1. Battery characteristics
For independent power supply systems, the following points must be considered when selecting batteries:
(1) How much energy needs to be stored in the battery.
(2) Charge rate and discharge rate.
(3) To what extent can the battery be discharged.
(4) What is the voltage required by the battery.
(5) How to judge whether the battery is overcharged or undercharged.
(6) Factors affecting the performance and life of the battery.
(7) Battery maintenance.
The cold cranking current (CCA) of a car battery is a key factor, but the ampere-hour capacity and cycle life are the main criteria for deep cycle batteries to be used in self-contained power systems. The selection of steel plate thickness, alloy mixture, separator design, electrolyte concentration and volume must be made with the premise of optimizing deep-cycle battery performance.
2. battery energy storage
The energy generated and used by various electrical equipment is usually measured in watt-hour (W·h) or kilowatt-hour (kw·h), but in order to reflect the capacity of battery storage, it is usually recorded in ampere-hour (A·h), called battery capacity.
Note: A h is not a unit of energy, it is a unit of electricity in the battery.
The energy of the battery depends on the battery capacity and voltage. The conversion formula between w h and A h is
The conversion formula between kW·h and A·h is
3. battery capacity
When fully charged, each battery has the ability to generate a certain amount of power. For a given discharge rate and battery temperature, the battery capacity is given by the manufacturer. The discharge rate (usually C20 or C100) refers to the number of hours the battery is powered at a certain current. Therefore, if the battery capacity is expressed as C20=100A·h, it can provide a current of 5A for 20h; if the battery capacity of a battery is expressed as C10=100A·h, it can provide a current of 10A for 10h; if a battery of a battery has a current of 10A The capacity is expressed as C100=100A·h, then a current of 1A for 100h can be continuously provided.
But battery capacity also depends on its discharge rate. For a given battery, the faster the discharge rate, the lower the usable capacity.
That is to say, if a battery has a capacity of C10=100A·h (10A, 10h), if it is discharged at 20A, it will not last for more than 5h, and may only last for 4h (even if the given battery capacity is 100A·h), so At the charging rate C4, the battery capacity is only 80A·h .
The capacities of several Exide energy storage batteries are shown in Table 1.
|model||discharge time||discharge time||discharge time||discharge time||discharge time|
4. Charge rate
The charging rate is specified by the battery manufacturer and depends on the state of charge. A deeply discharged battery can be charged at a high charge rate (such as C10 or faster) for a period of time, but as the battery approaches full charge, the charge rate must be reduced to C50 or lower to reduce moisture loss from the battery. In any case, always follow the battery manufacturer’s recommendations.
5. Cut-off voltage
As current flows from the battery, the voltage measured at the battery terminals drops, this minimum allowable voltage is called the cut-off voltage. When the voltage is below this cutoff voltage, the current may cause permanent damage to the battery and permanent loss of battery capacity.
When battery manufacturers specify the capacity at a specific discharge rate, they usually also specify the cut-off voltage. Different discharge rates and currents may have different cutoff voltages.
6. Depth of discharge (DOD)
Depth of discharge is a measure of how much of the total battery capacity is consumed, usually expressed as a percentage. For example, if the battery is discharged at 20A for 5h at the rated capacity C10=200A·h, the remaining capacity is 200-(5×20)=100(A·h). In this case, the initial capacity of 100/200 has been consumed, so the depth of discharge is 50%.
The difficulty with determining the exact depth of discharge is that the actual capacity always depends on the discharge current.
In the above example, if you discharge at 10A for 10h, the battery discharges 100A·h, but the depth of discharge is not necessarily 50%, because at 10A, the actual capacity of the battery is expected to be larger, because it is discharged at 20A less than the rated current. , so actually its depth of discharge is expected to be less than 50%.
The recommended maximum depth of discharge is generally around 70%, but the battery manufacturer’s specifications should also be consulted. Periodically discharging to this depth will greatly reduce the cycle life of the battery.
Battery capacity decays over time, and when a battery cannot be charged to more than 80% of its original capacity, it is considered to be at the end of its life and deteriorates rapidly thereafter. Battery life should preferably be expressed in number of cycles, but for most cycles the number of cycles varies depending on the depth of discharge.
Battery manufacturers specify their battery life for a specific depth of discharge, see Table 2.
|Depth of discharge/%||Battery life (cycles)||Battery life (cycles)|
|Century Yuasa SSR@C10||Exide Energy Storage @C10|
Other factors, including battery temperature, also play a role in the decay of battery capacity. It is reasonable to expect a lifespan of at least 10 years if properly configured, sized and maintained. Some batteries have been running for up to 15 years. The relationship between Exide A400 battery temperature and service life is shown in Figure 1.
