the battery’s specification for the manufacturer’s recommended equalization charging frequency. Sealed batteries do not require equalization charging, and may even be damaged by equalization charging.
17. Types of batteries
Most of the batteries used in the field of renewable energy are lead-acid batteries, which are divided into three types: power batteries, starting batteries and stationary batteries.
1) The power battery is a deep cycle battery with thicker plates. This design makes the battery peak current smaller, while allowing DOD up to 80%. Deep cycle batteries can withstand multiple periodic charge and discharge, and power batteries are mostly used in photovoltaic and wind power systems.
2) Starter batteries are not designed for deep cycle discharge. They have a large number of thinner plates that can provide a lot of current for a short period of time, but are not suitable for deep charge and discharge. Starter batteries are often used to start car engines.
3) Fixed batteries are usually used as uninterruptible power supplies (UPS). UPS batteries supply power to some important loads, such as computers, special medical instruments, and emergency lighting appliances. This type of battery is not suitable for frequent charge and discharge cycles.
(1) Fixed electrolyte battery
This is actually a sealed lead-acid battery, and its electrolyte is not liquid like a common flooded battery. There are two common fixed electrolyte design methods, colloidal battery cells and adsorbed fiberglass wool. In gel batteries, silicon dioxide is added to the electrolyte, which turns into a sticky gel when heated and cooled. Adsorbent glass wool (AGM) batteries adsorb the electrolyte on the glass wool, which is then placed between the plates and in contact with the plates.
Compared with vented batteries, fixed electrolyte batteries are more suitable for low current and low rate charging, so that the accumulation of gas in the case can be better controlled. Such batteries require precise control to prevent overcharging and overdischarging, otherwise the life of the battery will be shortened.
The biggest advantages of fixed-electrolyte batteries are that they do not spill liquids, are easy to transport, and are simple to maintain.
(2) Nickel-cadmium battery
Nickel-cadmium (NiCad) batteries are rechargeable batteries. Its positive plate is nickel hydroxide, and graphite is added to enhance its conductivity. The negative plate is made of cadmium oxide and the electrolyte is potassium hydroxide.
This type of battery is rechargeable and can be recharged more than 2000 times on average. The discharge voltage drop of nickel-cadmium batteries is lower than that of lead-acid batteries, as shown in Figure 14. A single nickel-cadmium battery has a voltage of 1.2V, while the single-cell voltage of a lead-acid battery is about 2.2V. Therefore, the series voltage of 10 nickel-cadmium battery cells is 12V, which is approximately equivalent to 6 lead-acid battery cells. series voltage. If the NiCd battery is only partially discharged and then recharged, when the cycle is repeated many times, the battery “remembers” the cycle as a normal characteristic, reducing battery capacity.
18. Hydrogen-oxygen fuel cell
Fuel cells convert chemical energy into electrical energy through chemical reactions. Hydrogen-oxygen batteries use hydrogen as fuel and oxygen as oxidant. A fuel cell is similar to a lead-acid battery, consisting of an anode (-), a cathode (+), and an electrolyte between the anode and the cathode.
As shown in Figure 15, the electrolyte is a potassium hydroxide (KOH) solution, and the anode is a nickel-containing porous carbon electrode to which hydrogen gas (H2) is fed. The cathode is a porous carbon electrode containing nickel and nickel oxide, into which oxygen (O2) is fed. Electrons move from the anode to the cathode of the battery via an external circuit, producing a DC current in the load. The metal in the anode acts as a catalyst for hydrogen, a substance that accelerates the reaction, but does not change itself in the reaction. In this example, the catalyst converts the hydrogen fuel into positive ions and free electrons. The electrolyte in a battery allows positive ions to pass through, but not electrons. The electrons pass through the external circuit and return to the positive (+) terminal of the battery. The positive ions start from the anode and pass through the electrolyte in the battery to the cathode. During the catalytic reaction between oxygen and the cathode, they recombine with the electrons returning to the battery to produce water (H2O) and a small amount of carbon dioxide (CO2).
Because fuel cells generate electricity through electrochemical conversion rather than the combustion of hydrogen, very few by-products from the combustion process are produced. As long as hydrogen fuel and oxygen are supplied, hydrogen-oxygen fuel cells can continue to generate electricity. This is the difference between a fuel cell and a secondary battery, that is, a fuel cell does not have the kind of charge and discharge process that a typical battery does.
Fuel cells have not been commercialized as power storage devices for renewable energy systems. However, they are already used in the aerospace sector as well as in many experimental applications such as hybrid vehicles.