7. days depth of discharge
Although the battery has a maximum allowable depth of discharge before it is damaged, in a self-powered system, the battery is usually discharged to a lower level each day, which is called the daily depth of discharge. Daily depth of discharge is usually kept below 20% to extend battery life.
8. Battery efficiency
During discharge, no battery is able to release all the energy it captured during charging. Reasons include:
(1) The charging process generates heat energy and dissipates it in the surrounding environment.
(2) The charging voltage is higher than the discharging voltage, which represents the loss of potential energy, so the energy is also lost.
(3) The process of splitting water to produce gas will dissipate energy. Therefore, the efficiency of the battery decreases as it approaches a fully charged state.
The efficiency of a cell can be expressed as ampere-hour efficiency (or coulombic efficiency as in AS4509.2) or watt-hour efficiency.
The ampere-hour efficiency calculation formula is
Ampere-hour efficiency = discharge ampere hour/charge ampere hour
The typical efficiency of a photovoltaic cell is 90%, but at equilibrium (during gassing) the state is greatly reduced. Efficiency varies with state of charge and also depends on instantaneous charging current and voltage.
The watt-hour efficiency of a battery reflects the true energy efficiency of the battery,
Its calculation formula is
Watt-hour efficiency = discharge watt-hour / charge watt-hour
Because the charge voltage is always greater than the discharge voltage at a particular discharge capacity, the watt-hour efficiency will always be less than the equivalent amp-hour efficiency of the same cell.
9. Discharge rate
Battery manufacturers determine a battery’s discharge rate not by amperage, but by battery discharge time, which usually refers to the time it takes to drop to a specified voltage (eg, 1.85V or 1.8V per cell). For example, a battery with a rated capacity of C20=100A·h and a discharge current of 5A will take 20h to discharge to the specified voltage (the specified voltage is determined by the manufacturer). Discharging the battery below the specified voltage will usually damage the battery. The faster the battery discharge rate, the less available energy is drawn from the battery. AS/NZS4509.2-2010 “Independent Power Supply Systems Part 2: Guidelines for System Design” stipulates that the discharge rate of batteries used in renewable energy systems can be C100, while the discharge rate required in larger grid-connected systems is C20 . The ampere-hour capacity of C100 is higher than that of C20 because of its slow discharge rate.
However, for Australia’s current independent power supply system, the typical discharge rate is C20~C50.
During charging and discharging, some cells in a battery system may have a different voltage than other cells. To equalize the state of charge of each cell, the system will charge to a fully charged state when the charging voltage approaches 2.5V per cell. Overcharging a battery pack will “equalize” the state of charge and voltage of all battery cells.
A charging battery cell can split water to produce hydrogen and oxygen to release gases. Gas evolution is accompanied by loss of moisture from the electrolyte. The hydrogen gas produced in the process could explode if there were sparks around. Gassing can even occur in uncharged batteries.
NOTE: Always be safe when working near batteries!
Outgassing also causes water loss, but it helps to mix the electrolyte. Over time, the electrolyte’s heavier acid tends to deposit to the bottom of the cell (called delamination), but gassing can hinder its deposition.
12. Self-discharge rate
Even when there is no load, there are always chemical reactions taking place in the battery cells, which reduce the capacity of the battery. The rate of charge loss in this process is called the self-discharge rate. Depending on the type of battery and its chemical composition, the self-discharge rate may be 1% to 3% per month, and as the battery ages, its self-discharge rate generally increases. In systems that do not include backup generators, the self-discharge rate should be one of the factors that determines the size of the system. The self-discharge rate also affects the overall efficiency of the battery.
13. State of Charge (SOC)
The state of charge represents the initial available capacity of the battery, expressed as a percentage of the rated capacity. For example, a battery with a depth of discharge (DOD) of 25% may reach a state of charge (SOC) of 75%.
DOD=(discharge capacity/rated capacity)×100%
SOC=(available capacity/rated capacity)×100%
Therefore, SOC plus DOD always equals 100%.
The relationship between DOD, SOC and battery capacity is shown in Figure 2.
14. Specific Gravity
Specific gravity is the ratio of electrolyte density to water density. Sulfuric acid is heavier than water, so batteries using sulfuric acid have a specific gravity greater than 1. During battery discharge, the specific gravity drop is linear with the state of charge, while the voltage is nonlinear with the state of charge (Figure 3). The specific gravity of the electrolyte can better reflect the state of charge of the battery cell. Measurements taken using a hydrometer sample from the electrolyte can be used to monitor the state of charge of the battery. When determining the state of charge from specific gravity, it is important to be aware of the battery’s specifications given by the manufacturer, while also remembering that the specific gravity of the battery electrolyte varies with temperature (Figure 5).
When the battery is in a low state of charge for a long time, lead sulfate in crystalline form is deposited on the surface of the electrodes. Self-discharge can lead to sulfation when the battery is not charged frequently. Sulfation can lead to permanent loss of battery capacity because the effects of lead sulfate produced at low state of charge for extended periods of time are difficult to reverse.
Often adding other additives to the battery alleviates this problem. But it is best to ensure frequent, full charging, and monthly equalization of the battery voltage to prevent sulfation of the battery. For sulfated batteries, charging may require prolonged slow charging with a charger.
16. Battery voltage
Each cell of a lead-acid battery generates a voltage of about 2V. The operating voltage of a battery cell is not constant due to factors such as the internal resistance and temperature of the battery cell. The internal resistance of the battery depends on the specific gravity of the electrolyte and the amount of lead sulfate as an electrical insulator. During the discharge process, the voltage across the battery drops due to the current flowing through the resistor.
As the discharge process progresses and the depth of discharge increases, the battery operating voltage bends and decreases along each curve. These dips are often referred to as the inflection points of the voltage curve, the first sign of the discharge process. The corresponding voltage is called the cut-off voltage. From this, it can be seen that the cut-off voltage depends on the discharge rate. After the inflection point of the voltage curve is exceeded, the battery cell can release less energy.
The charging voltage of a lead-acid battery depends on the battery’s state of charge, charging rate, and its operating history, as shown in Figure 3.
If the battery is charged with a constant current, then according to Figure 3, the charging voltage will vary with the state of charge.
In area A, during the charging process of the battery, lead sulfate is changed back to lead and lead dioxide; in area B, when charging is nearing the end, electrolysis begins, and the generated gas helps to mix the electrolyte; in area C, due to excessive gas evolution, Loss of electrolyte, so efficient charging is no longer possible.
17. Charge after deep discharge
The charging curve for a given depth of discharge and charging rate is shown in Figure 4.
18. State of charge determination
1) Specific gravity
The most common way to determine the state of charge is to measure the specific gravity of the battery using a hydrometer. When using a hydrometer to determine the state of charge, the curve provided by the manufacturer must be used, and the temperature of the battery should be considered and measured. Since acid is heavier than water, this method will be inaccurate if delamination occurs in the battery or if the battery is recharged after a deep discharge.
2) Open circuit voltage
When a battery is charging, the battery voltage is higher than the battery’s nominal voltage because a higher voltage is required to drive current to the battery. Likewise, as the battery discharges, the battery voltage drops.
Therefore, to use the battery voltage to determine the state of charge of the battery, the open circuit voltage must be used. It is necessary to disconnect all loads and charging devices, and allow at least 20 minutes for the battery voltage to stabilize. The state of charge can then be determined using a curve or table provided by the manufacturer. This is not always possible if the user uses electricity during the day and does not want to waste available solar energy.
3) Discharge and charge voltage
The manufacturer can provide the discharge and charge curves of the battery, which show different charge currents and the corresponding voltage at the depth of discharge (discharge) or state of charge (charge). Different temperatures correspond to different curves. Therefore, by measuring current, battery voltage and temperature, the state of charge can be determined.
19. Battery Maintenance
Batteries should be checked regularly for moisture loss. The water loss mainly comes from gassing. Distilled water should only be added to a single cell if required, and all electrodes should be ensured to be fully submerged in the electrolyte, never adding acid unless a leak occurs.
Batteries should be inspected for any acid leaks, terminal corrosion, casing cracks. Regulations regarding acid inhibition should comply with relevant Australian standards. The specific gravity of the electrolyte should be measured. If the difference in specific gravity between cells in a string is greater than 0.02, cell balancing should be performed.
All electrical connections to the battery should be checked regularly to ensure they are secure. The battery must be placed in a ventilated place, and the top of the battery should be kept clean and dry.
20. Temperature effects
During cold periods, when the battery temperature is low, the rate of chemical reactions is reduced and the penetration of ions into the sheet is slower, so less exposed material is available for the reaction, with a concomitant loss of available battery capacity. The battery capacity is usually given at a reference temperature of 25°C, at higher or lower temperatures, it should be corrected using the correction factor curve, as shown in Figures 5 and 6. Note that the correction factor depends on the discharge rate.