19.General safety protection regulations for battery use
When handling and handling batteries, observe the following safety precautions. But this is only part of the requirements and the manufacturer’s recommendations should also be followed.
1) The electrolyte in lead-acid batteries contains sulfuric acid and water. Sulfuric acid can corrode clothing and burn skin and eyes. An apron, rubber gloves, and safety goggles should be worn when working on the battery. Sulfuric acid spills can be neutralized with baking soda and water. If skin and eyes come into contact with electrolyte, rinse immediately with plenty of clean, cold water.
2) The electrolyte flowing out of the ruptured nickel-cadmium battery can be neutralized with vinegar and water.
3) The pole of the battery will generate a large current when it is short-circuited. Rings, watches, necklaces, and other jewelry should be removed when service personnel install and remove batteries. The tools used should have insulated handles to prevent short circuits if the poles are accidentally touched.
4) Lead-acid batteries produce hydrogen and oxygen when they are charged. Hydrogen is a flammable and explosive gas. The room where the battery is stored should be kept ventilated, and the use of devices that may generate open flames, sparks and other ignition sources, such as relays and switches, should be strictly controlled or removed from the battery room.
20. Series, parallel and series-parallel of batteries
When batteries or cells are connected in series, as shown in Figure 16, their voltages add up, while the capacity remains the same as a single battery. In this example, the output of the battery pack is 48V, 100A·h.
When batteries or cells are connected in parallel, as shown in Figure 17, the voltage remains the same as a battery, and the capacity accumulates. In this example, the output of 4 batteries in parallel is 12V, 400A·h.
Batteries can also be connected in series-parallel combination, as shown in Figure 18. This form of connection allows both voltage and current to be increased. In the analysis and calculation, the series rule is applied to the batteries connected in series first, and after the batteries connected in series are connected in parallel, the parallel rule is applied to the battery packs connected in parallel. In this example, two series-connected branches are connected in parallel, and the output is 24V, 200A·h.
Batteries can be connected in series to increase voltage and in parallel to increase current. When batteries are connected in series, the voltage ratings of the individual batteries can be different, but the rated current must be the same. When batteries are connected in parallel, the voltage ratings of the individual batteries must be the same, but the current ratings can be different.
Batteries with different degrees of aging and different states and specifications should not be mixed or stacked to increase capacity. Otherwise, the system will become unreliable due to different charge and discharge rates. When designing a battery backup system, if additional generation sources (photovoltaic cells or wind turbines) or increased loads are expected, the capacity requirements of the batteries need to be considered at the outset.
21. Scale design of battery packs
The following steps can be used to estimate stand-alone PV scale. The following steps can be used to estimate the size of the battery pack in a stand-alone photovoltaic system (or other type of system).
1) Calculate the daily average AC load W·h (this step is carried out before estimating the size of the battery pack, using the bill of the power company or accumulating the W·h of each energy-consuming project).
2) Divide the result of step 1) by the efficiency of the inverter.
3) Divide the result of step 2) by the DC voltage of the system.
4) The result of step 3) is the daily average (demand) A·h.
5) Multiply the result of step 4) by the number of days of self-sustainment.
6) If using lead-acid batteries, multiply the result of step 5) by the temperature coefficient.
7) Divide the result of step 6) by the DOD percentage (if it is not a lead acid battery, divide the result of step 5) by the DOD percentage).
8) Divide the result of step 7) by the rated A·h of the battery (selected from the system).
9) The result of step 8) is the required number of battery packs in parallel.
10) Divide the DC voltage of the system by the battery voltage.
11) The result of step 10) is the number of cells required in a series branch.
12) Multiply the result of step 11) by the result of step 8).
13) The result of step 12) is the total number of cells required for the battery pack.
Design a backup battery system for an independent photovoltaic array system (see Figure 19), and the output of the system is the AC power output by the inverter. The battery pack is required to provide power for self-sustaining day and night.
1) Daily load demand: 14kW h
2) Self-sustaining time: 2 days.
3) Battery system voltage: 24V.
4) Battery specification: 12V, 250A·h, lead-acid deep cycle battery.
5) DOD: 70%.
6) Minimum ambient temperature: 40°F (temperature coefficient 1.3).
7) Inverter efficiency: 90%.
NOTE: In Figure 19, the battery pack system is within the dashed box. The power generated by the PV array must meet the needs of battery charging and the system, which should be considered when sizing the PV array. When the power generation of the photovoltaic array cannot meet the total demand, only the important loads are supplied with power. Generators are usually employed to supplement power to meet demand at this time.