21. battery mix
The rated voltage of lead-acid batteries is 2V. Batteries can be arranged in series to generate higher voltages; or in parallel to increase capacity and maintain a constant voltage. As shown in Figure 7, if each battery has a capacity of 100A·h and a voltage of 2V, each string will have a voltage of 6V and a capacity of 100A·h, and two strings in parallel will obtain a capacity of 200A·h and a voltage of 6V.
The cells of the battery pack must have the same capacity and be from the same manufacturer.
Note: Batteries of different ages should not be used, otherwise equal charging current and charging voltage will not be obtained.
Fuses and isolation measures should be provided for each string.
22. Battery selection
The following factors need to be considered when selecting a battery (some of which are given by the manufacturer, so review their specifications carefully): system type and mode of operation; self-discharge rate; charging characteristics, internal resistance; maximum battery capacity; discharge magnitude and variation of current; days of storage required; size, weight, and installation location accessibility; energy storage density;
Maximum allowable depth of discharge; outgassing characteristics; daily depth of discharge requirements; susceptibility to freezing; ambient temperature and environmental conditions; sulfate sensitivity; cycle life; electrolyte concentration; availability of auxiliary hardware; terminal configuration; product reputation; Repair requirements; sealed or not; cost and warranty.
23. Installation Requirements
When the battery outgassed, the hydrogen gas escaped. It explodes extremely easily when mixed with oxygen and ignited. If the battery is installed in a poorly ventilated confined space and the positive and negative plates of the battery pack are close to each other (in this case, if a conductor is placed across the terminals, such as while a wrench is being serviced, there is a risk of sparking ), it will cause an explosion.
Therefore, for safety reasons, the following standards must be followed to install and place batteries:
(1) AS4086.2 “Secondary Batteries for Independent Power Supply Systems Part 2: Installation and Maintenance”.
(2) AS/NZS4509.1-2009 “Independent Power Supply System Part 1: Safety and Installation”.
(3) AS/NZS4509.2-2010 “Independent Power Supply System Part 2: System Design Guidelines”.
(4) All battery terminals and connecting lugs must be covered (shrouded) to ensure that no objects can come into contact with them. The shield should accept a test probe without exposing the terminals.
(5) The battery must be placed in a well-ventilated space and on a flat, firm surface.
24. Other considerations
(1) The battery should be kept away from water to prevent corrosion. Commercially available conductive greases, sprays, and terminal covers can be used to protect terminals from corrosion.
(2) The battery should be placed off the ground or concrete. If placed on the ground, the battery will absorb the temperature of the ground, which is generally lower than the optimal operating temperature of the battery. In addition, a battery in operation generates some heat, and when the bottom of the battery is cooler than the top, long-term thermal stress can cause the battery to fail prematurely.
(3) Pests should be isolated. Insulating materials such as plastic are often consumed and damaged by rodents.
(4) A fence should be placed to control the ambient temperature to be kept at 20~25℃ as much as possible. A thermometer should be provided for checking battery temperature.
(5) Safety equipment such as eye protection, eye wash bottles and sodium bicarbonate should be provided. There should be a high-quality hydrometer near the battery.
(6) Make sure that the hydrometer is very clean before being inserted into the battery.
(7) Safety signs should follow AS/NZS4086.2-2010 “Independent Power Supply System Part 2: System Design Guidelines” and must be clearly visible.
25. battery cost
Batteries cost anywhere from less than $100 to over $600 per 2V (usually depending on capacity) and represent a large percentage of the cost of a standalone power supply system. Determining the price of a battery starts with determining the required capacity, and then researching the products on the market, not only the price, but all the factors that go into the battery selection criteria (described above).
26. Battery fault current
In a DC system, a battery short circuit can generate extremely high currents for short periods of time, about 100 to 1000 times the typical discharge current in most applications. The short circuit current of the battery can be estimated by dividing the open circuit voltage by the internal resistance. Manufacturers will provide short-circuit current or fault current values from the information they publish or upon request.
Systems using batteries must be protected by bidirectional fuses. Without this protection, a short circuit could permanently damage the battery itself.
Figure 8 shows the discharge voltage and current versus time for a shorted VRLA battery.
It should be noted that stable minimum voltage and maximum current readings can be reached within 5~10ms. It can be seen from Figure 8 that the short-circuit current of 1754A is reached within 10ms, which is about 640 times the current at the 10h discharge rate (2.73A@C10).
Fault currents and short-circuit currents are extremely dangerous to personnel working on and near the battery. AS/NZS4509 “Independent Power Supply System” requires that all independent power supply systems must set up signs to display the battery system voltage and fault current under the installed battery capacity.